Core Java: Basics of Java Interview Questions
1) What is difference between JDK,JRE and JVM?
JVM
JVM is an acronym for Java
Virtual Machine, it is an abstract machine which provides the runtime
environment in which java bytecode can be executed. It is a specification.
JVMs are available for many
hardware and software platforms (so JVM is platform dependent).
JRE
JRE stands for Java Runtime
Environment. It is the implementation of JVM.
JDK
JDK is an acronym for Java
Development Kit. It physically exists. It contains JRE + development tools.
2) How many types of memory areas are
allocated by JVM?
Many types:
1. Class(Method)
Area
2. Heap
3. Stack
4. Program
Counter Register
5. Native
Method Stack
3) What is JIT compiler?
Just-In-Time(JIT)
compiler:It is used to improve the performance. JIT compiles parts of the
byte code that have similar functionality at the same time, and hence reduces
the amount of time needed for compilation.Here the term “compiler” refers to a
translator from the instruction set of a Java virtual machine (JVM) to the
instruction set of a specific CPU.
4) What is platform?
A platform is basically the
hardware or software environment in which a program runs. There are two types
of platforms software-based and hardware-based. Java provides software-based
platform.
5) What is the main difference between Java
platform and other platforms?
The Java platform differs
from most other platforms in the sense that it's a software-based platform that
runs on top of other hardware-based platforms.It has two components:
1. Runtime
Environment
2. API(Application
Programming Interface)
6) What gives Java its 'write once and run
anywhere' nature?
The bytecode. Java is
compiled to be a byte code which is the intermediate language between source
code and machine code. This byte code is not platform specific and hence can be
fed to any platform.
7) What is classloader?
The classloader is a
subsystem of JVM that is used to load classes and interfaces.There are many
types of classloaders e.g. Bootstrap classloader, Extension classloader, System
classloader, Plugin classloader etc.
8) Is Empty .java file name a valid source
file name?
Yes, save your java file by
.java only, compile it by javac .java and run by java
yourclassname Let's take a simple example:
1.
//save by .java only
2.
class A{
3.
public static void main(String args[]){
4.
System.out.println("Hello java");
5.
}
6.
}
7.
//compile by javac .java
8. //run by java A
compile it by javac
.java
run it by java A
9) Is delete,next,main,exit or null keyword in
java?
No.
10) If I don't 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.
11) What if I write static public void instead
of public static void?
Program compiles and runs
properly.
12) What is the default value of the local
variables?
The local variables are not
initialized to any default value, neither primitives nor object references.
Core Java - OOPs Concepts: Initial OOPs
Interview Questions
There is given more than 50
OOPs (Object-Oriented Programming and System) interview questions. But they
have been categorized in many sections such as constructor interview questions,
static interview questions, Inheritance Interview questions, Abstraction
interview question, Polymorphism interview questions etc. for better
understanding.
13) What is difference between object oriented
programming language and object based programming language?
Object based programming
languages follow all the features of OOPs except Inheritance. Examples of
object based programming languages are JavaScript, VBScript etc.
14) 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.
Core Java - OOPs Concepts: Constructor Interview
Questions
15) What is constructor?
- Constructor is just like a method
that is used to initialize the state of an object. It is invoked at the
time of object creation.
16) What is the purpose of default
constructor?
- The default constructor provides
the default values to the objects. The java compiler creates a default
constructor only if there is no constructor in the class.more details...
17) Does constructor return any value?
Ans:yes, that
is current instance (You cannot use return type yet it returns a value).more details...
18)Is constructor inherited?
No, constructor is not
inherited.
19) Can you make a constructor final?
No, constructor can't be
final.
Core Java - OOPs Concepts: static keyword
Interview Questions
20) What is static variable?
- static variable is used to refer
the common property of all objects (that is not unique for each object)
e.g. company name of employees,college name of students etc.
- static variable gets memory only
once in class area at the time of class loading.
21) What is static method?
