How interfaces are used for code reusability

[Shashank Prashar]

Hi all,

I am a newbie to java programming.
In an interview,i was asked about how the interfaces are used for code reusablity and we are also achieving multiple inheritence through interfaces.
we also do not write any code in interfaces methods..?


Warm Regards..

[shreyas Kulkarni]

If you implement any interface for your class like
class x implements yourInterface
then you have to write every abstract method you have in that interface. So you can force to override all those methods by using interface.

We can achieve multiple inheritance means, we can implement as many interfaces as we want but in java, we can extend only 1 class.. So sometimes its advisable to use interface instead of abstract class dependng on your design.

Yes, we cant write any code in methods of interface. Just declaration. The concrete class that implements interface has to write all the methods.
Hope you find it useful.

[Shashank Prashar]

Thanks Shreyas..

I know that we can achieve multiple inheritence through interfaces by implementing various interfaces in our class.
But basically my question was..

Code reusablity means..we have defined our code some where and then we are using it..

but the point is that we are not writing any code in the methods of interfaces..so where
we are achieving code resuability in that...and also we have to provide the body of that methods also..

[Tina Smith]

It's not the classes that implement the interface that are getting the code reusability. It's the classes that are using the interface.

For example, we have interface Shape, and implementations Triangle and Square.

interface Shape
{
    public void draw();
}

class Triangle implements Shape
{
     @Override
    public void draw()
    {
        System.out.println("Drawing triangle");
    }
}

class Square implements Shape
{
     @Override
    public void draw()
    {
        System.out.println("Drawing square");
    }
}

So the implementation code is different, but because they both implement Shape I'm allowed to do this

public static void main(String[] args)
{
    // Initialization of classes belonging to an interface.
    List<Shape> shapes = new ArrayList<Shape>();
    shapes.add( new Square() );
    shapes.add( new Triangle() );
    // Add other shapes of an infinite number of types.

    for( Shape shape : shapes )
    {
        shape.draw();
    }

    
}

instead of doing this

public static void main(String[] args)
{
    new Square().draw();
    new Triangle().draw();
    // ...all other types of shapes are handled individually....   
}

The point is that I don't have to know what kind of Shape I'm working with to use the Shape type in a general way; code reuse is all in the implementation.

[Shashank Prashar]

Thanks Tina..

for such a lovely example..