如果我们有原始的列表，为什么我们可以更改不可修改的列表？ [英] Why can we change the unmodifiable list if we have the original one?

问题描述

查看 Collections 类，我知道当我们使用 unmodifiableList（List list）或 unmodifiableCollection（Collection c）它不是创建一个新的对象，而是返回同一个对象的引用，并覆盖可以修改 List [添加， addall ， >， retainAll ...]

所以我运行这个测试：

By looking at the code of Collections class, i got to know that when we are using the method unmodifiableList(List list) or unmodifiableCollection(Collection c) it is not creating a new object but it is returning the reference of the same object and overriding the methods which can modify the List [ add, addall, remove, retainAll ... ]
So i ran this test:

List modifiableList = new ArrayList();
modifiableList.add ( 1 );   
List unmodifiableList = Collections.unmodifiableList( modifiableList );
// unmodifiableList.add(3);  // it will throw the exception 
modifiableList.add ( 2 );       
System.out.println( unmodifiableList );

结果为 [1,2] 。

现在的问题是为什么它是指同一个对象？为什么不创建新对象？

result is [ 1,2 ] .
Now the point is why it is referring to the same object? Why it don't create a new object?

推荐答案

（底部的任务答案）

当您创建不可修改的列表时，其目的是不应由以外的人（即API的客户端）修改。

When you create an unmodifiable list, the purpose is that it should not be modified by people other than you - i.e. clients of an API.

方法 unmodifiableList（..）创建一个类型为 UnmodifiableList 但这不是一个公共类），它获得原始列表，并将所有的方法委托给它除了的方法将修改它。

the method unmodifiableList(..) creates a new object of type UnmodifiableList (but this is not a public class), which gets the original list, and delegates all methods to it except the methods which would modify it.

正如文档中所述：

返回指定列表的不可修改视图。此方法允许模块为用户提供对内部列表的只读访问。

Returns an unmodifiable view of the specified list. This method allows modules to provide users with "read-only" access to internal lists.

所以，一个例子：你有一个 List 已经检测到并可以操作，并且您想要为他们提供您的API的客户端。但他不应该改变 。所以你有两个选择：

So, an example: You have a List of devices that your API has detected and can operate, and you want to give them a client of your API. But he is not supposed to change them. So you have two options:

• 给他一份 List 所以即使他修改了它，这不会改变你的列表


• 给他一个不可修改的集合 - 他不能修改它，

• give him a deep copy of your List, so that even if he modifies it, this does not change your list
• give him an unmodifiable collection - he can't modify it, and you spare the creation of a new collection.

现在这里提供问题标题的答案 - 不可修改的列表是一个 。因此，如果您需要向其中添加一个新项目，例如，您发现了一个刚刚插入的新设备，客户端将能够在其不可修改的视图中看到它。

And now here comes the answer to the title of your question - the unmodifiable list is a view of the original collection. So if you need to add a new item to it - say, you have discovered a new device that was just plugged-in, the clients will be able to see it in their unmodifiable view.

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