- A static method belongs to the
class rather than object of a class.
- A static method can be invoked
without the need for creating an instance of a class.
- static method can access static
data member and can change the value of it.
22) Why main method is static?
because object is not
required to call static method if It were non-static method,jvm creats object
first then call main() method that will lead to the problem of extra memory
allocation.more details...
23) What is static block?
- Is used to initialize the static
data member.
- It is excuted before main method
at the time of classloading.
24) Can we execute a program without main()
method?
Ans) Yes, one of the way is
static block.more details...
25) What if the static modifier is removed
from the signature of the main method?
Program compiles. But at
runtime throws an error "NoSuchMethodError".
26) What is difference between static (class)
method and instance method?
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static or class method
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instance method
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1)A
method i.e. declared as static is known as static method.
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A
method i.e. not declared as static is known as instance method.
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2)Object
is not required to call static method.
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Object
is required to call instance methods.
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3)Non-static
(instance) members cannot be accessed in static context (static method,
static block and static nested class) directly.
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static
and non-static variables both can be accessed in instance methods.
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4)For
example: public static int cube(int n){ return n*n*n;}
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For
example: public void msg(){...}.
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Core Java - OOPs Concepts: Inheritance Interview
Questions
27) What is this in java?
It is a keyword that that
refers to the current object.more details...
28)What is Inheritance?
Inheritance is a mechanism
in which one object acquires all the properties and behaviour of another object
of another class. It represents IS-A relationship. It is used for Code
Resusability and Method Overriding.
29) Which class is the superclass for every
class.
Object class.
30) Why multiple inheritance is not supported
in java?
- To reduce the complexity and
simplify the language, multiple inheritance is not supported in java in
case of class.more
details...
31) What is composition?
Holding the reference of
the other class within some other class is known as composition.
32) What is difference between aggregation and
composition?
Aggregation represents weak
relationship whereas composition represents strong relationship. For example:
bike has an indicator (aggregation) but bike has an engine (compostion).
33) Why Java does not support pointers?
Pointer is a variable that
refers to the memory address. They are not used in java because they are
unsafe(unsecured) and complex to understand.
34) What is super in java?
It is a keyword that refers
to the immediate parent class object.more details...
35) Can you use this() and super() both in a
constructor?
No. Because super() or
this() must be the first statement.
36)What is object cloning?
The object cloning is used
to create the exact copy of an object. more details...
Core Java - OOPs Concepts: Method Overloading
Interview Questions
37) What is method overloading?
If a class have multiple
methods by same name but different parameters, it is known as Method
Overloading. It increases the readability of the program.more details...
38) Why method overloading is not possible by
changing the return type in java?
Becauseof ambiguity.more details...
39) Can we overload main() method?
Yes, You can have many
main() methods in a class by overloading the main method.
Core Java - OOPs Concepts: Method Overriding
Interview Questions
40) What is method overriding:
If a subclass provides a
specific implementation of a method that is already provided by its parent
class, it is known as Method Overriding. It is used for runtime polymorphism
and to provide the specific implementation of the method.more details...
41) Can we override static method?
No, you can't override the
static method because they are the part of class not object.
42) Why we cannot override static method?
It is because the static
method is the part of class and it is bound with class whereas instance method
is bound with object and static gets memory in class area and instance gets
memory in heap.
43) Can we override the overloaded method?
Yes.
44) Difference between method Overloading and
Overriding.
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Method Overloading
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Method Overriding
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1)
Method overloading increases the readability of the program.
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Method
overriding provides the specific implementation of the method that is already
provided by its super class.
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2)
method overlaoding is occurs within the class.
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Method
overriding occurs in two classes that have IS-A relationship.
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3) In
this case, parameter must be different.
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In this
case, parameter must be same.
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45) Can you have virtual functions in Java?
Yes, all functions in Java
are virtual by default.
46) What is covariant return type?
