Call Method from default method in java interface

Question

This may be silly question. but i want to know there is some possibility to do this.

Suppose I have interface like

public interface GroupIdentifier {

    Integer getRevision();
}

And I need another method named as getNextRevision. So what i can do is, implement default method inside same interface and return next number.

EX :

public interface GroupIdentifier {

    //OUTER GET REVISION
    Integer getRevision();

    default GroupIdentifier getNextRevisionIdentifier() {
         return new GroupIdentifier() {
         //INNER GET REVISION
         public Integer getRevision() {
               //Here I want to return OUTER GET REVISION + 1
               return null;
         }
         };
    }
}

Is there some possibility to this.

Answer 1

I'm not sure what's the purpose of getNextRevisionIdentifier() returning an instance of GroupIdentifier .

If you want getNextRevisionIdentifier() to return the next identifier, let it return an Integer :

public interface GroupIdentifier {

    Integer getRevision();

    default Integer getNextRevisionIdentifier() {
        return getRevision () + 1;
    }

}

Answer 2

Return a new instance of GroupIdentifier for which you implement getRevision accordingly:

public interface GroupIdentifier {

    //OUTER GET REVISION
    Integer getRevision();

    default GroupIdentifier getNextRevisionIdentifier() {
        //INNER GET REVISION
        Integer outer = getRevision();
        return new GroupIdentifier() {
            @Override
            public Integer getRevision() {
                return outer + 1;
            }
        }; // can be simplified to return () -> outer + 1;
    }
}

This can be run, eg via

public static void main(String[] args) {
    GroupIdentifier groupIdentifier = new GroupIdentifier() {
        @Override
        public Integer getRevision() {
            return 1;
        }
    };
    System.out.println(groupIdentifier.getNextRevisionIdentifier().getRevision());
}

outputting

2

Answer 3

I believe what you wanted to write is this:

public interface GroupIdentifier {
    Integer getRevision();

    default GroupIdentifier getNextRevisionIdentifier() {
        return new GroupIdentifier() {
            public Integer getRevision() {
                return GroupIdentifier.this.getRevision() + 1;
            }
        };
    }
}

You just didn't find a way to refer to the enclosing instance's getRevision() . BTW if your interface is as you've posted it, with just a single abstract method, then you can implement it much more cleanly:

public interface GroupIdentifier {
    Integer getRevision();

    default GroupIdentifier getNextRevisionIdentifier() {
        return () -> getRevision() + 1;
    }
}

Answer 4

I have fixed my solution as below.

public interface GroupIdentifier {
    Integer getRevision();

    default GroupIdentifier getNextRevisionIdentifier(GroupIdentifier identifier) {
        return new GroupIdentifier() {
            @Override
            public Integer getRevision() {
                return identifier.getRevision() + 1;
            }
        };
    }
}