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1.
What
is the difference between a constructor and a method?
A constructor is a member function of a class
that is used to create objects of that class. It has the same name as the class
itself, has no return type, and is invoked using the new operator.
A method is an ordinary member function of a class. It has its
own name, a return type (which may be void), and is invoked using the dot
operator.
2.
What
is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to
identify and discard objects that are no longer needed by a program so that
their resources can be reclaimed and reused.
A Java object is subject to garbage collection when it becomes
unreachable to the program in which it is used.
3.
Describe
synchronization in respect to multithreading.
With respect to multithreading,
synchronization is the capability to control the access of multiple threads to
shared resources.
Without synchonization, it is possible for one thread to modify
a shared variable while another thread is in the process of using or updating
same shared variable. This usually leads to significant errors.
4.
What
is an abstract class?
Abstract class must be extended/subclassed (to
be useful). It serves as a template. A class that is abstract may not be
instantiated (ie. you may not call its constructor), abstract class may contain
static data.
Any class with an abstract method is automatically abstract
itself, and must be declared as such. A class may be declared abstract even if
it has no abstract methods. This prevents it from being instantiated.
5.
What
is the difference between an Interface and an Abstract class?
An abstract class can have instance methods
that implement a default behavior. An Interface can only declare constants and
instance methods, but cannot implement default behavior and all methods are
implicitly abstract.
An interface has all public members and no implementation. An
abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.
6.
Explain
different way of using thread?
The thread could be implemented by using runnable interface or
by inheriting from the Thread class. The former is more advantageous,
'cause when you are going for multiple inheritance, the only interface can
help.
7.
What
is an Iterator?
Some of the collection
classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a
collection of objects, operating on each object in turn.
Remember when using Iterators that they
contain a snapshot of the collection at the time the Iterator was obtained;
generally it is not advisable to modify the collection itself while traversing
an Iterator.
8.
State
the significance of public, private, protected, default modifiers both singly
and in combination and state the effect of package relationships on declared
items qualified by these modifiers.
public: Public class is visible in other packages,
field is visible everywhere (class must be public too)
private : Private variables or methods may be used
only by an instance of the same class that declares the variable or method, A
private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same
package and also available to all subclasses of the class that owns the
protected feature. This access is provided even to subclasses that reside in a
different package from the class that owns the protected feature.
What you get by default ie, without any access
modifier (ie, public private or protected). It means that it is visible to all
within a particular package.
9.
What
is static in java?
Static means one per
class, not one for each object no matter how many instance of a class might
exist. This means that you can use them without creating an instance of a
class.Static methods are implicitly final, because overriding is done based on
the type of the object, and static methods are attached to a class, not an
object.
A static method in a superclass can be
shadowed by another static method in a subclass, as long as the original method
was not declared final. However, you can't override a static method with a
nonstatic method. In other words, you can't change a static method into an
instance method in a subclass.
10.
What
is final class?
A final class can't be extended ie., final class
may not be subclassed. A final method can't be overridden when its class is
inherited. You can't change value of a final variable (is a constant).
11.
What
if the main() method is declared as private?
The program compiles properly but at runtime
it will give "main() method not public." message.
12.
What
if the static modifier is removed from the signature of the main() method?
Program compiles. But at runtime throws an
error "NoSuchMethodError".
13.
What
if I write static public void instead of public static void?
Program compiles and runs properly.
14.
What
if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error
"NoSuchMethodError".
15.
What
is the first argument of the String array in main() method?
The String array is empty. It does not have
any element. This is unlike C/C++ where the first element by default is the
program name.
16.
If
I do not provide any arguments on the command line, then the String array of
main() method will be empty or null?
It is empty. But not null.
17.
How
can one prove that the array is not null but empty using one line of code?
Print args.length. It will print 0. That means it is empty. But
if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.
18.
What
environment variables do I need to set on my machine in order to be able to run
Java programs?
CLASSPATH and PATH are the two
variables.
19.
Can
an application have multiple classes having main() method?
Yes it is possible.
While starting the application we mention the class name to be run. The JVM
will look for the Main method only in the class whose name you have mentioned.
Hence there is not conflict amongst the
multiple classes having main() method.
20.
Can
I have multiple main() methods in the same class?
No the program fails to compile. The compiler
says that the main() method is already defined in the class.
21.
Do
I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the
JVM.
22.
Can
I import same package/class twice? Will the JVM load the package twice at
runtime?
One can import the same package or same class
multiple times. Neither compiler nor JVM complains about it. And the JVM will
internally load the class only once no matter how many times you import the
same class.
23.
What
are Checked and UnChecked Exception?
A checked exception is
some subclass of Exception (or Exception itself), excluding class
RuntimeException and its subclasses. Making an exception checked forces client
programmers to deal with the possibility that the exception will be thrown.
Example: IOException thrown
by java.io.FileInputStream's read() method·
Unchecked exceptions
are RuntimeException and any of its subclasses. Class Error and its subclasses
also are unchecked. With an unchecked exception, however, the compiler doesn't
force client programmers either to catch the exception or declare it in a
throws clause. In fact, client programmers may not even know that the exception
could be thrown.
Example: StringIndexOutOfBoundsException thrown by String's charAt() method· Checked
exceptions must be caught at compile time. Runtime exceptions do not need to
be. Errors often cannot be.
24.
What
is Overriding?
When a class defines a
method using the same name, return type, and arguments as a method in its
superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of
the class, it is the new definition of the method that is called, and not the
method definition from superclass. Methods may be overridden to be more public,
not more private.
25.
Are
the imports checked for validity at compile time? Example: will the code
containing an import such as java.lang.ABCD compile?
Yes the imports are
checked for the semantic validity at compile time. The code containing above
line of import will not compile. It will throw an error saying, can not resolve
symbol
symbol : class ABCD
location: package io
import java.io.ABCD;
26.
Does
importing a package imports the subpackages as well? Example: Does importing
com.MyTest.* also import com.MyTest.UnitTests.*?
No you will have to import the subpackages
explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will
not import any class in any of it's subpackage.
27.
What
is the difference between declaring a variable and defining a variable?
In declaration we just
mention the type of the variable and it's name. We do not initialize it. But
defining means declaration + initialization.
Example: String s; is just a
declaration while String s = new String
("abcd"); Or String s = "abcd"; are both definitions.
28.
What
is the default value of an object reference declared as an instance variable?
The default value will be null unless we define
it explicitly.
29.
Can
a top level class be private or protected?
No. A top level class
cannot be private or protected. It can have either "public" or no
modifier. If it does not have a modifier it is supposed to have a default
access.
If a top level class is declared as private
the compiler will complain that the "modifier private is not allowed
here". This means that a top level class can not be private. Same is the
case with protected.
30.
What
type of parameter passing does Java support?
In Java the arguments are always passed by
value.
31.
Primitive
data types are passed by reference or pass by value?
Primitive data types are passed by value.
32.
Objects
are passed by value or by reference?
Java only supports pass by value. With
objects, the object reference itself is passed by value and so both the
original reference and parameter copy both refer to the same object.
33.
What
is serialization?
Serialization is a mechanism by which you can
save the state of an object by converting it to a byte stream.
34.
How
do I serialize an object to a file?
The class whose instances are to be serialized
should implement an interface Serializable. Then you pass the instance to
the ObjectOutputStream which is connected to a fileoutputstream.
This will save the object to a file.
35.
Which
methods of Serializable interface should I implement?
The serializable interface is an empty
interface, it does not contain any methods. So we do not implement any methods.
36.
How
can I customize the seralization process? i.e. how can one have a control over
the serialization process?
Yes it is possible to
have control over serialization process. The class should implement
Externalizable interface. This interface contains two methods namely readExternal and writeExternal.
You should implement these methods and write
the logic for customizing the serialization process.
37.
What
is the common usage of serialization?
Whenever an object is to be sent over the
network, objects need to be serialized. Moreover if the state of an object is
to be saved, objects need to be serilazed.
38.
What
is Externalizable interface?
Externalizable is an
interface which contains two methods readExternal and writeExternal.
These methods give you a control over the serialization mechanism.
Thus if your class implements this interface,
you can customize the serialization process by implementing these methods.
39.
When
you serialize an object, what happens to the object references included in the
object?
The serialization
mechanism generates an object graph for serialization. Thus it determines
whether the included object references are serializable or not. This is a
recursive process.
Thus when an object is serialized, all the
included objects are also serialized alongwith the original obect.
40.
What
one should take care of while serializing the object?
One should make sure that all the included
objects are also serializable. If any of the objects is not serializable then
it throws a NotSerializableException.
41.
What
happens to the static fields of a class during serialization?
There are three
exceptions in which serialization doesnot necessarily read and write to the
stream. These are
1. Serialization
ignores static fields, because they are not part of ay particular state state.
2. Base class fields
are only hendled if the base class itself is serializable.
3. Transient fields.
42.
Does
Java provide any construct to find out the size of an object?
