java - 类型参数的数据类型在协变和逆变中是如何确定的？

Question

我正在阅读 Maurice Naftalin 和 Philip Wadler 合着的 Java Generics and Collections 一书，在前两章中，我的脑袋被疑惑搞得一团糟。我无法找出答案。

通话中:

 public static <T> void copy(List<? super T> dst, List<? extends T> src) {
 for (int i = 0; i < src.size(); i++) {
 dst.set(i, src.get(i));
 }
}


 List<Object> objs = Arrays.<Object>asList(2, 3.14, "four");
 List<Integer> ints = Arrays.asList(5, 6);
 Collections.copy(objs, ints);
 assert objs.toString().equals("[5, 6, four]");

在调用函数“复制”期间:
第一个参数:?= 对象
第二个参数:?=Integer

但是 T 的数据类型是什么？ jvm 是如何根据删除实现决定的？

书上说:在Collections.copy(obj,ints)这一行，取类型参数T为Number。调用是允许的，因为 objs 的类型是 List

Answer 1

http://docs.oracle.com/javase/specs/jls/se7/jls7.pdf

从“简单”示例中，JLS 表示它会选择满足其生成的所有约束的最具体的类型。

15.12.2.7 根据实际参数推断类型参数

A supertype constraint T :> X implies that the solution is one of supertypes of X. Given several such constraints on T, we can intersect the sets of supertypes implied by each of the constraints, since the type parameter must be a member of all of them. We can then choose the most specific type that is in the intersection

Copy.java:11: incompatible types
found   : java.lang.Integer[]
required: java.lang.String[]
    String[] res = copy(Arrays.<Object>asList(2, 3.14, "four"), Arrays.asList(5, 6));
                       ^
1 error
➜  /tmp  cat Copy.java 
import java.util.*;
public class Copy {
public static <T> T[] copy(List<? super T> dst, List<? extends T> src) {
   for (int i = 0; i < src.size(); i++) {
      dst.set(i, src.get(i));
   }

   return null;
  }
  public static void main(String[] args) {
    String[] res = copy(Arrays.<Object>asList(2, 3.14, "four"), Arrays.asList(5, 6));
  }

}