Practical side of the ability to define a class within an interface in Java?

Question

What would be the practical side of the ability to define a class within an interface in Java:

interface IFoo
{
    class Bar
    {
        void foobar (         


        

Answer 1

With a static class inside an interface you have the possibility to shorten a common programming fragment: Checking if an object is an instance of an interface, and if so calling a method of this interface. Look at this example:

public interface Printable {
    void print();

    public static class Caller {
        public static void print(Object mightBePrintable) {
            if (mightBePrintable instanceof Printable) {
                ((Printable) mightBePrintable).print();
            }
        }
    }
}

Now instead of doing this:

void genericPrintMethod(Object obj) {
    if (obj instanceof Printable) {
        ((Printable) obj).print();
    }
}

You can write:

void genericPrintMethod(Object obj) {
   Printable.Caller.print(obj);
}