No, there is not sizeof operator in Java. So
there is not direct way to determine the size of an object directly in Java.
43.
What
are wrapper classes?
Java provides
specialized classes corresponding to each of the primitive data types. These
are called wrapper classes.
They are example: Integer, Character, Double
etc.
44.
Why
do we need wrapper classes?
It is sometimes easier
to deal with primitives as objects. Moreover most of the collection classes
store objects and not primitive data types. And also the wrapper classes
provide many utility methods also.
Because of these resons we need wrapper
classes. And since we create instances of these classes we can store them in
any of the collection classes and pass them around as a collection. Also we can
pass them around as method parameters where a method expects an object.
45.
What
are checked exceptions?
Checked exception are
those which the Java compiler forces you to catch.
Example: IOException are
checked exceptions.
46.
What
are runtime exceptions?
Runtime exceptions are those exceptions that
are thrown at runtime because of either wrong input data or because of wrong
business logic etc. These are not checked by the compiler at compile time.
47.
What
is the difference between error and an exception?
An error is an
irrecoverable condition occurring at runtime. Such as OutOfMemory error.
These JVM errors and
you can not repair them at runtime. While exceptions are conditions that occur
because of bad input etc. Example: FileNotFoundException will be thrown if the specified file
does not exist. Or a NullPointerException will take place if you try using a null
reference.
In most of the cases it is possible to recover
from an exception (probably by giving user a feedback for entering proper
values etc.).
48.
How
to create custom exceptions?
Your class should extend class Exception,
or some more specific type thereof.
49.
If
I want an object of my class to be thrown as an exception object, what should I
do?
The class should extend from Exception class.
Or you can extend your class from some more precise exception type also.
50.
If
my class already extends from some other class what should I do if I want an
instance of my class to be thrown as an exception object?
One can not do anytihng in this scenarion.
Because Java does not allow multiple inheritance and does not provide any
exception interface as well.
51.
How
does an exception permeate through the code?
An unhandled exception
moves up the method stack in search of a matching When an exception is thrown
from a code which is wrapped in a try block followed by one or more catch
blocks, a search is made for matching catch block. If a matching type is found
then that block will be invoked. If a matching type is not found then the
exception moves up the method stack and reaches the caller method.
Same procedure is repeated if the caller
method is included in a try catch block. This process continues until a catch
block handling the appropriate type of exception is found. If it does not find
such a block then finally the program terminates.
52.
What
are the different ways to handle exceptions?
There are two ways to
handle exceptions,
1. By wrapping the
desired code in a try block followed by a catch block to catch the exceptions.
and
2. List the desired exceptions in the throws
clause of the method and let the caller of the method hadle those exceptions.
53.
Is
it necessary that each try block must be followed by a catch block?
It is not necessary that each try block must be
followed by a catch block. It should be followed by either a
catch block or a finally block. And whatever exceptions are
likely to be thrown should be declared in the throws clause of the method.
54.
If
I write return at the end of the try block, will the finally block still
execute?
Yes even if you write return as the last
statement in the try block and no exception occurs, the
finally block will execute. The finally block will execute and then the control
return.
55.
If
I write System.exit(0); at the end of the try block, will the finally block
still execute?
No. In this case the finally block will not
execute because when you say System.exit(0); the control immediately goes out of the
program, and thus finally never executes.
56.
How
are Observer and Observable used?
Objects that subclass the Observable class
maintain a list of observers. When an Observable object is updated it invokes
the update() method of each of its observers to
notify the observers that it has changed state. The Observer interface is
implemented by objects that observe Observable objects.
57.
What
is synchronization and why is it important?
With respect to
multithreading, synchronization is the capability to control the access of
multiple threads to shared resources.
Without synchronization, it is possible for
one thread to modify a shared object while another thread is in the process of
using or updating that object's value. This often leads to significant errors.
58.
How
does Java handle integer overflows and underflows?
It uses those low order bytes of the result
that can fit into the size of the type allowed by the operation.
59.
Does
garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a
program will not run out of memory. It is possible for programs to use up
memory resources faster than they are garbage collected. It is also possible
for programs to create objects that are not subject to garbage collection.
60.
What
is the difference between preemptive scheduling and time slicing?
Under preemptive
scheduling, the highest priority task executes until it enters the waiting or
dead states or a higher priority task comes into existence.
Under time slicing, a task executes for a
predefined slice of time and then reenters the pool of ready tasks. The
scheduler then determines which task should execute next, based on priority and
other factors.
61.
When
a thread is created and started, what is its initial state?
A thread is in the ready state after it has
been created and started.
62.
What
is the purpose of finalization?
The purpose of finalization is to give an
unreachable object the opportunity to perform any cleanup processing before the
object is garbage collected.
63.
What
is the Locale class?
The Locale class is used to tailor program
output to the conventions of a particular geographic, political, or cultural
region.
64.
What
is the difference between a while statement and a do statement?
A while statement checks
at the beginning of a loop to see whether the next loop iteration should occur.
A do statement checks at the end of a loop to
see whether the next iteration of a loop should occur. The do statement will
always execute the body of a loop at least once.
65.
What
is the difference between static and non-static variables?
A static variable is associated with the class
as a whole rather than with specific instances of a class. Non-static variables
take on unique values with each object instance.
66.
How
are this() and super() used with constructors?
this() is used to
invoke a constructor of the same class. super() is used to
invoke a superclass constructor.
·
What is daemon thread
and which method is used to create the daemon thread?
Daemon thread is a low priority thread which runs intermittently
in the back ground doing the garbage collection operation for the java runtime system.setDaemon method is used to create a daemon thread.
·
Can applets
communicate with each other?
At this point in time applets may communicate
with other applets running in the same virtual machine. If the applets are of
the same class, they can communicate via shared static variables. If the
applets are of different classes, then each will need a reference to the same
class with static variables. In any case the basic idea is to pass the
information back and forth through a static variable.
An applet can also get
references to all other applets on the same page using the getApplets() method
of java.applet.AppletContext. Once you get the reference to an applet, you
can communicate with it by using its public members.
It is conceivable to have applets in different virtual machines
that talk to a server somewhere on the Internet and store any data that needs
to be serialized there. Then, when another applet needs this data, it could
connect to this same server. Implementing this is non-trivial.
·
What are the steps in
the JDBC connection?
While making a JDBC
connection we go through the following steps :
Step 1 : Register the database driver by using :
Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that
specific database\" );
Step 2 : Now create a database connection
using :
Connection con =
DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt =
Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();
·
How does a try
statement determine which catch clause should be used to handle an exception?
When an exception is thrown within the body of a try statement,
the catch clauses of the try statement are
examined in the order in which they appear. The first catch clause that is
capable of handling the exceptionis executed. The remaining catch clauses are
ignored.
·
Can an unreachable
object become reachable again?
An unreachable object may become reachable again. This can
happen when the object's finalize() method is invoked and the object
performs an operation which causes it to become accessible to reachable
objects.
·
What method must be
implemented by all threads?
All tasks must implement the run() method, whether
they are a subclass of Thread or implement the Runnable interface.
73.
What
are synchronized methods and synchronized statements?
Synchronized methods
are methods that are used to control access to an object. A thread only
executes a synchronized method after it has acquired the lock for the method's
object or class.
Synchronized statements are similar to
synchronized methods. A synchronized statement can only be executed after a
thread has acquired the lock for the object or class referenced in the
synchronized statement.
74.
What
is Externalizable?
Externalizable is an Interface that extends
Serializable Interface. And sends data into Streams in Compressed Format. It
has two methods, writeExternal(ObjectOuput
out) and readExternal(ObjectInput in).
75.
What
modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are
allowed for methods in interfaces.
76.
What
are some alternatives to inheritance?
Delegation is an
alternative to inheritance.
Delegation means that you include an instance
of another class as an instance variable, and forward messages to the instance.
It is often safer than inheritance because it forces you to think about each
message you forward, because the instance is of a known class, rather than a
new class, and because it doesn't force you to accept all the methods of the
super class: you can provide only the methods that really make sense. On the
other hand, it makes you write more code, and it is harder to re-use (because
it is not a subclass).
77.
What
does it mean that a method or field is "static"?
Static variables and
methods are instantiated only once per class. In other words they are class
variables, not instance variables. If you change the value of a static variable
in a particular object, the value of that variable changes for all instances of
that class.
Static methods can be referenced with the name
of the class rather than the name of a particular object of the class (though
that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.
78.
What
is the difference between preemptive scheduling and time slicing?
Under preemptive
scheduling, the highest priority task executes until it enters the waiting or
dead states or a higher priority task comes into existence. Under time slicing,
a task executes for a predefined slice of time and then reenters the pool of
ready tasks.
The scheduler then determines which task
should execute next, based on priority and other factors.
79.
What
is the catch or declare rule for method declarations?
If a checked exception may be thrown within
the body of a method, the method must either catch the exception or declare it
in its throws clause.
80.
Is
Empty .java file a valid source file?