Now, since java5, it is
possible to override any method by changing the return type if the return type
of the subclass overriding method is subclass type. It is known as covariant
return type. more details...
Core Java - OOPs Concepts: final keyword
Interview Questions
47) What is final variable?
If you make any variable as
final, you cannot change the value of final variable(It will be constant).more details...
48) What is final method?
Final methods can't be
overriden.more details...
49) What is final class?
Final class can't be
inherited. more details...
50) What is blank final variable?
A final variable, not
initalized at the time of declaration, is known as blank final variable.more details...
51) Can we intialize blank final variable?
Yes, only in constructor if
it is non-static. If it is static blank final variable, it can be initialized
only in the static block.more details...
52) Can you declare the main method as final?
Yes, such as, public static
final void main(String[] args){}.
Core Java - OOPs : Polymorphism Interview Questions
53) What is Runtime Polymorphism?
Runtime polymorphism or dynamic
method dispatch is a process in which a call to an overridden method is
resolved at runtime rather than at compile-time.
In this process, an overridden
method is called through the reference variable of a super class. The
determination of the method to be called is based on the object being referred
to by the reference variable.
54) Can you achieve Runtime Polymorphism by data members?
No.
55) What is the difference between static binding and
dynamic binding?
In case of static binding type
of object is determined at compile time whereas in dynamic binding type of
object is determined at runtime.
Core Java - OOPs Concepts : Abstraction Interview Questions
56) What is abstraction?
Abstraction is a process of
hiding the implementation details and showing only functionality to the user.
Abstraction lets you focus on
what the object does instead of how it does it.
57) What is the difference between abstraction and
encapsulation?
Abstraction hides the
implementation details whereas encapsulation wraps code and data into a single
unit.
58) What is abstract class?
A class that is declared as
abstract is known as abstract class. It needs to be extended and its method
implemented. It cannot be instantiated.
59) Can there be any abstract method without abstract
class?
No, if there is any abstract
method in a class, that class must be abstract.
60) Can you use abstract and final both with a method?
No, because abstract method
needs to be overridden whereas you can't override final method.
61) Is it possible to instantiate the abstract class?
No, abstract class can never be
instantiated.
62) What is interface?
Interface is a blueprint of a
class that have static constants and abstract methods.It can be used to achieve
fully abstraction and multiple inheritance.
63) Can you declare an interface method static?
No, because methods of an
interface is abstract by default, and static and abstract keywords can't be
used together.
64) Can an Interface be final?
No, because its implementation
is provided by another class.
65) What is marker interface?
An interface that have no data
member and method is known as a marker interface.For example Serializable,
Cloneable etc.
66) What is difference between abstract class and
interface?
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Abstract class
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Interface
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1)An abstract class can have method body (non-abstract methods).
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Interface have only abstract methods.
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2)An abstract class can have instance variables.
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An interface cannot have instance variables.
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3)An abstract class can have constructor.
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Interface cannot have constructor.
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4)An abstract class can have static methods.
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Interface cannot have static methods.
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5)You can extends one abstract class.
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You can implement multiple interfaces.
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67) Can we define private and protected modifiers for
variables in interfaces?
No, they are implicitly public.
68) When can an object reference be cast to an interface
reference?
An object reference can be cast
to an interface reference when the object implements the referenced interface.
Core Java - OOPs Concepts : Package Interview Questions
69) What is package?
A package is a group of similar
type of classes interfaces and sub-packages. It provides access protection and
removes naming collision.
70) Do I need to import java.lang package any time? Why ?
No. It is by default loaded
internally by the JVM.
71) 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.But
the JVM will internally load the class only once no matter how many times you
import the same class.
72) What is static import ?
By static import, we can access
the static members of a class directly, there is no to qualify it with the
class name.
Java : Exception Handling Interview Questions
There is given a list of
exception handling interview questions with answers. If you know any exception
handling interview question, kindly post it in the comment section.
73) What is Exception Handling?
