Try it:

import java.util.List; public class Test { public static void swap(List list, int i, int j) { list.set(i, list.set(j, list.get(i))); } }

Compilation error:

.\Test.java:7: 错误: 无法将接口 List中的方法 set应用到给定类型; list.set(i, list.set(j, list.get(i))); ^ 需要: int,CAP#1 找到: int,CAP#2 原因: 参数不匹配; Object无法转换为CAP#1 其中, E是类型变量: E扩展已在接口 List中声明的Object 其中, CAP#1,CAP#2是新类型变量: CAP#1从?的捕获扩展Object CAP#2从?的捕获扩展Object 1 个错误

Becauselist的类型是List, we don’t know the specific type, so we can only take out an Object from the list, but cannot insert it into the list.

So convert it to List.

or replace it withList:

public static  void swap(List list, int i, int j) {...}