Below are the difference between Abstract class and an Interface:
| Point of Distinction | Abstract Class | Interface |
|---|---|---|
| Multiple Inheritance | No. A class can extend only one abstract class. A child class can only extend a single class (abstract or concrete). | Yes. An interface can extend or a class can implement multiple other interfaces. |
| Types of Methods | An abstract class can have abstract and non-abstract method (default and static methods). | All Methods of an interface are abstract. In another words Abstract classes can have method stubs (methods without a body) and defined methods, whereas interfaces can only have methods stubs. |
| Types of Variables | Abstract class can have instance, final, non-final, static and non-static variables. | Interface has only static and final variables. |
| Visibility | An abstract class can have any visibility: public, private or protected. | An Interface visibility must be public or none. |
| Constructor | An abstract class can contain constructors. | An Interface cannot contain constructors. |
| Speed | Abstract class is faster than Interface. | Interface are slower as it requires extra indirection to find corresponding method in the actual class. |
| Static Members | Only complete members of an abstract class can be static. | Member of an interface can not be Static. |
| Keyword used to extend | For abstract we use extends keyword. | For Interface we use Implements keyword. |
| Keyword used to declare | Abstract Keyword. | Interface Keyword. |
| Implementation | Abstract class can provide the implementation of interface. | Interface can't provide the implementation of abstract class. |
| Code Visibility | An abstract class can provide complete, default code and/or just the details that have to be overridden | An interface cannot provide any code at all, just the signature |
| Add new method | 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 |
| Extend/implement | A class can extend only one abstract class. | A class can implement several interfaces. |
| Example | Check below code | Check below code |
Example
package com.javademo;
import java.io.*;
interface Shape
{
// Abstract methods inside Interface
void draw();
double area();
}
class Rectangle implements Shape
{
int length, width;
// Abstract Method can have constructor
Rectangle(int length, int width)
{
this.length = length;
this.width = width;
}
public void draw()
{
System.out.println("Rectangle has been drawn ");
}
public double area()
{
return (double)(length * width);
}
}
class Circle implements Shape
{
double pi = 3.14;
int radius;
//constructor
Circle(int radius)
{
this.radius = radius;
}
public void draw()
{
System.out.println("Circle has been drawn");
}
public double area()
{
return (double)((pi*radius*radius)/2);
}
}
package com.javademo;
class CI
{
public static void main (String[] args)
{
// Creating the Object of Rectangle class using shape interface reference.
Shape rect = new Rectangle(10,20);
System.out.println("Area of Rectangle: " + rect.area());
// Creating the Objects of circle class using Shape interface reference.
Shape circle = new Circle(5);
System.out.println("Area of Circle: " + circle.area());
}
}
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