java - 用正确的返回类型覆盖通用的 getter

Question

我在尝试制作一个小型系统时遇到了 Java 泛型的问题。

public void bar(){
    final Shape shape = new Shape();
    final Painter<Shape> shapePainter = shape.getPainter();

    final Circle circle = new Circle();
    final Painter<Circle> circlePainter = circle.getPainter();
}

class Shape {

    public Painter<? extends Shape> getPainter(){
        return new Painter<>(this);
    }

}

class Circle extends Shape {

    @Override
    public Painter<Circle> getPainter(){
        return new CirclePainter(this);
    }

}

class Painter<E extends Shape> {

    public Painter(final E element){
        // ...
    }

    public void paint(final E shape){
        // ...
    }
    
}

class CirclePainter extends Painter<Circle> {

    public CirclePainter(final Circle element){
        super(element);
    }

    @Override
    public void paint(final Circle shape){
        // ...
    }
}

编译失败，第 3 行(final Painter<Shape> shapePainter = shape.getPainter();)出现错误:

Incompatible types:

Required: Bar.Painter < org.example.Bar.Shape>

Found: Bar.Painter < capture< ? extends org.example.Bar.Shape>>

这可以通过将故障线路更改为:

final Painter<?> shapePainter = shape.getPainter();

但是，后续调用如下:

shapePainter.paint(shape);

会抛出另一个异常:

paint (capture< ? extends org.example.Bar.Shape>) in Painter cannot be applied

to (org.example.Bar.Shape)

---

我觉得我遗漏了 Java 泛型的一些简单部分，它们可以帮助解决这个问题。我试过摆弄 Shape#getPainter 的返回类型, 但这通常会在 Circle#getPainter 中留下编译器错误或在 bar似乎没有任何可能的解决方案。

如果使用泛型无法做到这一点，还有哪些其他解决方案可用？

Answer 1

您需要使用 CRTP 使整个类通用:

class Shape<T extends Shape<T> {

    public Painter<T> getPainter(){
        return new Painter<>(this);
    }

}
class Painter<E extends Shape<E>> { ... }