Java通用上限通配符错误

Question

我的域模型中有以下实体:

class Point {
    public String details;

    public Point() {
        details = "empty";
    }

    public Point(String details) {
        this.details = details;
    }
}

class BasicPoint<X, Y> extends Point {
    public X x;
    public Y y;

    public BasicPoint() {}

    public BasicPoint(X x, Y y) {
        this.x = x;
        this.y = y;
    }
}

class Collector<T> {
    T elem;

    public void collect(T elem) {
        this.elem = elem;
    }
}

我想对数据应用操作并将该数据作为 Point 或 BasicPoint 返回，如上面声明的那样，但编译器提示以下错误，尽管 OUT 扩展了 Point:

class Operator<OUT extends Point> {
    public Collector<OUT> operate(Collector<OUT> collector) {

        // compile error, collect(OUT) cannot be applied to BasicPoint<Integer, Integer>
        collector.collect(new BasicPoint<Integer, Integer>(1,2));

        return collector;
    }
}

主要方法应如下所示:

Collector<BasicPoint<Integer, Integer>> c = new Collector<>();
c = new Operator<BasicPoint<Integer, Integer>>().operate(c);
System.out.println(c.elem.getClass() == new BasicPoint<Integer, Integer>().getClass());

Answer 1

您的Operator是通用的，它不能在BasicPoint上应用collect，只能在OUT上应用。您可以使您的Collector抽象化:

abstract class Operator<OUT extends Point> {
    abstract Collector <OUT> operate(Collector<OUT> collector);
}

然后 BasicPointOperator 将包含您想要的实际实现:

class BasicPointOperator extends Operator<BasicPoint<Integer, Integer>> {

    @Override
    Collector<BasicPoint<Integer, Integer>> operate(Collector<BasicPoint<Integer, Integer>> collector) {
        collector.collect(new BasicPoint<Integer, Integer>(1,2));
        return collector;
    }
}

或PointOperator，更通用一点

class PointOperator extends Operator<Point> {

    @Override
    Collector<Point> operate(Collector<Point> collector) {
        collector.collect(new BasicPoint<Integer, Integer>(1,2));
        return collector;
    }
}