Exception Handling is a
mechanism to handle runtime errors.It is mainly used to handle checked
exceptions.
74) What is difference between Checked
Exception and Unchecked Exception?
1)Checked Exception
The classes that extend
Throwable class except RuntimeException and Error are known as checked
exceptions e.g.IOException,SQLException etc. Checked exceptions are checked at
compile-time.
2)Unchecked Exception
The classes that extend
RuntimeException are known as unchecked exceptions e.g.
ArithmeticException,NullPointerException etc. Unchecked exceptions are not
checked at compile-time.
75) What is the base class for Error and
Exception?
Throwable.
76) 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.
77) What is finally block?
- finally block is a block that is
always executed.more
details...
78) Can finally block be used without catch?
- Yes, by try block. finally must be
followed by either try or catch.more
details...
79) Is there any case when finally will not be
executed?
finally block will not be
executed if program exits(either by calling System.exit() or by causing a fatal
error that causes the process to abort).more details...
80) What is difference between throw and
throws?
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throw keyword
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throws keyword
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1)throw
is used to explicitly throw an exception.
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throws
is used to declare an exception.
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2)checked
exceptions can not be propagated with throw only.
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checked
exception can be propagated with throws.
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3)throw
is followed by an instance.
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throws
is followed by class.
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4)throw
is used within the method.
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throws
is used with the method signature.
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5)You
cannot throw multiple exception
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You can
declare multiple exception e.g. public void method()throws
IOException,SQLException.
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81) Can an exception be rethrown?
Yes.
82) Can subclass overriding method declare an
exception if parent class method doesn't throw an exception ?
Yes but only unchecked exception
not checked.
83) What is exception propagation ?
Forwarding the exception
object to the invoking method is known as exception propagation.
There is given a list of
string handling interview questions with short and pointed answers. If you know
any string handling interview question, kindly post it in the comment section.
84) What is the meaning of immutable in terms
of String?
The simple meaning of
immutable is unmodifiable or unchangeable. Once string object has been created,
its value can't be changed.
85) Why string objects are immutable in java?
Because java uses the
concept of string literal. Suppose there are 5 reference variables,all referes
to one object "sachin".If one reference variable changes the value of
the object, it will be affected to all the reference variables. That is why
string objects are immutable in java.
86) How many ways we can create the string object?
There are two ways to
create the string object, by string literal and by new keyword.
87) How many objects will be created in the
following code?
1.
String s1="Welcome";
2.
String s2="Welcome";
3. String s3="Welcome";
Only one object.
88) Why java uses the concept of string
literal?
To make Java more memory
efficient (because no new objects are created if it exists already in string
constant pool).
89)How many objects will be created in the
following code?
1. String s = new String("Welcome");
Two objects, one in string
constant pool and other in non-pool(heap).
90) What is the basic difference between
string and stringbuffer object?
String is an immutable
object. StringBuffer is a mutable object.
91) What is the difference between
StringBuffer and StringBuilder ?
StringBuffer is
synchronized whereas StringBuilder is not synchronized.
92) How can we create immutable class in java
?
We can create immutable
class as the String class by defining final class and
93) What is the purpose of toString() method
in java ?
The toString() method
returns the string representation of any object. If you print any object, java
compiler internally invokes the toString() method on the object. So overriding
the toString() method, returns the desired output, it can be the state of an
object etc. depends on your implementation.
Core Java : Nested classes and Interfaces
Interview Questions
94)What is nested class?
A class which is declared
inside another class is known as nested class. There are 4 types of nested
class member inner class, local inner class, annonymous inner class and static
nested class.
95) Is there any difference between nested
classes and inner classes?
Yes, inner classes are
non-static nested classes i.e. inner classes are the part of nested classes.
96) Can we access the non-final local
variable, inside the local inner class?
No, local variable must be
constant if you want to access it in local inner class.
97) What is nested interface ?
Any interface i.e. declared
inside the interface or class, is known as nested interface. It is static by
default.
