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,并将数据返回为Point或BasicPoint,如上所述,但编译器抱怨以下错误,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;
}
}
主要方法应该看起来像:
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());
您的Operator
是通用的,它不能在BasicPoint
上应用collect
,只能在OUT
上应用。你可以让你的Collector
摘要:
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;
}
}
或者PointOperator
,这是一种更通用的
class PointOperator extends Operator<Point> {
@Override
Collector<Point> operate(Collector<Point> collector) {
collector.collect(new BasicPoint<Integer, Integer>(1,2));
return collector;
}
}
问题是,除非类型参数是反变量的,否则不能将放入中。类似的东西
public Collector<?> operate(Collector<? super Point> collector) {
collector.collect(new BasicPoint<Integer, Integer>(1,2));
collector.collect(new Point());
return collector;
}