CopiesList.addAll方法抛出UnsupportedOperationException [英] CopiesList.addAll method throws UnsupportedOperationException

问题描述

List<String> hi = Collections.nCopies(10, "Hi");
List<String> are = Collections.nCopies(10, "Are");

hi.addAll(are);

hi.forEach(System.out::println);

输出

Exception in thread "main" java.lang.UnsupportedOperationException
  at java.util.AbstractList.add(Unknown Source)
  at java.util.AbstractList.add(Unknown Source)
  at java.util.AbstractCollection.addAll(Unknown Source)
  at com.practice.java8.lambdaexp.Test.main(Test.java:14)

我认为AbstractList.add()在JDK中没有实现.这就是为什么它不起作用.

I think AbstractList.add() doesn't have implementation in JDK. That is why it's not working.

但是，如果我使用CopiesList对象创建一个新的ArrayList对象，它将起作用，因为它将具有适当的add()实现.

But if I create a new ArrayList object using a CopiesList object it works, because it will have a proper add() implementation.

我的问题是:为什么 CopiesList 没有正确的addAll实现?

My question is: Why doesn't CopiesList have a proper implementation for addAll?

推荐答案

Collections.nCopies 返回一个不变的 List ，因此您不能向其中添加任何内容:

Collections.nCopies returns an immutable List, so you can't add anything to it:

返回一个不可变列表，该列表由指定对象的n个副本组成.

Returns an immutable list consisting of n copies of the specified object.

它返回一个不变的 List ，因为它仅包含您传递给其构造函数的元素的单个引用:

It returns an immutable List since it only contains a single reference of the element you pass to its constructor:

新分配的数据对象很小(它包含对该数据对象的单个引用).

这样做是为了提高性能和存储要求- Collections.nCopies(10，"Hi")和 Collections.nCopies(10000000，"Hi")占据相同的空间.

This is done for the benefit of performance and storage requirements - Collections.nCopies(10, "Hi") and Collections.nCopies(10000000, "Hi") occupy the same amount of space.

因此，将其实现为可变列表将非常困难.如果没有为该元素分配存储空间，如何将第 10000 个元素设置为某个新值?

Therefore it would be very difficult to implement it as a mutable List. How would you set, for example, the 10000'th element to some new value if there's no storage allocated for that element?

如果需要包含相同对象副本的可变列表，则必须将不可变列表传递给某些可变列表构造函数:

If you need a mutable List that contains copies of the same object, you have to pass the immutable List to some mutable List constructor:

List<String> mutable = new ArrayList<>(Collections.nCopies(10, "Hi"));

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