singletonList的用途是什么? [英] What is the use of singletonList?

问题描述

我一直在寻找一些优雅的解决方案，以从列表中删除空值.我遇到了后续帖子，该帖子说我可以使用list.removeAll(Collections.singletonList(null));

I was looking around for some elegant solution to removing null values from a List. I came across the following post, which says I can use list.removeAll(Collections.singletonList(null));

但是，这引发了UnsupportedOperationException，我认为这是因为removeAll()试图对不可变的单例集合进行某些变数运算.这是正确的吗?

This, however, throws an UnsupportedOperationException, which I'm assuming is because removeAll() is attempting to do some mutative operation on the immutable singleton collection. Is this correct?

如果是这种情况，此singletonList的典型用法是什么?当您确定不想对集合进行任何操作时，要表示大小为1的集合吗?

If this is the case, what would be a typical use of this singletonList? To represent a collection of size 1 when you're sure you don't want to actually do anything with the collection?

谢谢.

推荐答案

它的工作原理就像是一种魅力:

It works like a charm:

List<String> list = new ArrayList<String>();
list.add("abc");
list.add(null);
list.add("def");
list.removeAll(Collections.singletonList(null));
System.out.println(list);  //[abc, def]

实际上Collections.singletonList(null)是不可变的(不幸的是，它隐藏在Java ^[1] 中)，但是从您的list变量抛出了异常.显然，它也是不可变的，如下面的示例所示:

Indeed Collections.singletonList(null) is immutable (which is unfortunately hidden in Java^[1]), but the exception is thrown from your list variable. Apparently it is immutable as well, like in example below:

List<String> list = Arrays.asList("abc", null, "def");
list.removeAll(Collections.singletonList(null));

此代码将抛出UnsupportedOperationException.因此，您可以看到singletonList()在这种情况下很有用.当客户端代码需要只读列表(不会修改它)而您只想在其中传递一个元素时，请使用它. singletonList()是(线程)安全的(由于不可变)，快速且紧凑.

This code will throw an UnsupportedOperationException. So as you can see singletonList() is useful in this case. Use it when client code expects a read-only list (it won't modify it) but you only want to pass one element in it. singletonList() is (thread-)safe (due to immutability), fast and compact.

^[1] 例如在 scala 的问题中，存在一个可变且不可变的独立层次结构集合和API可以选择是否接受一个或另一个(或两者都接受，因为它们具有通用的基本接口)

^[1] E.g. in scala there is a separete hierarchy for mutable and immutable collections and API can choose whether it accept this or the other (or both, as they have common base interfaces)

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