98) Can a class have an interface?
Yes, it is known as nested
interface.
99) Can an Interface have a class?
Yes, they are static
implicitely.
Garbage Collection Interview Questions
117) What is Garbage Collection?
Garbage collection is a
process of reclaiming the runtime unused objects.It is performed for memory
management.
118) What is gc()?
gc() is a daemon
thread.gc() method is defined in System class that is used to send request to
JVM to perform garbage collection.
119) What is the purpose of finalize() method?
finalize() method is
invoked just before the object is garbage collected.It is used to perform
cleanup processing.
120) Can an unrefrenced objects be refrenced
again?
Yes.
121)What kind of thread is the Garbage
collector thread?
Daemon thread.
122)What is difference between final, finally
and finalize?
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final: final
is a keyword, final can be variable, method or class.You, can't change the
value of final variable, can't override final method, can't inherit final
class.
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finally: finally
block is used in exception handling. finally block is always executed.
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finalize():finalize()
method is used in garbage collection.finalize() method is invoked just before
the object is garbage collected.The finalize() method can be used to perform
any cleanup processing.
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123)What is the purpose of the Runtime class?
The purpose of the Runtime
class is to provide access to the Java runtime system.
124)How will you invoke any external process
in Java?
By
Runtime.getRuntime().exec(?) method.
I/O Interview Questions
125)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.
126)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.
Serialization Interview Questions
127) What is serialization?
Serialization is a process
of writing the state of an object into a byte stream.It is mainly used to
travel object's state on the network.
128) What is Deserialization?
Deserialization is the
process of reconstructing the object from the serialized state.It is the
reverse operation of serialization.
129) What is transient keyword?
If you define any data
member as transient,it will not be serialized.more details...
130)What is Externalizable?
Externalizable interface is
used to write the state of an object into a byte stream in compressed format.It
is not a marker interface.
131)What is the difference between
Serializalble and Externalizable interface?
Serializable is a marker
interface but Externalizable is not a marker interface.When you use
Serializable interface, your class is serialized automatically by default. But
you can override writeObject() and readObject() two methods to control more
complex object serailization process. When you use Externalizable interface,
you have a complete control over your class's serialization process.
Networking Interview Questions
132)How do I convert a numeric IP address like
192.18.97.39 into a hostname like java.sun.com?
By
InetAddress.getByName("192.18.97.39").getHostName() where
192.18.97.39 is the IP address.
Reflection Interview Questions
133) What is reflection?
Reflection is the process
of examining or modifying the runtime behaviour of a class at runtime.It is
used in:
- IDE (Integreted Development
Environment) e.g. Eclipse, MyEclipse, NetBeans.
- Debugger
- Test Tools etc.
134) Can you access the private method from
outside the class?
Yes, by changing the
runtime behaviour of a class if the class is not secured.
Miscellaneous Interview Questions
148)What are wrapper classes?
Wrapper classes are classes
that allow primitive types to be accessed as objects.
149)What is a native method?
A native method is a method
that is implemented in a language other than Java.
150)What is the purpose of the System class?
The purpose of the System class
is to provide access to system resources.
151)What comes to mind when someone mentions a shallow copy
in Java?
Object cloning.
152)What is singleton class?
Singleton class means that any
given time only one instance of the class is present, in one JVM.
AWT and SWING Interview Questions
153)Which containers use a border layout as their default
layout?
The Window, Frame and Dialog
classes use a border layout as their default layout.
154)Which containers use a FlowLayout as their default
layout?
The Panel and Applet classes
use the FlowLayout as their default layout.
155)What are peerless components?
The peerless components are
called light weight components.
156)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.
157)What is a lightweight component?
Lightweight components are the
one which doesn?t go with the native call to obtain the graphical units. They
share their parent component graphical units to render them. For example, Swing
components.
158)What is a heavyweight component?
For every paint call, there
will be a native call to get the graphical units.For Example, AWT.
159)What is an applet?
