Question

I like to know can we define an interface within an interface.
like

interface abc {
    void show();
    public interface xyz {
        void read();
    }
}

This was question asked in interview. Any practical use of this.

Answer 1

Yes, we can do it. The definition of the nested interface in java is as follows:

A nested interface is any interface whose declaration occurs within the body of another class or interface. A top-level interface is an interface that is not a nested interface.

Refer this for more.

Further …

One reason could be that the outer interface has a method that takes a callback implementation as an argument. The nested interface is, in that case, the contract that the callback method must implement. I don’t see a reason to declare that callback interface at the top level.

public interface Processor {
   void execute(NotificationListener listener);

    interface NotificationListener {
        void processingCompleted();
    }  
}

Another good reading at sun site about this topic is here

In particular, notice that when you implement an interface, you are
not required to implement any
interfaces nested within.