Yes. An empty .java file is a
perfectly valid source file.
81.
Can
a .java file contain more than one java classes?
Yes. A .java file contain
more than one java classes, provided at the most one of them is a public class.
82.
Is
String a primitive data type in Java?
No. String is not a primitive data type in
Java, even though it is one of the most extensively used object. Strings in
Java are instances of String class defined in java.lang package.
83.
Is
main a keyword in Java?
No. main is not a keyword in Java.
84.
Is
next a keyword in Java?
No. next is not a keyword.
85.
Is
delete a keyword in Java?
No. delete is not a keyword in Java. Java does not
make use of explicit destructors the way C++ does.
86.
Is
exit a keyword in Java?
No. To exit a program explicitly you use exit method in System
object.
87.
What
happens if you dont initialize an instance variable of any of the primitive
types in Java?
Java by default initializes it to the default
value for that primitive type. Thus an int will be initialized to 0(zero), a boolean will
be initialized to false.
88.
What
will be the initial value of an object reference which is defined as an
instance variable?
The object references are all initialized to
null in Java. However in order to do anything useful with these references, you
must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized
references.
89.
What
are the different scopes for Java variables?
The scope of a Java
variable is determined by the context in which the variable is declared. Thus a
java variable can have one of the three scopes at any given point in time.
1. Instance : - These
are typical object level variables, they are initialized to default values at
the time of creation of object, and remain accessible as long as the object
accessible.
2. Local : - These are
the variables that are defined within a method. They remain accessbile only
during the course of method excecution. When the method finishes execution,
these variables fall out of scope.
3. Static: - These are the class level
variables. They are initialized when the class is loaded in JVM for the first
time and remain there as long as the class remains loaded. They are not tied to
any particular object instance.
90.
What
is the default value of the local variables?
The local variables are not initialized to any
default value, neither primitives nor object references. If you try to use
these variables without initializing them explicitly, the java compiler will
not compile the code. It will complain abt the local varaible not being
initilized.
91.
How
many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
Only 2 objects are created, c1 and c3. The reference c2 is only declared
and not initialized.
92.
Can
a public class MyClass be defined in a source file named YourClass.java?
No. The source file name, if it contains a
public class, must be the same as the public class name itself with a .java extension.
93.
Can
main() method be declared final?
Yes, the main() method can be
declared final, in addition to being public static.
94.
What
is HashMap and Map?
Map is an Interface and Hashmap is the class
that implements Map.
95.
Difference
between HashMap and HashTable?
The HashMap class is
roughly equivalent to Hashtable, except that it is unsynchronized and permits
nulls. (HashMap allows null values as key and value whereas Hashtable doesnt
allow).
HashMap does not guarantee that the order of
the map will remain constant over time. HashMap is unsynchronized and Hashtable
is synchronized.
96.
Difference
between Vector and ArrayList?
Vector is synchronized whereas arraylist is
not.
97.
Difference
between Swing and Awt?
AWT are heavy-weight componenets. Swings are
light-weight components. Hence swing works faster than AWT.
98.
What
will be the default values of all the elements of an array defined as an
instance variable?
If the array is an
array of primitive types, then all the elements of the array will be
initialized to the default value corresponding to that primitive type.
Example: All the elements of an array of int
will be initialized to 0(zero), while that of boolean type will be
initialized to false. Whereas if the array is an array of
references (of any type), all the elements will be initialized to null.
Core
Java :: Interview
Questions and Answers
Home » Interview Questions » Technical
Interview » Core Java » Interview
Questions
1.
What
is the most important feature of Java?
Java is a platform independent language.
2.
What
do you mean by platform independence?
Platform independence means that we can write and compile the
java code in one platform (eg Windows) and can execute the class in any other
supported platform eg (Linux,Solaris,etc).
3.
What
is a JVM?
JVM is Java Virtual Machine which is a run time environment for
the compiled java class files.
4.
Are
JVM's platform independent?
JVM's are not platform independent. JVM's are platform specific
run time implementation provided by the vendor.
5.
What
is the difference between a JDK and a JVM?
JDK is Java Development Kit which is for development purpose and
it includes execution environment also. But JVM is purely a run time
environment and hence you will not be able to compile your source files using a
JVM.
6.
What
is a pointer and does Java support pointers?
Pointer is a reference handle to a memory location. Improper
handling of pointers leads to memory leaks and reliability issues hence Java
doesn't support the usage of pointers.
7.
What
is the base class of all classes?
java.lang.Object
8.
Does
Java support multiple inheritance?
Java doesn't support multiple inheritance.
9.
Is
Java a pure object oriented language?
Java uses primitive data types and hence is
not a pure object oriented language.
10.
Are
arrays primitive data types?
In Java, Arrays are objects.
11.
What
is difference between Path and Classpath?
Path and Classpath are operating system level
environment variales. Path is used define where the system can find the
executables(.exe) files and classpath is used to specify the location .class
files.
12.
What
are local variables?
Local varaiables are those which are declared
within a block of code like methods. Local variables should be initialised
before accessing them.
·
What are instance
variables?
Instance variables are those which are defined at the class level.
Instance variables need not be initialized before using them as they are
automatically initialized to their default values.
·
How to define a
constant variable in Java?
The variable should be declared as static and final. So only one copy of
the variable exists for all instances of the class and the value can't be
changed also.
static final int MAX_LENGTH = 50; is an example for constant.
static final int MAX_LENGTH = 50; is an example for constant.
·
Should a main() method
be compulsorily declared in all java classes?
No not required. main() method should be defined only if the
source class is a java application.
·
What is the return
type of the main() method?
Main() method doesn't return anything hence
declared void.
·
Why is the main()
method declared static?
main() method is called by the JVM even before
the instantiation of the class hence it is declared as static.
·
What is the arguement
of main() method?
main() method accepts an array of String object
as arguement.
19.
Can
a main() method be overloaded?
Yes. You can have any number of main() methods with
different method signature and implementation in the class.
20.
Can
a main() method be declared final?
Yes. Any inheriting class will not be able to
have it's own default main() method.
21.
Does
the order of public and static declaration matter in main() method?
No. It doesn't matter but void should always
come before main().
22.
Can
a source file contain more than one class declaration?
Yes a single source file can contain any
number of Class declarations but only one of the class can be declared as public.
23.
What
is a package?
Package is a collection of related classes and
interfaces. package declaration should be first statement in a java class.
24.
Which
package is imported by default?
java.lang package is imported by default even without a package declaration.
25.
Can
a class declared as private be accessed outside it's package?
Not possible.
26.
Can
a class be declared as protected?
The protected access modifier cannot be
applied to class and interfaces. Methods, fields can be declared protected, however methods and
fields in a interface cannot be declared protected.
27.
What
is the access scope of a protected method?
A protected method can be accessed by the classes
within the same package or by the subclasses of the class in any package.
28.
What
is the purpose of declaring a variable as final?
A final variable's value can't be changed. final variables should
be initialized before using them.
29.
What
is the impact of declaring a method as final?
A method declared as final can't be
overridden. A sub-class can't have the same method signature with a different
implementation.
30.
I
don't want my class to be inherited by any other class. What should i do?
You should declared your class as final. But you can't define
your class as final, if it is an abstract class. A class
declared as final can't be extended by any other class.
31.
Can
you give few examples of final classes defined in Java API?
java.lang.String, java.lang.Math are final classes.
32.
How
is final different from finally and finalize()?
final is a modifier
which can be applied to a class or a method or a variable. final class can't be
inherited, final method can't be overridden and final variable can't
be changed.
finally is an exception handling code section which gets executed whether an exception is raised or not by the try block code segment.
finalize() is a method of Object class which will be executed by the JVM just before garbage collecting object to give a final chance for resource releasing activity.
finally is an exception handling code section which gets executed whether an exception is raised or not by the try block code segment.
finalize() is a method of Object class which will be executed by the JVM just before garbage collecting object to give a final chance for resource releasing activity.
33.
Can
a class be declared as static?
We can not declare top
level class as static, but only inner class can be declared static.
public class Test
{
static class InnerClass
{
public static void InnerMethod()
{
System.out.println("Static Inner Class!"); }
}
public static void main(String args[])
{
Test.InnerClass.InnerMethod();
}
}
//output: Static Inner Class!
34.
When
will you define a method as static?
When a method needs to be accessed even before
the creation of the object of the class then we should declare the method
as static.
35.
What
are the restriction imposed on a static method or a static block of code?
A static method should not refer to instance variables
without creating an instance and cannot use "this" operator to refer
the instance.
36.
I
want to print "Hello" even before main() is executed. How will you
acheive that?
Print the statement inside a static block of
code. Static blocks get executed when the class gets loaded into the memory and
even before the creation of an object. Hence it will be executed before
the main() method. And it will be executed only
once.
37.
What
is the importance of static variable?
static variables are class level variables
where all objects of the class refer to the same variable. If one object
changes the value then the change gets reflected in all the objects.
