Hi Friends this Post is about Java interview questions and Core Java Interview questions . As you know now a days cracking job interviews are very tough and we have to learn lot of technologies to crack the interviews . Expectations of companies are growing as they want multiple skilled person who has knowledge in Java , Core Java , J2EE and Many More.
Java /Core Java is a Vast technology as it have loads of things in it . Its very hard for a person to learn all the competencies i am trying to summarize and Bring all the Java \Core Java Interview questions on one Platform . Its my try to help you folks for free and Help you crack Job interviews
Everyone who is in IT knows that we wont work on All the part of any technologies we will be working on a particular part but when we go for a Job Interview we have to Prepare all the supporting technologies too so we have to prepared for the same .
Some Tips which may Help you to crack the Job Interviews
1) Prepare you Cv\Resume Very well now what that means ...It means that what you have written in you Resume you should be well confident on that , Dont include that topics in your CV which you have very little knowledge It may have adverse effect if the Interviewer starts asking questions from that Topics.
2) First Questions in 99% Cases will be Brief Me about yourself or Tell me about yourself . You should Very well prepared for that Prepare it will short description of 3 5 Minutes ,Try to highlight your Strength , Tell your Roles and Responsibilities in Your Current Project . Explain what you work and how you work .
3) Don't Try to Bluff Interviewers or Make them Fool , Try to Answers the question what has been asked.
4) Tell no if you don't know the answers try to be truth as much as possible if Some questions is asked you have little knowledge then please tell the same .
All the best Folks for your Job Hunting. Please free to ask any questions in Comments.
1.What are the principle concepts of OOPS?
• The overriding method cannot have a more restrictive access modifier than the method being
24.What is a marker interface?
• You see that something in your design will change frequently.
35.How are this() and super() used with constructors?
• Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must
50.What are the types of Exceptions in Java
Java /Core Java is a Vast technology as it have loads of things in it . Its very hard for a person to learn all the competencies i am trying to summarize and Bring all the Java \Core Java Interview questions on one Platform . Its my try to help you folks for free and Help you crack Job interviews
Everyone who is in IT knows that we wont work on All the part of any technologies we will be working on a particular part but when we go for a Job Interview we have to Prepare all the supporting technologies too so we have to prepared for the same .
Some Tips which may Help you to crack the Job Interviews
1) Prepare you Cv\Resume Very well now what that means ...It means that what you have written in you Resume you should be well confident on that , Dont include that topics in your CV which you have very little knowledge It may have adverse effect if the Interviewer starts asking questions from that Topics.
2) First Questions in 99% Cases will be Brief Me about yourself or Tell me about yourself . You should Very well prepared for that Prepare it will short description of 3 5 Minutes ,Try to highlight your Strength , Tell your Roles and Responsibilities in Your Current Project . Explain what you work and how you work .
3) Don't Try to Bluff Interviewers or Make them Fool , Try to Answers the question what has been asked.
4) Tell no if you don't know the answers try to be truth as much as possible if Some questions is asked you have little knowledge then please tell the same .
All the best Folks for your Job Hunting. Please free to ask any questions in Comments.
1.What are the principle concepts of OOPS?
There are four principle concepts upon which object oriented design and programming rest. They are:
• Abstraction
• Polymorphism
• Inheritance
• Encapsulation (i.e. easily remembered as A-PIE).
2.What is Abstraction?
Abstraction refers to the act of representing essential features without including the background details
or explanations.
3.What is Encapsulation?
Encapsulation is a technique used for hiding the properties and behaviors of an object and allowing
outside access only as appropriate. It prevents other objects from directly altering or accessing the
properties or methods of the encapsulated object.
4.What is the difference between abstraction and encapsulation?
• Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation
(information hiding) prevents clients from seeing it’s inside view, where the behavior of the
abstraction is implemented.
• Abstraction solves the problem in the design side while Encapsulation is the Implementation.
• Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up
your abstraction to suit the developer needs.
5.What is Inheritance?
• Inheritance is the process by which objects of one class acquire the properties of objects of
another class.
• A class that is inherited is called a superclass.
• The class that does the inheriting is called a subclass.
• Inheritance is done by using the keyword extends.
• The two most common reasons to use inheritance are:
• To promote code reuse
• To use polymorphism
6.What is Polymorphism?
Polymorphism is briefly described as "one interface, many implementations." Polymorphism is a
characteristic of being able to assign a different meaning or usage to something in different contexts -
specifically, to allow an entity such as a variable, a function, or an object to have more than one form.
