Returning an objects subclass with generics

问题:

With an abstract class I want to define a method that returns "this" for the subclasses:

public abstract class Foo {
    ...
    public <T extends Foo> T eat(String eatCake) {
        ...
        return this;
    }
}  

public class CakeEater extends Foo {}

I want to be able to do things like:

CakeEater phil = new CakeEater();
phil.eat("wacky cake").eat("chocolate cake").eat("banana bread");

Arguably banana bread would throw an IllegalArgumentException with the message "Not a cake!"

回答1:

public abstract class Foo<T extends Foo<T>>  // see ColinD's comment
{
    public T eat(String eatCake) 
    {
        return (T)this;
    }
}

public class CakeEater extends Foo<CakeEater> 
{
    public void f(){}
}

Edit

There is no problem to require subclass behave in a certain way that's beyond what static typing can check. We do that all the time - pages and pages of plain english to specify how you write a subclass.

The other proposed solution, with covariant return type, must do the same - asking subclass implementers, in plain english, to return the type of this. That requirement cannot be specified by static typing.

回答2:

The tasteful approach from the client point of view (which is usually the one you want to take) is to use covariant return types which was added to support generics, as Michael Barker points out.

The slightly less tasteful, but more tasteful that a cast is to add a getThis method:

protected abstract T getThis();

public <T extends Foo> T eat(String eatCake) {
    ...
    return getThis();
}

回答3:

I don't think you need generics Java 5 (and later) has covariant return types, e.g.:

public abstract class Foo {
    ...
    public Foo eat(String eatCake) {
        ...
        return this;
    }
}  

public class CakeEater extends Foo {

    public CakeEater eat(String eatCake) {
        return this;
    }
}

回答4:

An approach I've used before to achieve similar behaviour is to have the subclass pass its type into a constructor of the (generified) parent type. By way of disclaimer I was generating the subclasses on the fly and inheritence was a bit of a cheat to keep my code generation simple, as always my first instinct is to try to remove the extends relationship altogether.