38.
Can
we declare a static variable inside a method?
Static varaibles are class level variables and
they can't be declared inside a method. If declared, the class will not
compile.
39.
What
is an Abstract Class and what is it's purpose?
A Class which doesn't provide complete
implementation is defined as an abstract class. Abstract classes enforce
abstraction.
40.
Can
a abstract class be declared final?
Not possible. An abstract class without being
inherited is of no use and hence will result in compile time error.
41.
What
is use of a abstract variable?
Variables can't be declared as abstract. only
classes and methods can be declared as abstract.
42.
Can
you create an object of an abstract class?
Not possible. Abstract classes can't be
instantiated.
43.
Can
a abstract class be defined without any abstract methods?
Yes it's possible. This is basically to avoid
instance creation of the class.
44.
Class
C implements Interface I containing method m1 and m2 declarations. Class C has
provided implementation for method m2. Can i create an object of Class C?
No not possible. Class C should provide
implementation for all the methods in the Interface I.
Since Class C didn't provide implementation for m1 method, it has
to be declared as abstract. Abstract classes can't be instantiated.
45.
Can
a method inside a Interface be declared as final?
No not possible. Doing so will result in
compilation error. public and abstract are the only
applicable modifiers for method declaration in an interface.
46.
Can
an Interface implement another Interface?
Intefaces doesn't provide implementation hence
a interface cannot implement another interface.
47.
Can
an Interface extend another Interface?
Yes an Interface can inherit another
Interface, for that matter an Interface can extend more than one Interface.
48.
Can
a Class extend more than one Class?
Not possible. A Class can extend only one
class but can implement any number of Interfaces.
49.
Why
is an Interface be able to extend more than one Interface but a Class can't
extend more than one Class?
Basically Java doesn't allow multiple
inheritance, so a Class is restricted to extend only one Class. But an
Interface is a pure abstraction model and doesn't have inheritance hierarchy
like classes(do remember that the base class of all classes is Object). So an
Interface is allowed to extend more than one Interface.
50.
Can
an Interface be final?
Not possible. Doing so so will result in
compilation error.
51.
Can
a class be defined inside an Interface?
Yes it's possible.
52.
Can
an Interface be defined inside a class?
Yes it's possible.
53.
What
is a Marker Interface?
An Interface which doesn't have any
declaration inside but still enforces a mechanism.
54.
Which
object oriented Concept is achieved by using overloading and overriding?
Polymorphism.
55.
Why
does Java not support operator overloading?
Operator overloading makes the code very
difficult to read and maintain. To maintain code simplicity, Java doesn't
support operator overloading.
56.
Can
we define private and protected modifiers for variables in interfaces?
No.
57.
What
is Externalizable?
Externalizable is an Interface that extends
Serializable Interface. And sends data into Streams in Compressed Format. It
has two methods, writeExternal(ObjectOuput
out) and readExternal(ObjectInput in)
58.
What
modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are
allowed for methods in interfaces.
59.
What
is a local, member and a class variable?
Variables declared
within a method are "local" variables.
Variables declared
within the class i.e not within any methods are "member" variables
(global variables).
Variables declared within the class i.e not
within any methods and are defined as "static" are class variables.
60.
What
is an abstract method?
An abstract method is a method whose
implementation is deferred to a subclass.
61.
What
value does read() return when it has reached the end of a file?
The read() method returns -1 when it has
reached the end of a file.
62.
Can
a Byte object be cast to a double value?
No, an object cannot be cast to a primitive
value.
63.
What
is the difference between a static and a non-static inner class?
A non-static inner class may have object
instances that are associated with instances of the class's outer class. A
static inner class does not have any object instances.
64.
What
is an object's lock and which object's have locks?
An object's lock is a mechanism that is used
by multiple threads to obtain synchronized access to the object. A thread may
execute a synchronized method of an object only after it has acquired the
object's lock. All objects and classes have locks. A class's lock is acquired
on the class's Class object.
65.
What
is the % operator?
It is referred to as the modulo or remainder
operator. It returns the remainder of dividing the first operand by the second
operand.
66.
When
can an object reference be cast to an interface reference?
An object reference be cast to an interface
reference when the object implements the referenced interface
67.
Which
class is extended by all other classes?
The Object class is extended by all other
classes.
68.
Which
non-Unicode letter characters may be used as the first character of an
identifier?
The non-Unicode letter characters $ and _ may appear as
the first character of an identifier
69.
What
restrictions are placed on method overloading?
Two methods may not have the same name and
argument list but different return types.
70.
What
is casting?
There are two types of casting, casting
between primitive numeric types and casting between object references. Casting
between numeric types is used to convert larger values, such as double values,
to smaller values, such as byte values. Casting between object references is
used to refer to an object by a compatible class, interface, or array type
reference.
71.
What
is the return type of a program's main() method?
void.
72.
If
a variable is declared as private, where may the variable be accessed?
A private variable may only be accessed within
the class in which it is declared.
73.
What
do you understand by private, protected and public?
These are accessibility modifiers. Private is the most
restrictive, while public is the least restrictive. There is no
real difference between protected and the default type (also known as
package protected) within the context of the same package, however the
protected keyword allows visibility to a derived class in a different package.
74.
What
is Downcasting ?
Downcasting is the casting from a general to a
more specific type, i.e. casting down the hierarchy
75.
What
modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared as
public, protected, private, static, final, or abstract.
76.
How
many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Unicode requires 16
bits and ASCII require 7 bits Although the ASCII character set uses only 7
bits, it is usually represented as 8 bits.
UTF-8 represents
characters using 8, 16, and 18 bit patterns.
UTF-16 uses 16-bit and larger bit patterns.
77.
What
restrictions are placed on the location of a package statement within a source
code file?
A package statement must appear as the first
line in a source code file (excluding blank lines and comments).
78.
What
is a native method?
A native method is a method that is
implemented in a language other than Java.
·
What are order of
precedence and associativity, and how are they used?
Order of precedence determines the order in which operators are
evaluated in expressions. Associatity determines whether an expression is
evaluated left-to-right or right-to-left.
·
Can an anonymous class
be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a
superclass, but may not be declared to do both.
·
What is the range of
the char type?
The range of the char type is 0 to 216 - 1 (i.e. 0 to 65535.)
·
What is the range of
the short type?
The range of the short type is -(215) to 215 - 1. (i.e. -32,768 to 32,767)
·
Why isn't there
operator overloading?
Because C++ has proven by example that operator overloading makes
code almost impossible to maintain.
·
What does it mean that
a method or field is "static"?
Static variables and methods are instantiated only once per
class. In other words they are class variables, not instance variables. If you
change the value of a static variable in a particular object, the value of that
variable changes for all instances of that class. Static methods can be
referenced with the name of the class rather than the name of a particular
object of the class (though that works too). That's how library methods
like System.out.println() work. out is a static field in the java.lang.System class.
85.
Is
null a keyword?
The null value is not a keyword.
86.
Which
characters may be used as the second character of an identifier, but not as the
first character of an identifier?
The digits 0 through 9 may not be used as the
first character of an identifier but they may be used after the first character
of an identifier.
87.
Is
the ternary operator written x : y ? z or x ? y : z ?
It is written x ? y : z.
88.
How
is rounding performed under integer division?
The fractional part of the result is
truncated. This is known as rounding toward zero.
89.
If
a class is declared without any access modifiers, where may the class be
accessed?
A class that is declared without any access
modifiers is said to have package access. This means that the class can only be
accessed by other classes and interfaces that are defined within the same
package.
90.
Does
a class inherit the constructors of its superclass?
A class does not inherit constructors from any
of its superclasses.
·
Name the eight
primitive Java types.
The eight primitive types are byte, char, short, int, long,
float, double, and boolean.
·
What restrictions are
placed on the values of each case of a switch statement?
During compilation, the values of each case of a switch statement must
evaluate to a value that can be promoted to an int value.
·
What is the difference
between a while statement and a do while statement?
A while statement checks at the beginning of a
loop to see whether the next loop iteration should occur. A do while statement checks
at the end of a loop to see whether the next iteration of a loop should occur.
The do whilestatement will always execute the body of a
loop at least once.
·
What modifiers can be
used with a local inner class?
A local inner class may be final or abstract.
·
When does the compiler
supply a default constructor for a class?
The compiler supplies a default constructor for a class if no
other constructors are provided.
·
If a method is
declared as protected, where may the method be accessed?
A protected method may only be accessed by classes or interfaces
of the same package or by subclasses of the class in which it is declared.
97.
What
are the legal operands of the instanceof operator?
The left operand is an object reference or
null value and the right operand is a class, interface, or array type.
98.
Are
true and false keywords?
The values true and false are not keywords.
99.
What
happens when you add a double value to a String?
The result is a String object.
100.
What
is the diffrence between inner class and nested class?
When a class is defined within a scope od
another class, then it becomes inner class. If the access modifier of the inner
class is static, then it becomes nested class.
