Question

This is a bit of a lazyweb question but you get the rep so 🙂

I have a Java class that returns instances of itself to allow chaining (e.g. ClassObject.doStuff().doStuff())

For instance:

public class Chainer {     public Chainer doStuff()     {        /* Do stuff ... */        return this;     } } 

I would like to extend this class. Is there a way, perhaps using generics, to extend this class without having to overwrite each method signature?

E.g. not:

public class ChainerExtender extends Chainer {     public ChainerExtender doStuff()     {        super.doStuff();        return this;     } } 

I have tried:

public class Chainer {     public <A extends Chainer> A doStuff()     {        /* Do stuff ... */        return (A)this;     } }  public class ChainerExtender extends Chainer {     public <A extends Chainer> A doStuff()     {        /* Do stuff ... */        return super.doStuff();     } } 

But this didn’t work giving the error:

type parameters of <A>A cannot be determined;  no unique maximal instance exists for type variable A with upper bounds A,Chainer 

Am I forced to have class declarations like:

public class Chainer<T extends Chainer<T>> {} public class ChainerExtender extends Chainer<ChainerExtender> 

As per this question?

Answer 1

Have you tried the straight-forward

public class Chainer {     public Chainer doStuff()     {        /* Do stuff ... */        return this;     } }  public class ChainerExtender extends Chainer {     @Override     public ChainerExtender doStuff()     {        /* Do stuff ... */        super.doStuff();        return this;     } } 

With Java 5, you can declare overriding methods to have co-variant return types, which means just what this says: a subclass can have an overriding method signature with a more specific return type.