What is the difference between an interface and abstract class?

Question

What exactly is the difference between an interface and abstract class?

Answer 1

If you have some common methods that can be used by multiple classes go for abstract classes. Else if you want the classes to follow some definite blueprint go for interfaces.

Following examples demonstrate this.

Abstract class in Java:

abstract class animals
{
    // They all love to eat. So let's implement them for everybody
    void eat()
    {
        System.out.println("Eating...");
    }
    // The make different sounds. They will provide their own implementation.
    abstract void sound();
}

class dog extends animals
{
    void sound()
    {
        System.out.println("Woof Woof");
    }
}

class cat extends animals
{
    void sound()
    {
        System.out.println("Meoww");
    }
}

Following is an implementation of interface in Java:

interface Shape
{
    void display();
    double area();
}

class Rectangle implements Shape 
{
    int length, width;
    Rectangle(int length, int width)
    {
        this.length = length;
        this.width = width;
    }
    @Override
    public void display() 
    {
        System.out.println("****\n* *\n* *\n****"); 
    }
    @Override
    public double area() 
    {
        return (double)(length*width);
    }
} 

class Circle implements Shape 
{
    double pi = 3.14;
    int radius;
    Circle(int radius)
    {
        this.radius = radius;
    }
    @Override
    public void display() 
    {
        System.out.println("O"); // :P
    }
    @Override
    public double area() 
    { 
        return (double)((pi*radius*radius)/2);
    }
}

Some Important Key points in a nutshell:

1. The variables declared in Java interface are by default final. Abstract classes can have non-final variables.

2. The variables declared in Java interface are by default static. Abstract classes can have non-static variables.

3. Members of a Java interface are public by default. A Java abstract class can have the usual flavors of class members like private, protected, etc..