Both are used to achieve abstraction in Java and can't be instantiated.
Abstraction :
It is the process of hiding the data and showing only essential information to the user.
Abstract Class :
abstract keyword is used to declare Abstract Class.
It can have abstract as well as non-abstract methods.
It doesn't support multiple inheritance.
It can extend another Java class and can implement interfaces just like any other class.
It can have non-final variables, final variables, static and non-static variables.
It can also provide implementation of the interface.
It can have access modifiers like private, public, protected, etc.
extends keyword is used to extend an abstract class.
//example of Abstract Class
public abstract class Game {
protected abstract void start();
protected abstract void stop();
}//Hockey is a class extending Game Abstract class so it needs to override //all the abstract methods of Game class.
public class Hockey extends Game {
@Override
protected abstract void start(){
//code implementation
}
@Override
protected abstract void stop(){
//code implementation
}
}//as abstract class can't be instantiated.
public static void main(String... args){
Game hockey= new Hockey();
game.start();
game.stop();
}Interface :
interface keyword is used to declare an Interface.
It can have abstract as well as default and static methods from Java 8.
It supports multiple inheritance.
It can extend another Java interfaces only.
It can have final variables and static variables.
It can't provide implementation of the abstract class.
It has public access modifier bydefault.
implements keyword is used to extend an interface by a class and extends is used when one interface extends another interface.
If we try to implement an interface with another interface, it will throw a compile-time error in Java because if interface is implemented then it's methods must be defined but interface never has the definition of any method.
//Interface extends interface
interface SecondInterface extends FirstInterface{
void c();
void d();
}
//example of an interface
interface FirstInterface{
void a();//bydefault, public and abstract
void b();
default void start() {
System.out.println("Default is starting...");
}
static void display() {
System.out.println("Displaying...");
}
} //Class implements interface
class Hockey implements FirstInterface{
@Override
public void a(){
//code implementation
}
@Override
public void b(){
//code implementation
}
}
public static void main(String... args){
FirstInterface hockey = new Hockey();//as interface can't be instantiated
hockey.a();
hockey.b();
hockey.start();
}Both of them should be used as per the requirement as both are very essential and play a very important role when you will proceed to learn design patterns.
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