可以在超类中声明一个返回类型为subclass的对象的方法吗？

Question

I have a class called Calculator, which is extended by many other classes. It has a method called intern() which reads as follows:

   public DailyValueCalculator intern() {
    if (TimeSeriesIterator.containsMatchingCalculator(this)) {
        TimeSeriesIterator.removeCalculator(this);
        return TimeSeriesIterator.getMatchingCalculator(this);
    } else {
        return this;
    }
}

If I have, say, a BetaCalculator called bc, I'd like to be able to intern it by saying

bc = bc.intern();

Currently though I have to say

bc = (BetaCalculator) bc.intern();

Is there any way to make it so that the intern() method would always have a return type of the type on which it was called? And if there isn't a way to do this, is there a reason why it's not possible?

Possible solutions I'm aware of though am unsatisfied with:

1. Rewriting the intern() method in every calculator I write

2. Passing the intern() method a Class and having it return a T (eg

   <T extends DailyValueCalculator> T intern(Class<T> returnType){...}

3. Giving DailyValueCalculator a Type <T> (ie making it DailyValueCalculator<T>) that all subclasses would be required to specify where T is equal to the type of the subclass and then having intern() return a T.

Answer 1

You can do it if you type your super-class with itself. Here is an example:

public abstract class SuperClass<S extends SuperClass<S>> {

    public S intern() {
        return (S) this;
    }

    public static class SubClass extends SuperClass<SubClass> {

        private void methodOnlyInSubClass() {
            System.out.println("subclass method");
        }

        public void myMethod() {
            SubClass sub = intern();
            sub.methodOnlyInSubClass();
        }

    }

}

Answer 2

No there's no way to do this. Think about it: I know you know that the intern method is just going to return back the same class, but how could the compiler?