java - 使用通配符调用通用方法无法编译

有人可以解释为什么以下代码片段无法编译吗？

public class Main {

    public static void main(String[] args) {

        Integer[] integers = {3, 5, 8};
        Set<Integer> s1 = new HashSet<Integer>(Arrays.asList(integers));

        Double[] doubles = {3.5, 5.5, 8.5};
        Set<Double> s2 = new HashSet<Double>(Arrays.asList(doubles));

        Set<Number> res1 = union(s1, s1);       // ->it does not compile
        Set<Number> res2 = union(s1, s2);       // ->it does not compile
    }

    static <E> Set<E> union(Set<? extends E> s1, Set<? extends E> s2)
    {
        Set<E> result = new HashSet<E>(s1);
        result.addAll(s2);
        return result;
    }

}

我已阅读错误消息:

Type mismatch: cannot convert from Set<Integer> to Set<Number>  Main.java   /TestingGenerics/src/com/al/testinggenerics line 17 Java Problem

Type mismatch: cannot convert from Set<Number&Comparable<?>> to Set<Number> Main.java   /TestingGenerics/src/com/al/testinggenerics line 18 Java Problem

只要Integer和Double从Number扩展，是什么导致问题？

最佳答案

如user3580294(+1)和my comment中所述:原因是Java 7类型推断不够智能。

但是，您可以通过省略通配符，而仍然保持所需的灵活性，从而胜过类型推断:

import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

public class TargetTypeInferenceTest
{
    public static void main(String[] args)
    {

        Integer[] integers = { 3, 5, 8 };
        Set<Integer> s1 = new HashSet<Integer>(Arrays.asList(integers));

        Double[] doubles = { 3.5, 5.5, 8.5 };
        Set<Double> s2 = new HashSet<Double>(Arrays.asList(doubles));

        Set<Number> res1 = union(s1, s1); // ->it does compile ;-)
        Set<Number> res2 = union(s1, s2); // ->it does compile ;-)
    }

    static <E, F extends E, G extends E> Set<E> union(Set<F> s1, Set<G> s2)
    {
        Set<E> result = new HashSet<E>(s1);
        result.addAll(s2);
        return result;
    }
}