101.
Can
an abstract class be final?
An abstract class may not be declared as final.
102.
What
is numeric promotion?
Numeric promotion is the conversion of a
smaller numeric type to a larger numeric type, so that integer and
floating-point operations may take place. In numerical promotion, byte, char,
and short values are converted to int values. The int values are also converted
to long values, if necessary. The long and float values are converted to double
values, as required.
103.
What
is the difference between a public and a non-public class?
A public class may be accessed outside of its
package. A non-public class may not be accessed outside of its package.
104.
To
what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is
false.
105.
What
is the difference between the prefix and postfix forms of the ++ operator?
The prefix form performs the increment
operation and returns the value of the increment operation. The postfix form
returns the current value all of the expression and then performs the increment
operation on that value.
106.
What
restrictions are placed on method overriding?
Overridden methods must have the same name,
argument list, and return type. The overriding method may not limit the access
of the method it overrides. The overriding method may not throw any exceptions
that may not be thrown by the overridden method.
107.
What
is a Java package and how is it used?
A Java package is a naming context for classes
and interfaces. A package is used to create a separate name space for groups of
classes and interfaces. Packages are also used to organize related classes and
interfaces into a single API unit and to control accessibility to these classes
and interfaces.
108.
What
modifiers may be used with a top-level class?
A top-level class may be public, abstract, or
final.
109.
What
is the difference between an if statement and a switch statement?
The if statement is used to select among two
alternatives. It uses a boolean expression to decide which alternative should
be executed. The switch statement is used to select among multiple
alternatives. It uses an int expression to determine which alternative should
be executed.
110.
What
are the practical benefits, if any, of importing a specific class rather than
an entire package (e.g. import java.net.* versus import java.net.Socket)?
It makes no difference in the generated class
files since only the classes that are actually used are referenced by the
generated class file. There is another practical benefit to importing single
classes, and this arises when two (or more) packages have classes with the same
name. Take java.util.Timer and javax.swing.Timer, for example. If
I import java.util.* and javax.swing.* and
then try to use "Timer", I get an error while compiling (the class
name is ambiguous between both packages). Let's say what you really wanted was
the javax.swing.Timer class, and the only classes you plan on
using in java.util are Collection and HashMap. In this
case, some people will prefer to import
java.util.Collection and import java.util.HashMap instead of importing java.util.*.
This will now allow them to use Timer, Collection, HashMap, and other javax.swing classes
without using fully qualified class names in.
111.
Can
a method be overloaded based on different return type but same argument type ?
No, because the methods can be called without
using their return type in which case there is ambiquity for the compiler.
112.
What
happens to a static variable that is defined within a method of a class ?
Can't do it. You'll get a compilation error.
113.
How
many static initializers can you have ?
As many as you want, but the static
initializers and class variable initializers are executed in textual order and
may not refer to class variables declared in the class whose declarations
appear textually after the use, even though these class variables are in scope.
114.
What
is the difference between method overriding and overloading?
Overriding is a method with the same name and
arguments as in a parent, whereas overloading is the same method name but
different arguments
115.
What
is constructor chaining and how is it achieved in Java ?
A child object constructor always first needs
to construct its parent (which in turn calls its parent constructor.). In Java
it is done via an implicit call to the no-args constructor as the first
statement.
116.
What
is the difference between the Boolean & operator and the &&
operator?
If an expression involving the Boolean & operator
is evaluated, both operands are evaluated. Then the & operator is applied
to the operand. When an expression involving the && operator is
evaluated, the first operand is evaluated. If the first operand returns a value
of true then the second operand is evaluated. The && operator is then
applied to the first and second operands. If the first operand evaluates to
false, the evaluation of the second operand is skipped.
117.
Which
Java operator is right associative?
The = operator is right associative.
118.
Can
a double value be cast to a byte?
Yes, a double value can be cast to a byte.
119.
What
is the difference between a break statement and a continue statement?
A break statement results in the termination of
the statement to which it applies (switch, for, do, or while). A continue statement is
used to end the current loop iteration and return control to the loop
statement.
120.
Can
a for statement loop indefinitely?
Yes, a for statement can loop indefinitely.
For example, consider the following: for(;;);
121.
To
what value is a variable of the String type automatically initialized?
The default value of an String type is null.
122.
What
is the difference between a field variable and a local variable?
A field variable is a variable that is
declared as a member of a class. A local variable is a variable that is
declared local to a method.
123.
How
are this() and super() used with constructors?
this() is used to
invoke a constructor of the same class. super() is used to
invoke a superclass constructor.
124.
What
does it mean that a class or member is final?
A final class cannot be inherited. A final
method cannot be overridden in a subclass. A final field cannot be changed
after it's initialized, and it must include an initializer statement where it's
declared.
125.
What
does it mean that a method or class is abstract?
An abstract class cannot be instantiated.
Abstract methods may only be included in abstract classes. However, an abstract
class is not required to have any abstract methods, though most of them do.
Each subclass of an abstract class must override the abstract methods of its
superclasses or it also should be declared abstract.
126.
What
is a transient variable?
Transient variable is a variable that may not
be serialized.
127.
How
does Java handle integer overflows and underflows?
It uses those low order bytes of the result
that can fit into the size of the type allowed by the operation.
128.
What
is the difference between the >> and >>> operators?
The >> operator carries the sign bit
when shifting right. The >>> zero-fills bits that have been shifted
out.
129.
Is
sizeof a keyword?
The sizeof operator is not a keyword.
Advanced
Java :: Interview
Questions and Answers
Home » Interview Questions » Technical
Interview » Advanced Java »
Interview Questions
1.
What
is a transient variable?
A transient variable is a variable that may not be serialized.
2.
Which
containers use a border Layout as their default layout?
The Window, Frame and Dialog classes use a
border layout as their default layout.
3.
Why
do threads block on I/O?
Threads block on I/O (that is enters the waiting state) so that
other threads may execute while the I/O Operation is performed.
4.
How
are Observer and Observable used?
Objects that subclass the Observable class maintain a
list of observers. When an Observable object is updated it invokes the update() method of each
of its observers to notify the observers that it has changed state. The
Observer interface is implemented by objects that observe Observable objects.
5.
What
is synchronization and why is it important?
With respect to multithreading, synchronization is the
capability to control the access of multiple threads to shared resources.
Without synchronization, it is possible for one thread to modify a shared object
while another thread is in the process of using or updating that object's
value. This often leads to significant errors.
6.
Can
a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on
the class's Class object..
·
What's new with the
stop(), suspend() and resume() methods in JDK 1.2?
The stop(), suspend() and resume() methods have
been deprecated in JDK 1.2.
·
Is null a keyword?
The null is not a keyword.
·
What is the preferred
size of a component?
The preferred size of a component is the minimum component size
that will allow the component to display normally.
·
What method is used to
specify a container's layout?
The setLayout() method is used to specify a container's
layout.
·
Which containers use a
FlowLayout as their default layout?
The Panel and Applet classes use
the FlowLayout as their default layout.
·
What state does a
thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead
state.
13.
What
is the Collections API?
The Collections API is a set of classes and
interfaces that support operations on collections of objects.
14.
Which
characters may be used as the second character of an identifier, but not as the
first character of an identifier?
The digits 0 through 9 may not be used as the
first character of an identifier but they may be used after the first character
of an identifier.
15.
What
is the List interface?
The List interface provides support for ordered
collections of objects.
16.
How
does Java handle integer overflows and underflows?
It uses those low order bytes of the result
that can fit into the size of the type allowed by the operation.
17.
What
is the Vector class?
The Vector class provides the capability to
implement a growable array of objects
18.
What
modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared
as public, protected, private, static, final, or abstract.
19.
What
is an Iterator interface?
The Iterator interface is used to step through the
elements of a Collection.
20.
What
is the difference between the >> and >>> operators?
The >> operator carries the sign bit when
shifting right. The >>> zero-fills bits that have been shifted
out.
21.
Which
method of the Component class is used to set the position and size of a
component?
setBounds() method
is used to set the position and size of a component.
22.
What
is the difference between yielding and sleeping?
When a task invokes its yield() method, it
returns to the ready state. When a task invokes its sleep()method, it returns to
the waiting state.
23.
Which
java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface
support event processing.
24.
Is
sizeof a keyword?
The sizeof operator is not a keyword.
·
What are wrapped
classes?
Wrapped classes are classes that allow primitive types to be
accessed as objects.
·
Does garbage
collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not
run out of memory. It is possible for programs to use up memory resources
faster than they are garbage collected. It is also possible for programs to
create objects that are not subject to garbage collection.
·
What restrictions are
placed on the location of a package statement within a source code file?
A package statement must appear as the first line in a source
code file (excluding blank lines and comments).
·
Can an object's
finalize() method be invoked while it is reachable?
An object's finalize() method cannot be invoked by the garbage
collector while the object is still reachable. However, an object's finalize() method may be
invoked by other objects.
