Java Generic类型：List<？之间的差异extends Number>和列表< T extends Number> [英] Java Generic type : difference between List <? extends Number> and List <T extends Number>

问题描述

Java Generic类型：有什么区别

Java Generic type : what is the difference between

(1) List <? extends Number>   
(2) List <T extends Number>

根据我的理解

（1 ）列表<？ extends Number> 是具有超类Number的
Readonly List ofunknown数据类型。我们只能读取元素但不能添加

(1) List <? extends Number> is the Readonly List of "unknown" data type with super class "Number". we can read the element only but can not add

（2）列表< T extends Number>
超类数字的数据类型列表。我们可以读取并添加元素到列表中

请参阅下面的代码示例

class TestGen{

        public static void main(String[] args) {

        double result = 0.0;

        List<Integer> intList = new ArrayList<Integer>();
        intList.add(10);
        intList.add(20);
        intList.add(30);

        result = TestGen.sumOfList1(intList);
        System.out.println("Result=" + result); 

        result = TestGen.sumOfList2(intList);
        System.out.println("Result=" + result); 
    }

    public static double sumOfList1(List<? extends Number> list) {
        double s = 0.0;
        for (Number n : list)
            s += n.doubleValue();
        return s;
    }


    public static <T extends Number> double sumOfList2(List<T> list) {

        double s = 0.0;

        // -- --------------------------------------------------------- 
        // getting error while trying to add new element 
        // list<T> is not applicable for argument(Integer) : Why ?
        list.add(new Integer(40));                      

        for (Number n : list)               
            s += n.doubleValue();

        return s;
    }
}

当我尝试添加整数时（或者甚至数字对象）进入sumOfList2然后得到错误。请解释这里有什么问题？

When I am trying to add the Integer (or even Number object) into the sumOfList2 then getting the error. Please explain what is wrong here ?

推荐答案

基本区别在于你使用 T extends Number 那么你可以参考类型 T

例如： list.add（（T）new Integer （40））;

如果您使用？扩展数然后你不能引用类型，但你仍然可以说（（List< Integer>）列表）.add（（int）s）;

Basic difference is if you use T extends Number then you can refer to the type T
For example: list.add((T) new Integer(40));
Where as if you use ? extends Number then you can't refer to the type, but you can still say ((List<Integer>)list).add((int) s);

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