An applet is a small java
program that runs inside the browser and generates dynamic contents.
160)Can you write a Java class that could be used both as
an applet as well as an application?
Yes. Add a main() method to the
applet.
Internationalization Interview Questions
161)What is Locale?
A Locale object represents a
specific geographical, political, or cultural region.
162)How will you load a specific locale?
By ResourceBundle.getBundle(?)
method.
Java Bean Interview Questions
163)What is a JavaBean?
are reusable software
components written in the Java programming language, designed to be manipulated
visually by a software development environment, like JBuilder or VisualAge for
Java.
RMI Interview Questions
164)Can RMI and Corba based applications interact?
Yes they can. RMI is available
with IIOP as the transport protocol instead of JRMP.
Java Multithreading Interview Questions
Multithreading and
Synchronization is considered as the typical chapter in java programming. In
game development company, mulithreading related interview questions are asked
mostly. A list of frequently asked java multithreading interview questions are
given below.
1) What is multithreading?
Multithreading is a process of
executing multiple threads simultaneously. Its main advantage is:
- Threads share the same address space.
- Thread is lightweight.
- Cost of communication between process
is low.
2) What is thread?
A thread is a lightweight
subprocess.It is a separate path of execution.It is called separate path of
execution because each thread runs in a separate stack frame.
3)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.
4) What does join() method?
The join() method waits for a
thread to die. In other words, it causes the currently running threads to stop
executing until the thread it joins with completes its task.
5) What is difference between wait() and sleep() method?
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wait()
|
sleep()
|
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1) The wait() method is defined in Object class.
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The sleep() method is defined in Thread class.
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2) wait() method releases the lock.
|
The sleep() method doesn't releases the lock.
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6) Is it possible to start a thread twice?
No, there is no possibility to
start a thread twice. If we does, it throws an exception.
7) Can we call the run() method instead of start()?
yes, but it will not work as a
thread rather it will work as a normal object so there will not be
context-switching between the threads.
8) What about the daemon threads?
The daemon threads are
basically the low priority threads that provides the background support to the
user threads. It provides services to the user threads.
9)Can we make the user thread as daemon thread if thread is
started?
No, if you do so, it will throw
IllegalThreadStateException
10)What is shutdown hook?
The shutdown hook is basically
a thread i.e. invoked implicitely before JVM shuts down. So we can use it
perform clean up resource.
11)When should we interrupt a thread?
We should interrupt a thread if
we want to break out the sleep or wait state of a thread.
12) What is synchronization?
Synchronization is the
capabilility of control the access of multiple threads to any shared
resource.It is used:
1. To
prevent thread interference.
2. To
prevent consistency problem.
13) What is the purpose of Synchronized block?
- Synchronized block is used to lock an
object for any shared resource.
- Scope of synchronized block is smaller
than the method.
14)Can Java object be locked down for exclusive use by a
given thread?
Yes. You can lock an object by
putting it in a "synchronized" block. The locked object is
inaccessible to any thread other than the one that explicitly claimed it.
15) What is static synchronization?
If you make any static method
as synchronized, the lock will be on the class not on object. more details...
16)What is the difference between notify() and notifyAll()?
The notify() is used to unblock
one waiting thread whereas notifyAll() method is used to unblock all the
threads in waiting state.
17)What is deadlock?
Deadlock is a situation when
two threads are waiting on each other to release a resource. Each thread
waiting for a resource which is held by the other waiting thread.
20 Java Collections Interview Questions
In java, collection interview
questions are mostly asked by the interviewers. Here is the list of mostly
asked collections interview questions with answers.