·
What is the immediate
superclass of the Applet class?
Panel.
·
What is the difference
between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest
priority task executes until it enters the waiting or dead states or a higher
priority task comes into existence. Under time slicing, a task executes for a
predefined slice of time and then reenters the pool of ready tasks.
The scheduler then determines which task should execute next,
based on priority and other factors.
·
Name three Component
subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.
·
What value does
readLine() return when it has reached the end of a file?
The readLine() method returns null when it has
reached the end of a file.
·
What is the immediate
superclass of the Dialog class?
Window.
·
What is clipping?
Clipping is the process of confining paint operations to a
limited area or shape.
·
What is a native
method?
A native method is a method that is implemented in a language
other than Java.
·
Can a for statement
loop indefinitely?
Yes, a for statement can loop indefinitely.
For example, consider the following:
for(;;)
;
37.
What
are order of precedence and associativity, and how are they used?
Order of precedence
determines the order in which operators are evaluated in expressions.
Associatity determines whether an expression
is evaluated left-to-right or right-to-left.
38.
When
a thread blocks on I/O, what state does it enter?
A thread enters the waiting state when it
blocks on I/O.
39.
To
what value is a variable of the String type automatically initialized?
The default value of an String type is null.
40.
What
is the catch or declare rule for method declarations?
If a checked exception may be thrown within
the body of a method, the method must either catch the exception or declare it
in its throws clause.
41.
What
is the difference between a MenuItem and a CheckboxMenuItem?
The CheckboxMenuItem class extends the MenuItem class to support
a menu item that may be checked or unchecked.
42.
What
is a task's priority and how is it used in scheduling?
A task's priority is an integer value that
identifies the relative order in which it should be executed with respect to
other tasks. The scheduler attempts to schedule higher priority tasks before
lower priority tasks.
43.
What
class is the top of the AWT event hierarchy?
The java.awt.AWTEvent class is the highest-level class in the
AWT event-class hierarchy.
44.
When
a thread is created and started, what is its initial state?
A thread is in the ready state after it has
been created and started.
45.
Can
an anonymous class be declared as implementing an interface and extending a
class?
An anonymous class may implement an interface
or extend a superclass, but may not be declared to do both.
46.
What
is the immediate superclass of Menu?
MenuItem.
47.
What
is the purpose of finalization?
The purpose of finalization is to give an
unreachable object the opportunity to perform any cleanup processing before the
object is garbage collected.
48.
Which
class is the immediate superclass of the MenuComponent class?
Object.
49.
What
invokes a thread's run() method?
After a thread is started, via its start() method or that
of the Thread class, the JVM invokes the
thread's run() method when the thread is initially executed.
50.
What
is the difference between the Boolean & operator and the &&
operator?
If an expression
involving the Boolean & operator is evaluated, both operands are
evaluated. Then the &operator is applied to the operand. When an
expression involving the && operator is evaluated, the first operand
is evaluated.
If the first operand returns a value of true
then the second operand is evaluated. The && operator is then
applied to the first and second operands. If the first operand evaluates to
false, the evaluation of the second operand is skipped.
51.
Name
three subclasses of the Component class.
Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent.
52.
What
is the GregorianCalendar class?
The GregorianCalendar class provides support for traditional
Western calendars.
53.
Which
Container method is used to cause a container to be laid out and redisplayed?
validate() method is used
to cause a container to be laid out and redisplayed.
54.
What
is the purpose of the Runtime class?
The purpose of the Runtime class is to
provide access to the Java runtime system.
55.
How
many times may an object's finalize() method be invoked by the garbage
collector?
An object's finalize() method may only
be invoked once by the garbage collector.
56.
What
is the purpose of the finally clause of a try-catch-finally statement?
The finally clause is used to provide the
capability to execute code no matter whether or not an exception is thrown or
caught.
57.
What
is the argument type of a program's main() method?
A program's main() method takes an
argument of the String[] type.
58.
Which
Java operator is right associative?
The = operator is right associative.
59.
Can
a double value be cast to a byte?
Yes, a double value can be
cast to a byte.
60.
What
must a class do to implement an interface?
It must provide all of the methods in the
interface and identify the interface in its implements clause.
61.
What
method is invoked to cause an object to begin executing as a separate thread?
The start() method of the Thread class
is invoked to cause an object to begin executing as a separate thread.
62.
Name
two subclasses of the TextComponent class.
TextField and TextArea.
63.
Which
containers may have a MenuBar?
Frame.
64.
How
are commas used in the intialization and iteration parts of a for statement?
Commas are used to separate multiple
statements within the initialization and iteration parts of a forstatement.
65.
What
is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(), notify(), and notifyAll() methods are used to provide an efficient
way for threads to wait for a shared resource. When a thread executes an
object's wait() method, it enters the waiting state. It
only enters the ready state after another thread invokes the object's notify() or notifyAll() methods..
66.
What
is an abstract method?
An abstract method is a method whose
implementation is deferred to a subclass.
67.
How
are Java source code files named?
A Java source code
file takes the name of a public class or interface that is defined within the file.
A source code file may contain at most one public class or interface. If a
public class or interface is defined within a source code file, then the source
code file must take the name of the public class or interface.
If no public class or interface is defined
within a source code file, then the file must take on a name that is different
than its classes and interfaces. Source code files use the .java extension.
68.
What
is the relationship between the Canvas class and the Graphics class?
A Canvas object provides access to a Graphics
object via its paint() method.
69.
What
are the high-level thread states?
The high-level thread states are ready,
running, waiting, and dead.
70.
What
value does read() return when it has reached the end of a file?
The read() method returns -1 when it has
reached the end of a file.
71.
Can
a Byte object be cast to a double value?
No. An object cannot be cast to a primitive
value.
72.
What
is the difference between a static and a non-static inner class?
A non-static inner
class may have object instances that are associated with instances of the
class's outer class.
A static inner class does not have any object
instances.
73.
What
is the difference between the String and StringBuffer classes?
String objects are
constants. StringBuffer objects are not constants.
74.
If
a variable is declared as private, where may the variable be accessed?
A private variable may only be accessed within
the class in which it is declared.
75.
What
is an object's lock and which object's have locks?
An object's lock is a
mechanism that is used by multiple threads to obtain synchronized access to the
object. A thread may execute a synchronized method of an object only after it
has acquired the object's lock.
All objects and classes have locks. A class's
lock is acquired on the class's Class object.
76.
What
is the Dictionary class?
The Dictionary class provides the capability to store
key-value pairs.
77.
How
are the elements of a BorderLayout organized?
The elements of a BorderLayout are
organized at the borders (North, South, East, and West) and the center of a
container.
78.
What
is the % operator?
It is referred to as the modulo or remainder
operator. It returns the remainder of dividing the first operand by the second
operand.
79.
When
can an object reference be cast to an interface reference?
An object reference be cast to an interface
reference when the object implements the referenced interface.
80.
What
is the difference between a Window and a Frame?
The Frame class extends Window to define a main
application window that can have a menu bar.
81.
Which
class is extended by all other classes?
The Object class is extended by all other classes.
82.
Can
an object be garbage collected while it is still reachable?
A reachable object cannot be garbage collected.
Only unreachable objects may be garbage collected..
83.
Is
the ternary operator written x : y ? z or x ? y : z ?
It is written x ? y : z.
84.
What
is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific
properties, such as ascent and descent, of a Font object.
85.
How
is rounding performed under integer division?
The fractional part of the result is
truncated. This is known as rounding toward zero.
86.
What
happens when a thread cannot acquire a lock on an object?
If a thread attempts to execute a synchronized
method or synchronized statement and is unable to acquire an object's lock, it
enters the waiting state until the lock becomes available.
87.
What
is the difference between the Reader/Writer class hierarchy and the
InputStream/OutputStream class hierarchy?
The Reader/Writer class hierarchy is character-oriented,
and the InputStream/OutputStream class hierarchy is byte-oriented.
88.
What
classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that
may be assigned to the Throwable type. This includes the Error and Exception
types.
89.
If
a class is declared without any access modifiers, where may the class be
accessed?
A class that is declared without any access
modifiers is said to have package access. This means that the class can only be
accessed by other classes and interfaces that are defined within the same
package.
90.
What
is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian
calendar.
91.
What
is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is
used associate keys with values.
92.
Does
a class inherit the constructors of its superclass?
A class does not inherit constructors from any
of its superclasses.
93.
For
which statements does it make sense to use a label?
The only statements for which it makes sense
to use a label are those statements that can enclose a break or continue statement.
94.
What
is the purpose of the System class?
The purpose of the System class is to provide
access to system resources.
95.
Which
TextComponent method is used to set a TextComponent to the read-only state?
setEditable().
96.
How
are the elements of a CardLayout organized?
The elements of a CardLayout are stacked, one
on top of the other, like a deck of cards.
97.
Is
&&= a valid Java operator?
No. It is not a valid java operator.
98.
Name
the eight primitive Java types.
