我一直在使用Java 8 lambdas来轻松地过滤集合。但是我没有找到一种简洁的方法来检索结果，将其作为同一语句中的新列表。以下是我目前为止最简洁的方法:
List<Long> sourceLongList = Arrays.asList(1L, 10L, 50L, 80L, 100L, 120L, 133L, 333L);
List<Long> targetLongList = new ArrayList<>();
sourceLongList.stream().filter(l -> l > 100).forEach(targetLongList::add);
网上的示例没有回答我的问题，因为它们没有生成新的结果列表就停止了。一定有更简洁的方法。我本来希望，Stream类有toList()， toSet()，…
是否有一种方法，变量targetLongList可以直接由第三行分配?
当前回答
如果有人(比如我)正在寻找处理对象而不是基本类型的方法，那么就使用mapToObj()
String ss = "An alternative way is to insert the following VM option before "
+ "the -vmargs option in the Eclipse shortcut properties(edit the "
+ "field Target inside the Shortcut tab):";
List<Character> ll = ss
.chars()
.mapToObj(c -> new Character((char) c))
.collect(Collectors.toList());
System.out.println("List type: " + ll.getClass());
System.out.println("Elem type: " + ll.get(0).getClass());
ll.stream().limit(50).forEach(System.out::print);
打印:
List type: class java.util.ArrayList
Elem type: class java.lang.Character
An alternative way is to insert the following VM o
其他回答
您所做的可能是最简单的方法，前提是您的流保持顺序—否则您将不得不在forEach之前调用sequential()。
[later edit: the reason the call to sequential() is necessary is that the code as it stands (forEach(targetLongList::add)) would be racy if the stream was parallel. Even then, it will not achieve the effect intended, as forEach is explicitly nondeterministic—even in a sequential stream the order of element processing is not guaranteed. You would have to use forEachOrdered to ensure correct ordering. The intention of the Stream API designers is that you will use collector in this situation, as below.]
另一种选择是
targetLongList = sourceLongList.stream()
.filter(l -> l > 100)
.collect(Collectors.toList());
在Java 16中有一个新的方法Stream.toList():
List<Long> targetLongList = sourceLongList
.stream()
.filter(l -> l > 100)
.toList();
String joined =
Stream.of(isRead?"read":"", isFlagged?"flagged":"", isActionRequired?"action":"", isHide?"hide":"")
.filter(s -> s != null && !s.isEmpty())
.collect(Collectors.joining(","));
更新:
另一种方法是使用collections . tolist:
targetLongList =
sourceLongList.stream().
filter(l -> l > 100).
collect(Collectors.toList());
之前的解决方案:
另一种方法是使用collections . tocollection:
targetLongList =
sourceLongList.stream().
filter(l -> l > 100).
collect(Collectors.toCollection(ArrayList::new));
如果您有一个原语数组，您可以使用Eclipse collections中提供的原语集合。
LongList sourceLongList = LongLists.mutable.of(1L, 10L, 50L, 80L, 100L, 120L, 133L, 333L);
LongList targetLongList = sourceLongList.select(l -> l > 100);
如果你不能从List中更改sourceLongList:
List<Long> sourceLongList = Arrays.asList(1L, 10L, 50L, 80L, 100L, 120L, 133L, 333L);
List<Long> targetLongList =
ListAdapter.adapt(sourceLongList).select(l -> l > 100, new ArrayList<>());
如果您想使用LongStream:
long[] sourceLongs = new long[]{1L, 10L, 50L, 80L, 100L, 120L, 133L, 333L};
LongList targetList =
LongStream.of(sourceLongs)
.filter(l -> l > 100)
.collect(LongArrayList::new, LongArrayList::add, LongArrayList::addAll);
注意:我是Eclipse Collections的贡献者。