1) What is the difference between ArrayList and Vector?
|
No.
|
ArrayList
|
Vector
|
|
1)
|
ArrayList is not synchronized.
|
Vector is synchronized.
|
|
2)
|
ArrayList is not a legacy class.
|
Vector is a legacy class.
|
|
3)
|
ArrayList increases its size by 50% of the array size.
|
Vector increases its size by doubling the array size.
|
2) What is the difference between ArrayList and LinkedList?
|
No.
|
ArrayList
|
LinkedList
|
|
1)
|
ArrayList uses a dynamic array.
|
LinkedList uses doubly linked list.
|
|
2)
|
ArrayList is not efficient for manipulation because a lot of
shifting is required.
|
LinkedList is efficient for manipulation.
|
|
3)
|
ArrayList is better to store and fetch data.
|
LinkedList is better to manipulate data.
|
3) What is the difference between Iterator and
ListIterator?
Iterator traverses the elements
in forward direction only whereas ListIterator traverses the elements in
forward and backward direction.
|
No.
|
Iterator
|
ListIterator
|
|
1)
|
Iterator traverses the elements in forward direction only.
|
ListIterator traverses the elements in backward and forward
directions both.
|
|
2)
|
Iterator can be used in List, Set and Queue.
|
ListIterator can be used in List only.
|
4) What is the difference between Iterator and Enumeration?
|
No.
|
Iterator
|
Enumeration
|
|
1)
|
Iterator can traverse legacy and non-legacy elements.
|
Enumeration can traverse only legacy elements.
|
|
2)
|
Iterator is fail-fast.
|
Enumeration is not fail-fast.
|
|
3)
|
Iterator is slower than Enumeration.
|
Enumeration is faster than Iterator.
|
5) What is the difference between List and Set?
List can contain duplicate
elements whereas Set contains only unique elements.
6) What is the difference between HashSet and TreeSet?
HashSet maintains no
order whereas TreeSet maintains ascending order.
7) What is the difference between Set and Map?
Set contains values only
whereas Map contains key and values both.
8) What is the difference between HashSet and HashMap?
HashSet contains only values
whereas HashMap contains entry(key,value). HashSet can be iterated but HashMap
need to convert into Set to be iterated.
9) What is the difference between HashMap and TreeMap?
HashMap maintains no
order but TreeMap maintains ascending order.
10) What is the difference between HashMap and Hashtable?
|
No.
|
HashMap
|
Hashtable
|
|
1)
|
HashMap is not synchronized.
|
Hashtable is synchronized.
|
|
2)
|
HashMap can contain one null key and multiple null values.
|
Hashtable cannot contain any null key or null value.
|
11) What is the difference between Collection and
Collections?
Collection is an interface
whereas Collections is a class. Collection interface provides normal
functionality of data structure to List, Set and Queue. But, Collections class
is to sort and synchronize collection elements.
12) What is the difference between Comparable and
Comparator?
|
No.
|
Comparable
|
Comparator
|
|
1)
|
Comparable provides only one sort of sequence.
|
Comparator provides multiple sort of sequences.
|
|
2)
|
It provides one method named compareTo().
|
It provides one method named compare().
|
|
3)
|
It is found in java.lang package.
|
it is found in java.util package.
|
|
4)
|
If we implement Comparable interface, actual class is modified.
|
Actual class is not modified.
|
13) What is the advantage of Properties file?
If you change the value in
properties file, you don't need to recompile the java class. So, it makes the
application easy to manage.
14) What does the hashCode() method?
The hashCode() method returns a
hash code value (an integer number).
The hashCode() method returns
the same integer number, if two keys (by calling equals() method) are same.
But, it is possible that two
hash code numbers can have different or same keys.
15) Why we override equals() method?
The equals method is used to
check whether two objects are same or not. It needs to be overridden if we want
to check the objects based on property.
For example, Employee is a
class that has 3 data members: id, name and salary. But, we want to check the
equality of employee object on the basis of salary. Then, we need to override
the equals() method.
16) How to synchronize List, Set and Map elements?
Yes, Collections class provides
methods to make List, Set or Map elements as synchronized:
|
public static List synchronizedList(List l){}
|
|
public static Set synchronizedSet(Set s){}
|
|
public static SortedSet synchronizedSortedSet(SortedSet s){}
|
|
public static Map synchronizedMap(Map m){}
|
|
public static SortedMap synchronizedSortedMap(SortedMap m){}
|
17) What is the advantage of generic collection?
