forEach 不修改 java(8) 集合
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forEach not modify java(8) collection
提问by user1409534
Say I have an Integer list and I'm using Java 8 forEach method on the list to double its values. Say I have the following code:
假设我有一个整数列表，我在列表上使用 Java 8 forEach 方法将其值加倍。假设我有以下代码：
List<Integer> l = Arrays.asList(2,3,6,1,9);
l.forEach(p->p*=2);
As forEach method take Consumer and calls it accept methos. I print the list after runnig the above code and the original list doesn't change.
至于 forEach 方法采用 Consumer 并调用它接受方法。我在运行上面的代码后打印列表，原始列表没有改变。
As far as I understand Stream doesn't alter the source but here I just call accept method on each element...
据我了解 Stream 不会改变源，但在这里我只是在每个元素上调用 accept 方法......
Thank u in advace
提前谢谢你
采纳答案by Stephen C
The reason that forEach does not mutate the list comes down to the specification:
forEach 不改变列表的原因归结为规范：
The javadoc for forEachsays:
javadoc forforEach说：
default void forEach(Consumer<? super T> action)..... The default implementation behaves as if:
for (T t : this) action.accept(t);
default void forEach(Consumer<? super T> action)..... 默认实现的行为就像：
for (T t : this) action.accept(t);
As you can see:
如你看到的：
- The
actionis aConsumer; i.e. it doesn't generate a value. - The semantics don't allow for the
thiscollection to be updated.
- 这
action是一个Consumer; 即它不产生值。 - 语义不允许
this更新集合。
回答by Masudul
Try to use mapinsted of forEachto alter original List.
尝试使用mapinsted offorEach更改原始List.
List<Integer> list = Arrays.asList(2,3,6,1,9);
list=list.stream().map(p -> p * 2).collect(Collectors.toList());
System.out.println(list);
回答by nosid
The method forEachonly iterates through the elements of the list without changing them, If you want to change the elements, you can use the method replaceAll:
该方法forEach仅遍历列表中的元素而不更改它们，如果要更改元素，可以使用该方法replaceAll：
List<Integer> l = Arrays.asList(2,3,6,1,9);
l.replaceAll(p->p*2);