7.How does Java implement polymorphism?
(Inheritance, Overloading and Overriding are used to achieve Polymorphism in java).
Polymorphism manifests itself in Java in the form of multiple methods having the same name.
(overloaded methods).
• In other cases, multiple methods have the same name, same return type, and same formal
argument list (overridden methods).
8.Explain the different forms of Polymorphism.
There are two types of polymorphism one is Compile time polymorphism and the other is run time
polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is
done using inheritance and interface.
Note: From a practical programming viewpoint, polymorphism manifests itself in three distinct forms
in Java:
• Method overloading
• Method overriding through inheritance
• Method overriding through the Java interface
9.What is runtime polymorphism or dynamic method dispatch?
In Java, 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 superclass. The determination of the method to be
called is based on the object being referred to by the reference variable.
10.What is Dynamic Binding?
Binding refers to the linking of a procedure call to the code to be executed in response to the call.
Dynamic binding (also known as late binding) means that the code associated with a given procedure
call is not known until the time of the call at run-time. It is associated with polymorphism and
inheritance.
11.What is method overloading?
Method Overloading means to have two or more methods with same name in the same class with
different arguments. The benefit of method overloading is that it allows you to implement methods that
support the same semantic operation but differ by argument number or type.
Note:
• Overloaded methods MUST change the argument list
• Overloaded methods CAN change the return type
• Overloaded methods CAN change the access modifier
• Overloaded methods CAN declare new or broader checked exceptions
• A method can be overloaded in the same class or in a subclass
12.What is method overriding?
Method overriding occurs when sub class declares a method that has the same type arguments as a
method declared by one of its superclass. The key benefit of overriding is the ability to define behavior
that’s specific to a particular subclass type.
Note:
• The overriding method cannot have a more restrictive access modifier than the method being
overridden (Ex: You can’t override a method marked public and make it protected).
• You cannot override a method marked final
• You cannot override a method marked static
|
Yes, derived classes still can override the overloaded methods. Polymorphism can still happen.
Compiler will not binding the method calls since it is overloaded, because it might be overridden now
or in the future.
15.Is it possible to override the main method?
NO, because main is a static method. A static method can't be overridden in Java.
16.How to invoke a superclass version of an Overridden method?
To invoke a superclass method that has been overridden in a subclass, you must either call the method
directly through a superclass instance, or use the super prefix in the subclass itself. From the point of
the view of the subclass, the super prefix provides an explicit reference to the superclass'
implementation of the method.
// From subclass
super.overriddenMethod();
super is a keyword which is used to access the method or member variables from the superclass. If a
method hides one of the member variables in its superclass, the method can refer to the hidden variable
through the use of the super keyword. In the same way, if a method overrides one of the methods in its
superclass, the method can invoke the overridden method through the use of the super keyword.
Note:
• You can only go back one level.
• In the constructor, if you use super(), it must be the very first code, and you cannot access any
this.xxx variables or methods to compute its parameters.
18.How do you prevent a method from being overridden?
To prevent a specific method from being overridden in a subclass, use the final modifier on the method
declaration, which means "this is the final implementation of this method", the end of its inheritance
hierarchy.
public final void exampleMethod() {
// Method statements
}
19.What is an Interface?
An interface is a description of a set of methods that conforming implementing classes must have.
Note:
• You can’t mark an interface as final.
• Interface variables must be static.
• An Interface cannot extend anything but another interfaces.
20.Can we instantiate an interface?
You can’t instantiate an interface directly, but you can instantiate a class that implements an interface.
21.Can we create an object for an interface?
Yes, it is always necessary to create an object implementation for an interface. Interfaces cannot be
instantiated in their own right, so you must write a class that implements the interface and fulfill all the
methods defined in it.
22.Do interfaces have member variables?
Interfaces may have member variables, but these are implicitly public, static, and final- in other words,
interfaces can declare only constants, not instance variables that are available to all implementations
and may be used as key references for method arguments for example.
23.What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
Interfaces
|
|
An abstract class can provide complete, default
code and/or just the details that have to be
overridden.
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An interface cannot provide any code at all,just the
signature.
|
In case of abstract class, a class may extend only
one abstract class.
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A Class may implement several interfaces.
|
An abstract class can have non-abstract methods.
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All methods of an Interface are abstract.
