仅当流不为空后缀时,Custom Collector才能在定界符,后缀和前缀上加入流 [英] Custom Collector to join stream on delimiter, suffix and prefix only if stream is not empty suffix
问题描述
我有一串字符串:
Stream<String> stream = ...;
我想使用创建一个字符串
I want to create a string using
stream.collect(Collectors.joining(',', '[', ']'))
仅我想返回无字符串";如果流中不包含任何元素.
only I want to return "No Strings" if the stream does not contain any elements.
我注意到String java.util.stream.Stream.collect(Collector<? super String, ?, String> collector)
方法采用类型为java.util.stream.Collector< T,A,R>
I notice that String java.util.stream.Stream.collect(Collector<? super String, ?, String> collector)
method takes an argument of type java.util.stream.Collector<T, A, R>
对于我的项目,我需要在许多地方使用此功能,因此我需要一个实现Collector
接口的类.
For my project I need to this functionality in many places so I would need a class the implements the Collector
interface.
我知道这可以通过Stream到List然后检查List.size()== 0来完成吗?然后根据需要将该列表再次转换为流.
I know this could be done by Stream to a List and then checking on List.size() == 0? and then convert the list to a stream again if needed.
List<String> list = stream.collect(Collectors.toList());
if (list.size() == 0) {
return "No Strings";
}
return list.stream().collect(Collectors.joining(",", "[", "]"));`
这就是我想发生的事情
List emptyList<String> = new ArrayList<>; System.out.println(emptyList.stream().collect(Collectors.joining(",", "[", "]")));
[]
No Strings
推荐答案
老实说,我会采用您当前的方法(测试是否为空).
Honestly, I would go with your current approach (testing for emptiness).
但是,如果您真的想使用直接收集器,则可以使用 StringJoiner
的Javadoc作为制作自定义收集器的指南:
But if you really wanted to use a straight collector, you could use the source code of Collections.joining
and the Javadoc of StringJoiner
as a guide to make a custom collector:
Collector.of(
() -> new StringJoiner(",", "[", "]").setEmptyValue("No strings"),
StringJoiner::add,
StringJoiner::merge,
StringJoiner::toString)
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