If we use generic class, we
don't need typecasting. It is typesafe and checked at compile time.
18) What is hash-collision in Hashtable and how it is handled
in Java?
Two different keys with the
same hash value is known as hash-collision. Two different entries will be kept
in a single hash bucket to avoid the collision.
19) What is the Dictionary class?
The Dictionary class provides
the capability to store key-value pairs.
20) What is the default size of load factor in hashing
based collection?
The default size of load factor
is 0.75. The default capacity is computed as initial capacity *
load factor. For example, 16 * 0.75 = 12. So, 12 is the default capacity of
Map.
21) What is the difference between Array and ArrayList?
22) What is the difference between length of Array and size of
ArrayList?
23) How to convert ArrayList to Array and Array to ArrayList?
24) How to make Java ArrayList Read-Only?
25) How to remove duplicates from ArrayList?
26) How to reverse ArrayList?
27) How to sort ArrayList in descending order?
28) How to synchronize ArrayList?
29) When to use ArrayList and LinkedList?
JDBC Interview Questions
A list of frequently asked jdbc
interview questions with answers are given below.
1) What is JDBC?
JDBC is a Java API that is used
to connect and execute query to the database. JDBC API uses jdbc drivers to
connects to the database.
2) What is JDBC Driver?
JDBC Driver is a software
component that enables java application to interact with the database.There are
4 types of JDBC drivers:
1. JDBC-ODBC
bridge driver
2. Native-API
driver (partially java driver)
3. Network
Protocol driver (fully java driver)
4. Thin
driver (fully java driver)
3) What are the steps to connect to the database in java?
- Registering the driver class
- Creating connection
- Creating statement
- Executing queries
- Closing connection
4) What are the JDBC API components?
The java.sql package contains
interfaces and classes for JDBC API.
Interfaces:
- Connection
- Statement
- PreparedStatement
- ResultSet
- ResultSetMetaData
- DatabaseMetaData
- CallableStatement etc.
Classes:
- DriverManager
- Blob
- Clob
- Types
- SQLException etc.
5) What are the JDBC statements?
There are 3 JDBC statements.
1. Statement
2. PreparedStatement
3. CallableStatement
6) What is the difference between Statement and
PreparedStatement interface?
In case of Statement, query is
complied each time whereas in case of PreparedStatement, query is complied only
once. So performance of PreparedStatement is better than Statement.
7) How can we execute stored procedures and functions?
By using Callable
statement interface, we can execute procedures and functions.
8) What is the role of JDBC DriverManager class?
The DriverManager class manages
the registered drivers. It can be used to register and unregister drivers. It
provides factory method that returns the instance of Connection.
9) What does the JDBC Connection interface?
The Connection
interface maintains a session with the database. It can be used for
transaction management. It provides factory methods that returns the instance
of Statement, PreparedStatement, CallableStatement and DatabaseMetaData.
10) What does the JDBC ResultSet interface?
The ResultSet object represents
a row of a table. It can be used to change the cursor pointer and get the
information from the database.
11) What does the JDBC ResultSetMetaData interface?
The ResultSetMetaData interface
returns the information of table such as total number of columns, column name,
column type etc.
12) What does the JDBC DatabaseMetaData interface?
The DatabaseMetaData interface
returns the information of the database such as username, driver name, driver
version, number of tables, number of views etc.
13) Which interface is responsible for transaction
management in JDBC?
The Connection
interface provides methods for transaction management such as
commit(), rollback() etc.
14) What is batch processing and how to perform batch
processing in JDBC?
By using batch processing
technique in JDBC, we can execute multiple queries. It makes the performance
fast.
15) How can we store and retrieve images from the database?
By using PreparedStatement
interface, we can store and retrieve images.
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