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An abstract class can have instance variables.
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An Interface cannot have instance variables.
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An abstract class can have any visibility: public,
private, protected.
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An Interface visibility must be public (or) none.
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If we add a new method to an abstract class then
we have the option of providing default
implementation and therefore all the existing
code might work properly.
|
If we add a new method to an Interface then we
have to track down all the implementations of the
interface and define implementation for the new
method.
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An abstract class can contain constructors .
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An Interface cannot contain constructors .
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Abstract classes are fast.
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Interfaces are slow as it requires extra indirection
to find corresponding method in the actual class.
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24.What is a marker interface?
Marker interfaces are those which do not declare any required methods, but signify their compatibility
with certain operations. The java.io.Serializable interface and Cloneable are typical marker interfaces.
These do not contain any methods, but classes must implement this interface in order to be serialized
and de-serialized.
25.What is an abstract class?
Abstract classes are classes that contain one or more abstract methods. An abstract method is a method
that is declared, but contains no implementation.
Note:
• If even a single method is abstract, the whole class must be declared abstract.
• Abstract classes may not be instantiated, and require subclasses to provide implementations for
the abstract methods.
• You can’t mark a class as both abstract and final.
26.Can we instantiate an abstract class?
An abstract class can never be instantiated. Its sole purpose is to be extended (subclassed).
|
28.When should I use abstract classes and when should I use interfaces?
Use Interfaces when…
|
• You see that something in your design will change frequently.
• If various implementations only share method signatures then it is better to use Interfaces.
• you need some classes to use some methods which you don't want to be included in the class,
then you go for the interface, which makes it easy to just implement and make use of the
methods defined in the interface.
Use Abstract Class when…
• If various implementations are of the same kind and use common behavior or status then
abstract class is better to use.
• When you want to provide a generalized form of abstraction and leave the implementation task
with the inheriting subclass.
• Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a
good choice for nonleaf classes in class hierarchies.
29.When you declare a method as abstract, can other nonabstract methods access it?
Yes, other nonabstract methods can access a method that you declare as abstract.
30.Can there be an abstract class with no abstract methods in it?
Yes, there can be an abstract class without abstract methods.
31.What is Constructor?
• A constructor is a special method whose task is to initialize the object of its class.
• It is special because its name is the same as the class name.
• They do not have return types, not even void and therefore they cannot return values.
• They cannot be inherited, though a derived class can call the base class constructor.
• Constructor is invoked whenever an object of its associated class is created.
32.How does the Java default constructor be provided?
If a class defined by the code does not have any constructor, compiler will automatically provide one
no-parameter-constructor (default-constructor) for the class in the byte code. The access modifier
(public/private/etc.) of the default constructor is the same as the class itself.
33.Can constructor be inherited?
No, constructor cannot be inherited, though a derived class can call the base class constructor.
No return type, not even void
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void or a valid return type
|
|
Name
|
Same name as the class (first letter is
capitalized by convention) -- usually a
noun
|
Any name except the class. Method
names begin with a lowercase letter
by convention -- usually the name of
an action
|
this
|
Refers to another constructor in the
same class. If used, it must be the first
line of the constructor
|
Refers to an instance of the owning
class. Cannot be used by static
methods.
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super
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Calls the constructor of the parent
class. If used, must be the first line of
the constructor
|
Calls an overridden method in the
parent class
|
Inheritance
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Constructors are not inherited
|
Methods are inherited
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Class Methods
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Instance Methods
|
Class methods are methods which are declared as
static. The method can be called without creating
an instance of the class
|
Instance methods on the other hand require an
instance of the class to exist before they can be
called, so an instance of a class needs to be
created by using the new keyword.
Instance methods operate on specific instances of
classes.
|
Class methods can only operate on class members
and not on instance members as class methods are
unaware of instance members.
|
Instance methods of the class can also not be
called from within a class method unless they are
being called on an instance of that class.
|
Class methods are methods which are declared as
static. The method can be called without creating
an instance of the class.
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Instance methods are not declared as static.
|
35.How are this() and super() used with constructors?
• Constructors use this to refer to another constructor in the same class with a different parameter
list.
• Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must
use it in the first line; otherwise, the compiler will complain.
|
37.How are this() and super() used with constructors?
• Constructors use this to refer to another constructor in the same class with a different parameter
list.
public
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protected
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default
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private
|
|
Accessible to class
from same package?
|
yes
|
yes
|
yes
|
no
|
Accessible to class
from different package?
|
yes
|
no, unless it is a subclass
|
no
|
no
|
• Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must
use it in the first line; otherwise, the compiler will complain.
