拥有有什么用？爪哇文[副本]

Question

我想知道的用途？在java泛型中。通过研究占位符T和通配符？，我想知道？，浏览了几个网站/网页和书籍，但没有理解它。所以我创建了下面的类来研究这些差异。

import java.util.List;

public class Generics2 {

    public <T> void method1(List<T> list){
        System.out.println(list);
    }

    public <T extends Number> void method2(List<T> list){
        System.out.println(list);
    }

    /*public <T super Integer> void method3(List<T> list){

    }*///super does not work.

    public void method4(List<?> list){
        System.out.println(list);
    }

    public void method5(List<? extends Number> list){
        System.out.println(list);
    }

    public void method6(List<? super Integer> list){
        System.out.println(list);
    }

    public <T> void copy1(List<T> list1, List<T> list2){
        //copy elements from list1 to list2

    }//It does not span well with copy of one type of elements from list1 to other type elements in list2, where the list elements 
    //between the two are not same but are related through inheritance.

    public <T1,T2> void copy2(List<T1> list1,List<T2> list2){
        //copy elements from list1 to list2, right now we do not bother about the exceptions or errors that might generate.
    }//Our intention here is not to copy elements with relation between T1 and T2. We intend to explore the differences on T and ?

    public void copy3(List<?> list1,List<?> list2){
        //copy elements from list1 to list2, right now we do not bother about the exceptions or errors that might generate.
    }//Our intention here is not to copy elements with relation between T1 and T2. We intend to explore the differences on T

    public <T1 extends Object, T2 extends Object> void copy4(List<T1> list1, List<T2> list2){
        //copy elements from list1 to list2
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub

    }

}

null

Answer 1

？表示：未指定类型。含义：如果代码中没有其他原因要求您具有“命名”类型参数；您可以/应该使用？来表达这一点。

实际上，我以前问过这个问题，所以您可能会发现这里的答案很有帮助：Java泛型：通配符<？>与类型参数 ？