Interface inside Class

Question

Q1. Can I have an interface inside a class in java?

Q2. Can I have an class inside an interface?

If yes, then in which situations should such classes/interfaces used.

Answer 1

Q1. Yes Q2. Yes.

• Inside your class you may need multiple implementations of an interface, which is only relevant to this particular class. In that case make it an inner interface, rather than a public / package-private one

• In your interface you can define some data holder classes that are to be used by implementations and clients.

One example of the latter:

public interface EmailService {

    void send(EmailDetails details);

    class EmailDetails {
        private String from;
        private String to;
        private String messageTemplate;
        // etc...
    }
}

Answer 2

I have faced providing common complex operations for all classes implementing an interface, that use obviously the operations of the interface.

Until Java 8 is not out...

See http://datumedge.blogspot.hu/2012/06/java-8-lambdas.html (Default Methods)

A workaround for this is:

public interface I
{
    public Class U{
        public static void complexFunction(I i, String s){
             i.f();
             i.g(s) 
        }
    }
}

Then you can easily call common functionality (after importing I.U)

U.complexFunction(i,"my params...");

May further refined, with some more typical coding:

public interface I
{
    public Class U{
        I me;
        U(I me){
            this.me = me;
        }

        public void complexFunction(String s){
             me.f();
             me.g(s) 
        }
    }
    U getUtilities();
}

class implementationOfI implements I{
    U u=new U(this);
    U getUtilities(){ return u; }
}

calling then

I i = new implementationOfI();
i.getUtilities().complexFunction(s);

A further spicy tricks

1. is providing U as abstract, giving the chance for local implementation of certain functions for U...
2. reimplementing U with a local class and local constructor using I.this instead of forcing parameter pass...
3. using a static U... however then each operation must get I i as parameter...
4. using enum instead of class, for static functions only, not requiring any further objects (equivalent to the first method)

The reason for this is to put the operations into a single module instead of having utilities modules hanging around, allowing extendable the worst, duplicated implementation.