The eight primitive types are byte, char, short, int, long, float, double, and boolean.
99.
Which
class should you use to obtain design information about an object?
The Class class is used to obtain information
about an object's design.
100.
What
is the relationship between clipping and repainting?
When a window is repainted by the AWT painting
thread, it sets the clipping regions to the area of the window that requires
repainting.
101.
Is
"abc" a primitive value?
The String literal "abc" is not a
primitive value. It is a String object.
102.
What
is the relationship between an event-listener interface and an event-adapter
class?
An event-listener
interface defines the methods that must be implemented by an event handler for
a particular kind of event.
An event adapter provides a default
implementation of an event-listener interface.
·
What restrictions are
placed on the values of each case of a switch statement?
During compilation, the values of each case of a switch statement must
evaluate to a value that can be promoted to an int value.
·
What modifiers may be
used with an interface declaration?
An interface may be declared as public or abstract.
·
Is a class a subclass
of itself?
A class is a subclass of itself.
·
What is the
highest-level event class of the event-delegation model?
The java.util.EventObject class is the highest-level class in the
event-delegation class hierarchy.
·
What event results
from the clicking of a button?
The ActionEvent event is generated as the result of the
clicking of a button.
·
How can a GUI
component handle its own events?
A component can handle its own events by implementing the
required event-listener interface and adding itself as its own event listener.
109.
How
are the elements of a GridBagLayout organized?
The elements of a GridBagLayout are
organized according to a grid. However, the elements are of different sizes and
may occupy more than one row or column of the grid. In addition, the rows and
columns may have different sizes.
110.
What
advantage do Java's layout managers provide over traditional windowing systems?
Java uses layout managers to lay out
components in a consistent manner across all windowing platforms. Since Java's
layout managers aren't tied to absolute sizing and positioning, they are able
to accomodate platform-specific differences among windowing systems.
111.
What
is the Collection interface?
The Collection interface provides support for the
implementation of a mathematical bag - an unordered collection of objects that
may contain duplicates.
112.
What
modifiers can be used with a local inner class?
A local inner class may be final or abstract.
113.
What
is the difference between static and non-static variables?
A static variable is
associated with the class as a whole rather than with specific instances of a
class.
Non-static variables take on unique values
with each object instance.
114.
What
is the difference between the paint() and repaint() methods?
The paint() method
supports painting via a Graphics object. The repaint() method is used
to cause paint() to be invoked by the AWT painting
thread.
115.
What
is the purpose of the File class?
The File class is used to create objects that
provide access to the files and directories of a local file system.
116.
Can
an exception be rethrown?
Yes, an exception can be rethrown.
117.
Which
Math method is used to calculate the absolute value of a number?
The abs() method is used to calculate absolute
values.
118.
How
does multithreading take place on a computer with a single CPU?
The operating system's task scheduler
allocates execution time to multiple tasks. By quickly switching between
executing tasks, it creates the impression that tasks execute sequentially.
119.
When
does the compiler supply a default constructor for a class?
The compiler supplies a default constructor
for a class if no other constructors are provided.
120.
When
is the finally clause of a try-catch-finally statement executed?
The finally clause of the try-catch-finally
statement is always executed unless the thread of execution terminates or an
exception occurs within the execution of the finally clause.
·
Which class is the
immediate superclass of the Container class?
Component.
·
If a method is
declared as protected, where may the method be accessed?
A protected method may only be accessed by classes
or interfaces of the same package or by subclasses of the class in which it is
declared.
·
How can the Checkbox
class be used to create a radio button?
By associating Checkbox objects with a CheckboxGroup.
·
Which non-Unicode
letter characters may be used as the first character of an identifier?
The non-Unicode letter characters $ and _ may appear as
the first character of an identifier
·
What restrictions are
placed on method overloading?
Two methods may not have the same name and argument list but
different return types.
·
What happens when you
invoke a thread's interrupt method while it is sleeping or waiting?
When a task's interrupt() method is executed, the task enters the
ready state. The next time the task enters the running state, an InterruptedException is thrown.
·
What is casting?
There are two types of casting, casting
between primitive numeric types and casting between object references.
Casting between
numeric types is used to convert larger values, such as double values, to
smaller values, such as byte values.
Casting between object references is used to refer to an object
by a compatible class, interface, or arraytype reference.
·
What is the return
type of a program's main() method?
A program's main() method has a void return type.
·
Name four Container
classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane.
·
What is the difference
between a Choice and a List?
A Choice is displayed in a compact form that
requires you to pull it down to see the list of available choices. Only one
item may be selected from a Choice.
A List may be displayed in such a way that several List items
are visible. A List supports the selection of one or more List items.
·
What class of
exceptions are generated by the Java run-time system?
The Java runtime system generates RuntimeException and
Error exceptions.
·
What class allows you
to read objects directly from a stream?
The ObjectInputStream class supports the reading of objects
from input streams.
133.
What
is the difference between a field variable and a local variable?
A field variable is a
variable that is declared as a member of a class.
A local variable is a variable that is
declared local to a method.
134.
Under
what conditions is an object's finalize() method invoked by the garbage
collector?
The garbage collector invokes an
object's finalize() method when it detects that the object
has become unreachable.
135.
What
is the relationship between a method's throws clause and the exceptions that
can be thrown during the method's execution?
A method's throws clause must declare any
checked exceptions that are not caught within the body of the method.
136.
What
is the difference between the JDK 1.02 event model and the event-delegation
model introduced with JDK 1.1?
The JDK 1.02 event
model uses an event inheritance or bubbling approach. In this model, components
are required to handle their own events. If they do not handle a particular
event, the event is inherited by (or bubbled up to) the component's container.
The container then either handles the event or it is bubbled up to its
container and so on, until the highest-level container has been tried.
In the event-delegation model, specific
objects are designated as event handlers for GUI components. These objects
implement event-listener interfaces. The event-delegation model is more
efficient than the event-inheritance model because it eliminates the processing
required to support the bubbling of unhandled events.
137.
How
is it possible for two String objects with identical values not to be equal
under the == operator?
The == operator compares two objects to
determine if they are the same object in memory. It is possible for two String
objects to have the same value, but located indifferent areas of memory.
138.
Why
are the methods of the Math class static?
So they can be invoked as if they are a
mathematical code library.
139.
What
Checkbox method allows you to tell if a Checkbox is checked?
getState().
140.
What
state is a thread in when it is executing?
An executing thread is in the running state.
141.
What
are the legal operands of the instanceof operator?
The left operand is an object reference or
null value and the right operand is a class, interface, or arraytype.
142.
How
are the elements of a GridBagLayout organized?
The elements of a GridBagLayout are
of equal size and are laid out using the squares of a grid.
143.
What
an I/O filter?
An I/O filter is an object that reads from one
stream and writes to another, usually altering the data in some way as it is
passed from one stream to another.
144.
If
an object is garbage collected, can it become reachable again?
Once an object is garbage collected, it ceases
to exist. It can no longer become reachable again.
145.
What
is the Set interface?
The Set interface provides methods for accessing
the elements of a finite mathematical set. Sets do not allow duplicate
elements.
146.
What
classes of exceptions may be thrown by a throw statement?
A throw statement may throw any expression that
may be assigned to the Throwable type.
147.
What
are E and PI?
E is the base of
the natural logarithm and PI is mathematical value pi.
148.
Are
true and false keywords?
The values true and false are not
keywords.
149.
What
is a void return type?
A void return type indicates that a method does
not return a value.
150.
What
is the purpose of the enableEvents() method?
The enableEvents() method
is used to enable an event for a particular object. Normally, an event is
enabled when a listener is added to an object for a particular event.
The enableEvents() method is used by objects that handle
events by overriding their event-dispatch methods.
151.
What
is the difference between the File and RandomAccessFile classes?
The File class
encapsulates the files and directories of the local file system.
The RandomAccessFile class provides the methods needed to
directly access data contained in any part of a file.
152.
What
happens when you add a double value to a String?
The result is a String object.
153.
What
is your platform's default character encoding?
If you are running Java on English Windows
platforms, it is probably Cp1252. If you are running Java on English Solaris
platforms, it is most likely 8859_1.
154.
Which
package is always imported by default?
The java.lang
package is always
imported by default.
155.
What
interface must an object implement before it can be written to a stream as an
object?
An object must implement the Serializable or Externalizable interface
before it can be written to a stream as an object.
156.
How
are this and super used?
this is used to refer to the current object
instance.
super is used to refer
to the variables and methods of the superclass of the current object instance.
157.
What
is a compilation unit?
A compilation unit is a Java source code file.
158.
What
interface is extended by AWT event listeners?
All AWT event listeners extend the java.util.EventListener interface.
159.
What
restrictions are placed on method overriding?
Overridden methods
must have the same name, argument list, and return type.
The overriding method
may not limit the access of the method it overrides.
The overriding method may not throw any
exceptions that may not be thrown by the overridden method.
160.
How
can a dead thread be restarted?
