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Java is one of the most popular programming language. There is a growing demand for Java Developer jobs in technology companies.

This article contains technical most important core java questions that an interviewer asks for Java technology and related topics. Each question is accompanied with an answer so that you can prepare for job interview and clear your concepts in short time.

we have covered almost 500+ important core Java interview questions for freshers and experienced candidates. This post on CORE JAVA Important Questions is prepared to help you understand the basic concepts of Java programming for interview purposes. All the important JAVA concepts are explained here with examples for your easy understanding.This tutorial covers JAVA topics like basic Java definitions, OOP concepts, Access specifiers, Collections, Exceptions, Threads, Serialization, etc., with examples to make you get ready perfectly to face any JAVA interview confidently.

All the best !

Q1. What is the difference between an Abstract class and Interface?

1. Abstract classes may have some executable methods and methods left unimplemented. Interfaces contain no implementation code.

2. An class can implement any number of interfaces, but subclass at most one abstract class.

3. An abstract class can have non abstract methods. All methods of an interface are abstract.

4. An abstract class can have instance variables. An interface cannot.

5. An abstract class can define constructor. An interface cannot.

6. An abstract class can have any visibility: public, protected, private or none (package). An interface's visibility must be public or none (package).

7. An abstract class inherits from Object and includes methods such as clone() and equals().

Q2. What are checked and unchecked exceptions?

Java defines two kinds of exceptions :

Checked exceptions : Exceptions that inherit from the Exception class are checked exceptions. Client code has to handle the checked exceptions thrown by  the API, either in a catch clause or by forwarding it outward with the throws clause. Examples - SQLException, IOxception.

Unchecked exceptions : RuntimeException also extends from Exception. However, all of the exceptions that inherit from RuntimeException get special treatment.

There is no requirement for the client code to deal with them, and hence they are called unchecked exceptions. Example Unchecked exceptions are NullPointerException, OutOfMemoryError, DivideByZeroException typically, programming errors.

Q3. What is a user defined exception?

User-defined exceptions may be implemented by

defining a class to respond to the exception and

embedding a throw statement in the try block where the exception can occur or declaring that the method throws the exception (to another method where it is handled).

The developer can define a new exception by deriving it from the Exception class as follows: 

public class MyException extends Exception {
/* class definition of constructors (but NOT the exception handling code) goes here*/

public MyException() {



public MyException( String errorMessage ) { super( errorMessage );



The throw statement is used to signal the occurance of the exception within a try block. Often, exceptions are instantiated in the same statement in which they are thrown using the syntax.

throw new MyException("I threw my own exception.") To handle the exception within the method where it is thrown, a catch statement that handles MyException, must follow the try block. If the developer does not want to handle the exception in the method itself, the method must pass the exception using the syntax:

public myMethodName() throws MyException

Q4. What is the difference between C++ & Java?

Well as Bjarne Stroustrup says ", despite the syntactic similarities, C++ and Java are very different languages. In many ways, Java seems closer to Smalltalk than to C++..". Here are few I discovered:

Java is multithreaded

Java has no pointers

Java has automatic memory management (garbage collection)

Java is platform independent (Stroustrup may differ by saying "Java is a platform"

Java has built-in support for comment documentation

Java has no operator overloading

Java doesn’t provide multiple inheritance

There are no destructors in Java

Q5. What are statements in JAVA ?

Statements are equivalent to sentences in natural languages. A statement forms a complete unit of execution. The following types of expressions can be made into a statement by terminating the expression with a semicolon

Assignment expressions

Any use of ++ or --

Method calls

Object creation expressions

These kinds of statements are called expression statements. In addition to these kinds of expression statements, there are two other kinds of statements. A declaration statement declares a variable. A control flow statement regulates the order in which statements get executed. The for loop and the if statement are both examples of control flow statements.

Q6.What is JAR file?

JavaARchive files are a big glob of Java classes, images, audio, etc., compressed to make one simple, smaller file to ease Applet downloading. Normally when a browser encounters an applet, it goes and downloads all the files, images, audio, used by the Applet separately. This can lead to slower downloads.

Q7.What is JNI?

JNI is an acronym of Java Native Interface. Using JNI we can call functions which are written in other languages from Java. Following are its advantages and disadvantages.


You want to use your existing library which was previously written in other language.

You want to call Windows API function.

For the sake of execution speed.

You want to call API function of some server product which is in c or c++ from java client.


You can’t say write once run anywhere.

Difficult to debug runtime error in native code.

Potential security risk.

You can’t call it from Applet.

Q8.What is serialization?

Quite simply, object serialization provides a program the ability to read or write a whole   object to and  from a raw byte stream. It allows Java objects and primitives to be encoded into  a byte stream suitable for streaming to some type of network or to a file-system, or more generally, to a transmission medium or storage facility. A seralizable object must implement the Serilizable interface. We use ObjectOutputStream to write this object to a stream and ObjectInputStream to read it from the stream.

Q9. Why there are some null interface in java ? What does it mean ? Give me some null interfaces in JAVA?

Null interfaces act as markers..they just tell the compiler that the objects of this class need to be treated differently..some marker interfaces are : Serializable, Remote, Cloneable

Q10. Is synchronised a modifier or indentifier? what is it ?

It's a modifier. 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.

Q11. What is singleton class?where is it used?

Singleton is a design pattern meant to provide one and only one instance of an object. Other objects can get a reference to this instance through a static method (class constructor is kept private). Why do we need one? Sometimes it is necessary, and often sufficient, to create a single instance of a given class. This has advantages in memory management, and for Java, in garbage collection. Moreover, restricting the number of instances may be necessary or  desirable for technological or business reasons--for example, we may only want a single instance of a pool of database connections.

Q12. What is a compilation unit?

The smallest unit of source code that can be compiled, i.e. a .java file

Q13. Is string a wrapper class?

String is a class, but not a wrapper class. Wrapper classes like (Integer) exist for each primitive type. They can be used to convert a primitive data value into an object, and vicaversa.

Q14. Why java is not a 100% oops?

Many people say this because Java uses primitive types such as int, char, double. But then all the rest are objects. Confusing question.

Q15. Why java does not have multiple inheritance?

The Java design team strove to make Java:

Simple, object oriented, and familiar

Robust and secure

Architecture neutral and portable

High performance

Interpreted, threaded, and dynamic

The reasons for omitting multiple inheritance from the Java language mostly stem from the "simple, object oriented, and familiar" goal. As a simple language, Java's creators wanted a language that most developers could grasp without extensive training. To that end, they worked to make the language as similar to C++ as possible (familiar) without carrying over C++'s unnecessary complexity (simple).

In the designers' opinion, multiple inheritance causes more problems and confusion than it solves. So they cut multiple inheritance from the language (just as they cut operator overloading). The designers' extensive C++ experience taught them that multiple inheritance just wasn't worth the headache.

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