用什么说&lt ;?扩展SomeObject>而不是< SomeObject> [英] Whats the use of saying <? extends SomeObject> instead of <SomeObject>

问题描述

  List< ;?扩展SomeObject>升; 
  

基本上这个列表接受SomeObject的所有对象 - SomeObject本身或它的继承者。但是根据polyphophism，继承者也可以像SomeObject那样成为seens，所以这也可以工作：

  List< SomeObject>升; 
  

那么为什么有人会使用第一个选项，而第二个选项是明确定义的并且几乎相同？

解决方案

  List< SomeObject>升; 
  

在这里你不能说 List< SomeObject> l = new ArrayList< SubClassOfSomeObjectClass> ;; （不允许）
适用于

 列表与LT ;?扩展SomeObject>升; 
  

您可以说

列表< ;?扩展SomeObject> l = new ArrayList< SubClassOfSomeObject> ;; （ allowed ）

但是请注意，在名单< ;?扩展SomeObject> l = new ArrayList< SubClassOfSomeObject> ;; 您无法向您的列表添加任何内容l ，因为？代表未知课程（课程无效）。

更新： c $ c>如果我不能添加任何东西，我可以做什么？

现在考虑一个你有编写一个函数来打印列表，但请注意，它只能接受一个带有SomeObject的子类的对象的列表。在这种情况下，如上所述，您不能使用

public void printList（List someList）

那么你会怎么做？你会做类似于

  public void printList（List< ;? extends SomeObject> someList）{
 for（SomeObject myObj ：someList）{
 //在myObj 
上处理读取操作} 
  

So I was looking over some Java code and stumbled upon:

List<? extends SomeObject> l;

basically this list accepts all objects that are some kind of SomeObject - SomeObject itself or its inheritors. But according to polymophism, it's inheritors can also be seens as SomeObject, so this would work as well:

List<SomeObject> l;

So why would someone use the first option when the second is clearly defined and virtually identical?

解决方案

List<SomeObject> l;

In this you cannot say List<SomeObject> l = new ArrayList<SubClassOfSomeObjectClass>;(not allowed) wheres for

List<? extends SomeObject> l;

you can say

List<? extends SomeObject> l = new ArrayList<SubClassOfSomeObject>;(allowed)

But note that in List<? extends SomeObject> l = new ArrayList<SubClassOfSomeObject>; you cannot add anything to your list l because ? represents unknown class (Except null of-course).

Update: For your question in the comment What could I possibly do with a list if I cannot add anything to it?

Now consider a case in which you have to write a function to print your list but mind you it must only accept a List having objects which are subclasses of your SomeObject. In this case as I stated above you cannot use

public void printList(List<SubClassOfSomeObjectClass> someList)

So what would you do? You would do something like

    public void printList(List<? extends SomeObject> someList) {
        for(SomeObject myObj : someList) {
             //process read operations on  myObj
        }

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