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，并按上述声明将数据作为Point或BasicPoint返回，但是尽管OUT扩展了Point，编译器仍在抱怨以下错误：

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 ，而只能在OUT 。 您可以使您的Collector抽象：

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 ，这有点通用

class PointOperator extends Operator<Point> {

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

Answer 2

问题是，除非类型参数是反变的 ，否则您不能放东西。 像这样

public Collector<?>  operate(Collector<? super Point> collector) {
    collector.collect(new BasicPoint<Integer, Integer>(1,2));
    collector.collect(new Point());

    return collector;
 }