根据Java中的子类Type定义泛型类父类方法

Question

Is it possible to dynamically identify T as a return type depending on subclass Type? I want something like the following:

public class Parent {
        public <T extends Parent> T foo() {
                return (T)this;
        }
}

public class Child extends Parent {
        public void childMethod() {
                System.out.println("childMethod called");
        }
}

And then to call:

Child child = new Child();
child.foo().childMethod();

Without defining the type like so:

Child child = new Child();
child.foo().<Child>childMethod(); // compiles fine

Thanks in advance!

Answer 1

You want this:

public class Parent<T extends Parent<T>> {
    public T foo() {
        return (T)this;
    }
}

public class Child extends Parent<Child> {
    public void childMethod() {
        System.out.println("childMethod called");
    }
}

Child child = new Child();
child.foo().childMethod(); // compiles

Answer 2

It is impossible in the Java type system for Parent to refer to the exact class of this . However, it can have a type parameter (say T ) that subclasses can specify, as either themselves, or some other type (whatever they want), and use an abstract method to delegate the task of obtaining an instance of a that type T to the subclass.

public abstract class Parent<T> {
    // the implementer is responsible for how to get an instance of T
    public abstract T getT();
    // in this case, foo() is kind of redundant
    public T foo() {
        return getT();
    }
}

public class Child extends Parent<Child> {
    public Child getT() {
        return this;
    }
    public void childMethod() {
        System.out.println("childMethod called");
    }
}

Child child = new Child();
child.foo().childMethod(); // compiles