38.What are Access Specifiers?
One of the techniques in object-oriented programming is encapsulation. It concerns the hiding of data
in a class and making this class available only through methods. Java allows you to control access to
classes, methods, and fields via so-called access specifiers..
39.What are Access Specifiers available in Java?
Java offers four access specifiers, listed below in decreasing accessibility:
• Public- public classes, methods, and fields can be accessed from everywhere.
• Protected- protected methods and fields can only be accessed within the same class to which the
methods and fields belong, within its subclasses, and within classes of the same package.
• Default(no specifier)- If you do not set access to specific level, then such a class, method, or
field will be accessible from inside the same package to which the class, method, or field
belongs, but not from outside this package.
• Private- private methods and fields can only be accessed within the same class to which the
methods and fields belong. private methods and fields are not visible within subclasses and are
|
40.What is final modifier?
The final modifier keyword makes that the programmer cannot change the value anymore. The actual
meaning depends on whether it is applied to a class, a variable, or a method.
• final Classes- A final class cannot have subclasses.
• final Variables- A final variable cannot be changed once it is initialized.
• final Methods- A final method cannot be overridden by subclasses.
41.What are the uses of final method?
There are two reasons for marking a method as final:
• Disallowing subclasses to change the meaning of the method.
• Increasing efficiency by allowing the compiler to turn calls to the method into inline Java code.
42.What is static block?
Static block which exactly executed exactly once when the class is first loaded into JVM. Before going
to the main method the static block will execute.
Variables that have only one copy per class are known as static variables. They are not attached to a
particular instance of a class but rather belong to a class as a whole. They are declared by using the
static keyword as a modifier.
static type varIdentifier;
where, the name of the variable is varIdentifier and its data type is specified by type.
Note: Static variables that are not explicitly initialized in the code are automatically initialized with a
default value. The default value depends on the data type of the variables.
44.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.
45.What are static methods?
Methods declared with the keyword static as modifier are called static methods or class methods. They
are so called because they affect a class as a whole, not a particular instance of the class. Static methods
are always invoked without reference to a particular instance of a class.
Note:The use of a static method suffers from the following restrictions:
• A static method can only call other static methods.
• A static method must only access static data.
• A static method cannot reference to the current object using keywords super or this.
46.What is an exception?
An exception is an event, which occurs during the execution of a program, that disrupts the normal
flow of the program's instructions.
47.What is error?
An Error indicates that a non-recoverable condition has occurred that should not be caught. Error, a
subclass of Throwable, is intended for drastic problems, such as OutOfMemoryError, which would be
reported by the JVM itself.
48.Which is superclass of Exception?
"Throwable", the parent class of all exception related classes.
49.What are the advantages of using exception handling?
Exception handling provides the following advantages over "traditional" error management techniques:
• Separating Error Handling Code from "Regular" Code.
• Propagating Errors Up the Call Stack.
• Grouping Error Types and Error Differentiation.
50.What are the types of Exceptions in Java
There are two types of exceptions in Java, unchecked exceptions and checked exceptions.
• Checked exceptions: A checked exception is some subclass of Exception (or Exception itself),
excluding class RuntimeException and its subclasses. Each method must either handle all
checked exceptions by supplying a catch clause or list each unhandled checked exception as a
thrown exception.
• Unchecked exceptions: All Exceptions that extend the RuntimeException class are unchecked
exceptions. Class Error and its subclasses also are unchecked.
51.Why Errors are Not Checked?
A unchecked exception classes which are the error classes (Error and its subclasses) are exempted from
compile-time checking because they can occur at many points in the program and recovery from them
is difficult or impossible. A program declaring such exceptions would be pointlessly.
52.Why Runtime Exceptions are Not Checked?
The runtime exception classes (RuntimeException and its subclasses) are exempted from compile-time
checking because, in the judgment of the designers of the Java programming language, having to
declare such exceptions would not aid significantly in establishing the correctness of programs. Many
of the operations and constructs of the Java programming language can result in runtime exceptions.
The information available to a compiler, and the level of analysis the compiler performs, are usually
not sufficient to establish that such run-time exceptions cannot occur, even though this may be obvious
to the programmer. Requiring such exception classes to be declared would simply be an irritation to
programmers.
