Top Java interview Questions
that will get you ready for your next Interview
1) what are static blocks and static initalizers in Java ?
Static blocks or static initializers are used to initalize static fields in java. we declare static blocks when we want to intialize static fields in our class. Static blocks gets executed exactly once when the class is loaded. Static blocks are executed even before the constructors are executed.
2) How to call one constructor from the other constructor ?
With in the same class if we want to call one constructor from other we use this() method. Based on the number of parameters we pass appropriate this() method is called.
Restrictions for using this method :
1) this must be the first statement in the constructor
2)we cannot use two this() methods in the constructorz
3) What is method overriding in java ?
If we have methods with same signature (same name, same signature, same return type) in super class and subclass then we say subclass method is overridden by superclass. When to use overriding in java If we want same method with different behaviour in superclass and subclass then we go for overriding. When we call overridden method with subclass reference subclass method is called hiding the superclass method.
4) What is super keyword in java ?
Variables and methods of super class can be overridden in subclass . In case of overriding , a subclass object call its own variables and methods. Subclass cannot access the variables and methods of superclass because the overridden variables or methods hides the methods and variables of super class.
But still java provides a way to access super class members even if its members are overridden. Super is used to access superclass variables, methods, constructors.
Super can be used in two forms :
1) First form is for calling super class constructor.
2) Second one is to call super class variables,methods.
Super if present must be the first statement.
5) Difference between method overloading and method overriding in java ?
| Method Overloading | Method Overriding |
|---|---|
| 1) Method Overloading occurs within the same class | 1) Method Overriding occurs between two classes, superclass and subclass |
| 2) Since it involves only one class, inheritance is not involved. | 2) Since method overriding occurs between superclass and subclass, inheritance is involved. |
| 3) In overloading, the return type need not be the same. | 3) In overriding, the return type must be the same. |
| 4) Parameters must be different when we do overloading. | 4) Parameters must be the same. |
| 5) Static polymorphism can be achieved using method overloading. | 5) Dynamic polymorphism can be achieved using method overriding. |
| 6) In overloading, one method can't hide the another. | 6) In overriding, subclass method hides that of the superclass method. |
| Interface | Abstract Class |
|---|---|
| 1) Interface contains only abstract methods | 1) Abstract class can contain abstract methods, concrete methods, or both |
| 2) Access Specifiers for methods in interface must be public | 2) Except private, we can have any access specifier for methods in an abstract class |
| 3) Variables defined must be public, static, final | 3) Except private, variables can have any access specifiers |
| 4) Multiple Inheritance in Java is implemented using interface | 4) We cannot achieve multiple inheritance using an abstract class |
| 5) To implement an interface, we use the 'implements' keyword | 5) To implement an interface, we use the 'implements' keyword |
7) Why java is platform independent?
The most unique feature of java is platform independent. In any programming language soruce code is compiled in to executable code . This cannot be run across all platforms. When javac compiles a java program it generates an executable file called .class file .class file contains byte codes. Byte codes are interpreted only by JVM’s . Since these JVM’s are made available across all platforms by Sun Microsystems, we can execute this byte code in any platform. Byte code generated in windows environment can also be executed in linux environment. This makes java platform independent.
8) What is method overloading in java ?
A class having two or more methods with same name but with different arguments then we say that those methods are overloaded. Static polymorphism is achieved in java using method overloading. Method overloading is used when we want the methods to perform similar tasks but with different inputs or values. When an overloaded method is invoked java first checks the method name, and the number of
arguments ,type of arguments; based on this compiler executes this method.
Compiler decides which method to call at compile time. By using overloading static polymorphism or static binding can be achieved in java.
Note : Return type is not part of method signature. we may have methods with different return types but return type alone is not sufficient to call a method in java.
9) What is difference between c++ and Java ?
| Java | C++ |
|---|---|
| 1) Java is platform independent | 1) C++ is platform dependent. |
| 2) There are no pointers in Java | 2) There are pointers in C++. |
| 3) There is no operator overloading in Java | 3) C++ has operator overloading. |
| 4) There is garbage collection in Java | 4) There is no garbage collection |
| 5) Supports multithreading | 5) Doesn't support multithreading |
| 6) There are no templates in Java | 6) There are templates in C++ |
| 7) There are no global variables in Java | 7) There are global variables in C++ |
10) What is JIT compiler ?
JIT compiler stands for Just in time compiler. JIT compiler compiles byte code in to executable code. JIT a part of JVM .JIT cannot convert complete java program in to executable code it converts as and when it is needed during execution.
11) What is bytecode in java ?
When a javac compiler compiler compiles a class it generates .class file. This .class file contains set of instructions called byte code. Byte code is a machine independent language and contains set of instructions which are to be executed only by JVM. JVM can understand this byte codes.
12) Difference between this() and super() in java ?
this() is used to access one constructor from another with in the same class while super() is used to access superclass constructor. Either this() or super() exists it must be the first statement in the constructor.
13) What is a class ?
Classes are fundamental or basic unit in Object Oriented Programming .A class is kind of blueprint or template for objects. Class defines variables, methods. A class tells what type of objects we are creating.
For example take Department class tells us we can create department type objects. We can create any number of department objects. All programming constructs in java reside in class. When JVM starts running it first looks for the class when we compile. Every Java application must have atleast one class and one main method.
Class starts with class keyword. A class definition must be saved in class file that has same as class name.
File name must end with .java extension.
Example:
public class FirstClass {public static void main(String[] args)
{System.out.println(“My First class”); } } 14) What is an object ?
Add a Comment