Java 通过 lambda 表达式应用过滤器后如何获取流的大小？
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How to get the size of a Stream after applying a filter by lambda expression?
提问by Abdennour TOUMI
Consider the following code:
考虑以下代码：
List<Locale> locales = Arrays.asList(
new Locale("en", "US"),
new Locale("ar"),
new Locale("en", "GB")
);
locales.stream().filter(l -> l.getLanguage() == "en");
How do I get the size of the localesArrayListafterapplying filter, given that locales.size()gives me the size beforeapplying filter?
考虑到申请前的尺寸，我如何获得申请localesArrayList后的尺寸？filterlocales.size()filter
采纳答案by Alexis C.
When you get a stream from the list, it doesn't modify the list. If you want to get the size of the stream after the filtering, you call count()on it.
当您从列表中获取流时，它不会修改列表。如果你想在过滤后获得流的大小，你可以调用count()它。
long sizeAfterFilter =
locales.stream().filter(l -> l.getLanguage().equals("en")).count();
If you want to get a new list, just call .collect(toList())on the resulting stream. If you are not worried about modifying the list in place, you can simply use removeIfon the List.
如果您想获得一个新列表，只需调用.collect(toList())结果流即可。如果您不担心就地修改列表，您可以简单地removeIf在List.
locales.removeIf(l -> !l.getLanguage().equals("en"));
Note that Arrays.asListreturns a fixed-size list so it'll throw an exception but you can wrap it in an ArrayList, or simply collect the content of the filtered stream in a List(resp. ArrayList) using Collectors.toList()(resp. Collectors.toCollection(ArrayList::new)).
请注意，Arrays.asList返回一个固定大小的列表，以便它会抛出一个异常，但你可以在包裹它ArrayList，或者干脆收集过滤流的内容List（相应地ArrayList）使用Collectors.toList()（相应的Collectors.toCollection(ArrayList::new)）。
回答by Bohemian
Use the count()method:
使用count()方法：
long matches = locales.stream()
.filter(l -> l.getLanguage() == "en")
.count();
Note that you are comparing Strings using ==. Prefer using .equals(). While ==will work when comparing interned Strings, it fails otherwise.
请注意，您正在使用==. 更喜欢使用.equals(). 虽然==在比较实习字符串时会起作用，否则会失败。
FYI it can be coded using only method references:
仅供参考，它可以仅使用方法引用进行编码：
long matches = locales.stream()
.map(Locale::getLanguage)
.filter("en"::equals)
.count();
