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:
- Class(Method) Area
- Heap
- Stack
- Program Counter Register
- 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:
- Runtime Environment
- 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?
|
static or class method
|
instance method
|
|
1)A
method i.e. declared as static is known as static method.
|
A
method i.e. not declared as static is known as instance method.
|
|
2)Object
is not required to call static method.
|
Object
is required to call instance methods.
|
|
3)Non-static
(instance) members cannot be accessed in static context (static method,
static block and static nested class) directly.
|
static
and non-static variables both can be accessed in instance methods.
|
|
4)For
example: public static int cube(int n){ return n*n*n;}
|
For
example: public void msg(){...}.
|
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.
|
Method Overloading
|
Method Overriding
|
|
1)
Method overloading increases the readability of the program.
|
Method
overriding provides the specific implementation of the method that is already
provided by its super class.
|
|
2)
method overlaoding is occurs within the class.
|
Method
overriding occurs in two classes that have IS-A relationship.
|
|
3) In
this case, parameter must be different.
|
In this
case, parameter must be same.
|
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){}.
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?
|
Abstract class
|
Interface
|
|
1)An abstract class can have method body (non-abstract methods).
|
Interface have only abstract methods.
|
|
2)An abstract class can have instance variables.
|
An interface cannot have instance variables.
|
|
3)An abstract class can have constructor.
|
Interface cannot have constructor.
|
|
4)An abstract class can have static methods.
|
Interface cannot have static methods.
|
|
5)You can extends one abstract class.
|
You can implement multiple interfaces.
|
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.
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?
|
throw keyword
|
throws keyword
|
|
1)throw
is used to explicitly throw an exception.
|
throws
is used to declare an exception.
|
|
2)checked
exceptions can not be propagated with throw only.
|
checked
exception can be propagated with throws.
|
|
3)throw
is followed by an instance.
|
throws
is followed by class.
|
|
4)throw
is used within the method.
|
throws
is used with the method signature.
|
|
5)You
cannot throw multiple exception
|
You can
declare multiple exception e.g. public void method()throws
IOException,SQLException.
|
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.
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?
|
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.
|
|
finally: finally
block is used in exception handling. finally block is always executed.
|
|
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.
|
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.
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.
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