Proper Use of "<T extends SuperClass>"

Question

I am not familiar with Generics. Is this the correct use of " <T extends SuperClass>"? And do you agree that codes after using generics are better?

Before using Generics
=================================================

public abstract class SuperSample {

    public void getSomething(boolean isProcessA) {
        doProcessX();
        if(isProcessA){
            doProcessY(new SubASample());
        }else{
            doProcessY(new SubBSample());
        }
    }

    protected abstract void doProcessX();

    protected void doProcessY(SubASample subASample) {
        // Nothing to do
    }

    protected void doProcessY(SubBSample subBSample) {
        // Nothing to do
    }

}

public class SubASample extends SuperSample {

    @Override
    protected void doProcessX() {
        System.out.println("doProcessX in SubASample");
    }

    @Override
    protected void doProcessY(SubASample subASample) {
        System.out.println("doProcessY in SubASample");
    }

}

public class Sample {

    public static void main(String[] args) {
        SubASample subASample = new SubASample();
        subASample.getSomething(true);
    }

}

After using Generics
=================================================

public abstract class SuperSample {

    public void getSomething(boolean isProcessA) {
        doProcessX();
        if(isProcessA){
            doProcessY(new SubASample());
        }else{
            doProcessY(new SubBSample());
        }
    }

    protected abstract void doProcessX();

    protected abstract <T extends SuperSample> void doProcessY(T subSample);

}

public class SubASample extends SuperSample {

    @Override
    protected void doProcessX() {
        System.out.println("doProcessX in SubASample");
    }

    @Override
    protected <T extends SuperSample> void doProcessY(T subSample) {
        System.out.println("doProcessY in SubASample");
    }

}

public class Sample {

    public static void main(String[] args) {
        SubASample subASample = new SubASample();
        subASample.getSomething(true);
    }

}

Answer 1

If you want to do what I think you want to do, I don’t think this is the right way (*). If you want a method that processes its own type to be implemented in each subclass, you can use the CRTP trick :

abstract class Super<S extends Super<S>> {
   abstract void process(S s);
}

class SubA extends Super<SubA> {
   void process(SubA s){ /* do something */ }
}

class SubB extends Super<SubB> {
   void process(SubB s){ /* do something */ }
}

Note that this template provides a common signature for subclasses, for example. class SubA extends Super<SubB>will not compile.

Java itself uses this trick in java.lang.Enum, by the way.

(*) , , , .

Answer 2

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