有人可以解释一下为什么以下代码片段无法编译吗?
只要
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;
}
}
我已经阅读了错误信息:
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
继承自Number
,这个问题是什么原因引起的?