A dead thread cannot be restarted.
161.
What
happens if an exception is not caught?
An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup being
invoked, which eventually results in the termination of the program in which it
is thrown.
162.
What
is a layout manager?
A layout manager is an object that is used to
organize components in a container.
·
Which arithmetic
operations can result in the throwing of an ArithmeticException?
Integer / and % can result in
the throwing of an ArithmeticException.
·
What are three ways in
which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by
blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or
by invoking an object's wait() method. It can also enter the waiting
state by invoking its (deprecated) suspend() method.
·
Can an abstract class
be final?
An abstract class may not be declared as final.
·
What is the
ResourceBundle class?
The ResourceBundle class is used to store locale-specific
resources that can be loaded by a program to tailor the program's appearance to
the particular locale in which it is being run.
·
What happens if a
try-catch-finally statement does not have a catch clause to handle an exception
that is thrown within the body of the try statement?
The exception propagates up to the next higher level try-catch
statement (if any) or results in the program's termination.
·
What is numeric
promotion?
Numeric promotion is the conversion of a smaller numeric type to
a larger numeric type, so that integer and floating-point operations may take
place. In numerical promotion, byte, char, and short values are converted to int values. The int
values are also converted to long values, if necessary. The long and float values are
converted to double values, as required.
169.
What
is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component,
but not a Container. A ScrollPane is a Container.
A ScrollPane handles its own events and performs its
own scrolling.
170.
What
is the difference between a public and a non-public class?
A public class may be
accessed outside of its package.
A non-public class may not be accessed outside
of its package.
171.
To
what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is false.
172.
Can
try statements be nested?
Try statements may
be tested.
173.
What
is the difference between the prefix and postfix forms of the ++ operator?
The prefix form
performs the increment operation and returns the value of the increment
operation.
The postfix form returns the current value all
of the expression and then performs the increment operation on that value.
174.
What
is the purpose of a statement block?
A statement block is used to organize a
sequence of statements as a single statement group.
175.
What is a Java package and how is it used?
A Java package is a
naming context for classes and interfaces. A package is used to create a
separate name space for groups of classes and interfaces.
Packages are also used to organize related
classes and interfaces into a single API unit and to control accessibility to
these classes and interfaces.
176.
What
modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.
177.
What
are the Object and Class classes used for?
The Object class is the highest-level class in the
Java class hierarchy. The Class class is used to represent the classes
and interfaces that are loaded by a Java program..
178.
How
does a try statement determine which catch clause should be used to handle an
exception?
When an exception is thrown within the body of
a try statement, the catch clauses of
the try statement are examined in the order in
which they appear. The first catch clause that is capable of handling the
exception is executed. The remaining catch clauses are ignored.
179.
Can
an unreachable object become reachable again?
An unreachable object may become reachable
again. This can happen when the object's finalize() method is
invoked and the object performs an operation which causes it to become
accessible to reachable objects.
180.
When
is an object subject to garbage collection?
An object is subject to garbage collection
when it becomes unreachable to the program in which it is used.
181.
What
method must be implemented by all threads?
All tasks must implement the run() method, whether
they are a subclass of Thread or implement the Runnable interface.
182.
What
methods are used to get and set the text label displayed by a Button object?
getLabel() and setLabel().
183.
Which
Component subclass is used for drawing and painting?
Canvas.
184.
What
are synchronized methods and synchronized statements?
Synchronized methods
are methods that are used to control access to an object. A thread only
executes a synchronized method after it has acquired the lock for the method's
object or class.
Synchronized statements are similar to
synchronized methods. A synchronized statement can only be executed after a
thread has acquired the lock for the object or class referenced in the
synchronized statement.
185.
What
are the two basic ways in which classes that can be run as threads may be
defined?
A thread class may be declared as a subclass
of Thread, or it may implement the Runnable interface.
186.
What
are the problems faced by Java programmers who don't use layout managers?
Without layout managers, Java programmers are
faced with determining how their GUI will be displayed across multiple
windowing systems and finding a common sizing and positioning that will work
within the constraints imposed by each windowing system.
Data
Structures :: Interview Questions and Answers
Home » Interview Questions » Technical
Interview » Data Structures »
Interview Questions
1.
What
is data structure?
A data structure is a way of organizing data that considers not
only the items stored, but also their relationship to each other. Advance
knowledge about the relationship between data items allows designing of
efficient algorithms for the manipulation of data.
2.
List
out the areas in which data structures are applied extensively?
1.
Compiler Design,
2.
Operating System,
3.
Database Management
System,
4.
Statistical analysis
package,
5.
Numerical Analysis,
6.
Graphics,
7.
Artificial
Intelligence,
8.
Simulation
3.
What
are the major data structures used in the following areas : RDBMS, Network data
model and Hierarchical data model.
1.
RDBMS = Array (i.e.
Array of structures)
2.
Network data model =
Graph
3.
Hierarchical data
model = Trees
4.
If
you are using C language to implement the heterogeneous linked list, what
pointer type will you use?
The heterogeneous linked list contains different data types in
its nodes and we need a link, pointer to connect them. It is not possible to
use ordinary pointers for this. So we go for void pointer. Void pointer is
capable of storing pointer to any type as it is a generic pointer type.
5.
Minimum
number of queues needed to implement the priority queue?
Two. One queue is used for actual storing of data and another
for storing priorities.
6.
What
is the data structures used to perform recursion?
Stack. Because of its LIFO (Last In First Out)
property it remembers its 'caller' so knows whom to return when the function
has to return. Recursion makes use of system stack for storing the return
addresses of the function calls.
Every recursive function has its equivalent iterative
(non-recursive) function. Even when such equivalent iterative procedures are
written, explicit stack is to be used.
·
What are the notations
used in Evaluation of Arithmetic Expressions using prefix and postfix forms?
Polish and Reverse Polish notations.
·
Convert the expression
((A + B) * C - (D - E) ^ (F + G)) to equivalent Prefix and Postfix notations.
1.
Prefix
Notation: - * +ABC ^ - DE
+ FG
2.
Postfix
Notation: AB + C * DE - FG
+ ^ -
·
Sorting is not
possible by using which of the following methods? (Insertion, Selection,
Exchange, Deletion)
Sorting is not possible in Deletion. Using insertion we can perform insertion
sort, using selection we can perform selection sort, using exchange we can
perform the bubble sort (and other similar sorting methods). But no sorting
method can be done just using deletion.
·
What are the methods
available in storing sequential files ?
1.
Straight merging,
2.
Natural merging,
3.
Polyphase sort,
4.
Distribution of
Initial runs.
·
List out few of the
Application of tree data-structure?
1.
The manipulation of
Arithmetic expression,
2.
Symbol Table
construction,
3.
Syntax analysis.
·
List out few of the
applications that make use of Multilinked Structures?
1.
Sparse matrix,
2.
Index generation.
·
In tree construction
which is the suitable efficient data structure? (Array, Linked list, Stack,
Queue)
Linked list is the suitable efficient data structure.
·
What is the type of
the algorithm used in solving the 8 Queens problem?
Backtracking.
·
In an AVL tree, at
what condition the balancing is to be done?
If the 'pivotal value' (or the 'Height factor') is greater than
1 or less than -1.
·
What is the bucket
size, when the overlapping and collision occur at same time?
One. If there is only one entry possible in the bucket, when the
collision occurs, there is no way to accommodate the colliding value. This
results in the overlapping of values.
·
Classify the Hashing
Functions based on the various methods by which the key value is found.
1.
Direct method,
2.
Subtraction method,
3.
Modulo-Division
method,
4.
Digit-Extraction
method,
5.
Mid-Square method,
6.
Folding method,
7.
Pseudo-random method.
·
What are the types of
Collision Resolution Techniques and the methods used in each of the type?
1.
Open
addressing (closed hashing), The methods used include: Overflow block.
2.
Closed
addressing (open hashing), The methods used include: Linked list, Binary tree.
·
In RDBMS, what is the
efficient data structure used in the internal storage representation?
B+ tree. Because in B+ tree, all the data is stored only in leaf
nodes, that makes searching easier. This corresponds to the records that shall
be stored in leaf nodes.
·
What is a spanning
Tree?
A spanning tree is a tree associated with a network. All the
nodes of the graph appear on the tree once. A minimum spanning tree is a
spanning tree organized so that the total edge weight between nodes is
minimized.
·
Does the minimum
spanning tree of a graph give the shortest distance between any 2 specified
nodes?
No. The Minimal spanning tree assures that the total weight of
the tree is kept at its minimum. But it doesn't mean that the distance between
any two nodes involved in the minimum-spanning tree is minimum.
·
Which is the simplest
file structure? (Sequential, Indexed, Random)
Sequential is the simplest file structure.
·
Whether Linked List is
linear or Non-linear data structure?
According to Access strategies Linked list is a linear
one.
According to Storage Linked List is a Non-linear one.
According to Storage Linked List is a Non-linear one.
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