In this article, we will discuss the difference between Abstract Class and Interface in Java with examples. I have covered the abstract class and interface in separate tutorials of OOPs Concepts so I would recommend you to read them first, before going though the differences.
1. Abstract class in java
2. Interface in Java
| Abstract Class | Interface | |
| 1 | An abstract class can extend only one class or one abstract class at a time | An interface can extend any number of interfaces at a time |
| 2 | An abstract class can extend another concrete (regular) class or abstract class | An interface can only extend another interface |
| 3 | An abstract class can have both abstract and concrete methods | An interface can have only abstract methods |
| 4 | In abstract class keyword “abstract” is mandatory to declare a method as an abstract | In an interface keyword “abstract” is optional to declare a method as an abstract |
| 5 | An abstract class can have protected and public abstract methods | An interface can have only have public abstract methods |
| 6 | An abstract class can have static, final or static final variable with any access specifier | interface can only have public static final (constant) variable |
Each of the above mentioned points are explained with an example below:
Abstract class vs interface in Java
Difference No.1: Abstract class can extend only one class or one abstract class at a time
class Example1{
public void display1(){
System.out.println("display1 method");
}
}
abstract class Example2{
public void display2(){
System.out.println("display2 method");
}
}
abstract class Example3 extends Example1{
abstract void display3();
}
class Example4 extends Example3{
public void display3(){
System.out.println("display3 method");
}
}
class Demo{
public static void main(String args[]){
Example4 obj=new Example4();
obj.display3();
}
}
Output:
display3 method
Interface can extend any number of interfaces at a time
//first interface
interface Example1{
public void display1();
}
//second interface
interface Example2 {
public void display2();
}
//This interface is extending both the above interfaces
interface Example3 extends Example1,Example2{
}
class Example4 implements Example3{
public void display1(){
System.out.println("display2 method");
}
public void display2(){
System.out.println("display3 method");
}
}
class Demo{
public static void main(String args[]){
Example4 obj=new Example4();
obj.display1();
}
}
Output:
display2 method
Difference No.2: Abstract class can be extended(inherited) by a class or an abstract class
class Example1{
public void display1(){
System.out.println("display1 method");
}
}
abstract class Example2{
public void display2(){
System.out.println("display2 method");
}
}
abstract class Example3 extends Example2{
abstract void display3();
}
class Example4 extends Example3{
public void display2(){
System.out.println("Example4-display2 method");
}
public void display3(){
System.out.println("display3 method");
}
}
class Demo{
public static void main(String args[]){
Example4 obj=new Example4();
obj.display2();
}
}
Output:
Example4-display2 method
Interfaces can be extended only by interfaces. Classes has to implement them instead of extend
interface Example1{
public void display1();
}
interface Example2 extends Example1{
}
class Example3 implements Example2{
public void display1(){
System.out.println("display1 method");
}
}
class Demo{
public static void main(String args[]){
Example3 obj=new Example3();
obj.display1();
}
}
Output:
display1 method
Difference No.3: Abstract class can have both abstract and concrete methods
abstract class Example1 {
abstract void display1();
public void display2(){
System.out.println("display2 method");
}
}
class Example2 extends Example1{
public void display1(){
System.out.println("display1 method");
}
}
class Demo{
public static void main(String args[]){
Example2 obj=new Example2();
obj.display1();
}
}
Interface can only have abstract methods, they cannot have concrete methods
interface Example1{
public abstract void display1();
}
class Example2 implements Example1{
public void display1(){
System.out.println("display1 method");
}
}
class Demo{
public static void main(String args[]){
Example2 obj=new Example2();
obj.display1();
}
}
Output:
display1 method
Difference No.4: In abstract class, the keyword ‘abstract’ is mandatory to declare a method as an abstract
abstract class Example1{
public abstract void display1();
}
class Example2 extends Example1{
public void display1(){
System.out.println("display1 method");
}
public void display2(){
System.out.println("display2 method");
}
}
class Demo{
public static void main(String args[]){
Example2 obj=new Example2();
obj.display1();
}
}
In interfaces, the keyword ‘abstract’ is optional to declare a method as an abstract because all the methods are abstract by default
interface Example1{
public void display1();
}
class Example2 implements Example1{
public void display1(){
System.out.println("display1 method");
}
public void display2(){
System.out.println("display2 method");
}
}
class Demo{
public static void main(String args[]){
Example2 obj=new Example2();
obj.display1();
}
}
Difference No.5: Abstract class can have protected and public abstract methods
abstract class Example1{
protected abstract void display1();
public abstract void display2();
public abstract void display3();
}
class Example2 extends Example1{
public void display1(){
System.out.println("display1 method");
}
public void display2(){
System.out.println("display2 method");
}
public void display3(){
System.out.println("display3 method");
}
}
class Demo{
public static void main(String args[]){
Example2 obj=new Example2();
obj.display1();
}
}
Interface can have only public abstract methods
interface Example1{
void display1();
}
class Example2 implements Example1{
public void display1(){
System.out.println("display1 method");
}
public void display2(){
System.out.println("display2 method");
}
}
class Demo{
public static void main(String args[]){
Example2 obj=new Example2();
obj.display1();
}
}
Difference No.6: Abstract class can have static, final or static final variables with any access specifier
abstract class Example1{
private int numOne=10;
protected final int numTwo=20;
public static final int numThree=500;
public void display1(){
System.out.println("Num1="+numOne);
}
}
class Example2 extends Example1{
public void display2(){
System.out.println("Num2="+numTwo);
System.out.println("Num2="+numThree);
}
}
class Demo{
public static void main(String args[]){
Example2 obj=new Example2();
obj.display1();
obj.display2();
}
}
Interface can have only public static final (constant) variable
interface Example1{
int numOne=10;
}
class Example2 implements Example1{
public void display1(){
System.out.println("Num1="+numOne);
}
}
class Demo{
public static void main(String args[]){
Example2 obj=new Example2();
obj.display1();
}
}
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