是否有一种简洁的方法在流上迭代,同时访问流中的索引?
String[] names = {"Sam","Pamela", "Dave", "Pascal", "Erik"};
List<String> nameList;
Stream<Integer> indices = intRange(1, names.length).boxed();
nameList = zip(indices, stream(names), SimpleEntry::new)
.filter(e -> e.getValue().length() <= e.getKey())
.map(Entry::getValue)
.collect(toList());
与这里给出的LINQ示例相比,这似乎相当令人失望
string[] names = { "Sam", "Pamela", "Dave", "Pascal", "Erik" };
var nameList = names.Where((c, index) => c.Length <= index + 1).ToList();
有更简洁的方式吗?
此外,似乎拉链已经移动或被拆除…
这个问题(流方式获取第一个元素匹配布尔值的索引)已将当前问题标记为重复,所以我无法回答它;我在这里回答。
下面是获得匹配索引的通用解决方案,不需要外部库。
如果你有一个清单。
public static <T> int indexOf(List<T> items, Predicate<T> matches) {
return IntStream.range(0, items.size())
.filter(index -> matches.test(items.get(index)))
.findFirst().orElse(-1);
}
像这样叫它:
int index = indexOf(myList, item->item.getId()==100);
如果使用集合,试试这个。
public static <T> int indexOf(Collection<T> items, Predicate<T> matches) {
int index = -1;
Iterator<T> it = items.iterator();
while (it.hasNext()) {
index++;
if (matches.test(it.next())) {
return index;
}
}
return -1;
}
使用列表,你可以尝试一下
List<String> strings = new ArrayList<>(Arrays.asList("First", "Second", "Third", "Fourth", "Fifth")); // An example list of Strings
strings.stream() // Turn the list into a Stream
.collect(HashMap::new, (h, o) -> h.put(h.size(), o), (h, o) -> {}) // Create a map of the index to the object
.forEach((i, o) -> { // Now we can use a BiConsumer forEach!
System.out.println(String.format("%d => %s", i, o));
});
输出:
0 => First
1 => Second
2 => Third
3 => Fourth
4 => Fifth
如果您不介意使用第三方库,Eclipse Collections有zipWithIndex和forEachWithIndex可供跨多种类型使用。下面是针对JDK类型和Eclipse Collections类型使用zipWithIndex的一组解决方案。
String[] names = { "Sam", "Pamela", "Dave", "Pascal", "Erik" };
ImmutableList<String> expected = Lists.immutable.with("Erik");
Predicate<Pair<String, Integer>> predicate =
pair -> pair.getOne().length() <= pair.getTwo() + 1;
// JDK Types
List<String> strings1 = ArrayIterate.zipWithIndex(names)
.collectIf(predicate, Pair::getOne);
Assert.assertEquals(expected, strings1);
List<String> list = Arrays.asList(names);
List<String> strings2 = ListAdapter.adapt(list)
.zipWithIndex()
.collectIf(predicate, Pair::getOne);
Assert.assertEquals(expected, strings2);
// Eclipse Collections types
MutableList<String> mutableNames = Lists.mutable.with(names);
MutableList<String> strings3 = mutableNames.zipWithIndex()
.collectIf(predicate, Pair::getOne);
Assert.assertEquals(expected, strings3);
ImmutableList<String> immutableNames = Lists.immutable.with(names);
ImmutableList<String> strings4 = immutableNames.zipWithIndex()
.collectIf(predicate, Pair::getOne);
Assert.assertEquals(expected, strings4);
MutableList<String> strings5 = mutableNames.asLazy()
.zipWithIndex()
.collectIf(predicate, Pair::getOne, Lists.mutable.empty());
Assert.assertEquals(expected, strings5);
下面是一个使用forEachWithIndex的解决方案。
MutableList<String> mutableNames =
Lists.mutable.with("Sam", "Pamela", "Dave", "Pascal", "Erik");
ImmutableList<String> expected = Lists.immutable.with("Erik");
List<String> actual = Lists.mutable.empty();
mutableNames.forEachWithIndex((name, index) -> {
if (name.length() <= index + 1)
actual.add(name);
});
Assert.assertEquals(expected, actual);
如果您将上述lambdas更改为匿名内部类,那么所有这些代码示例都可以在Java 5 - 7中工作。
注意:我是Eclipse Collections的提交者
与https://github.com/poetix/protonpack
你可以做到,zip:
String[] names = {"Sam","Pamela", "Dave", "Pascal", "Erik"};
List<String> nameList;
Stream<Integer> indices = IntStream.range(0, names.length).boxed();
nameList = StreamUtils.zip(indices, stream(names),SimpleEntry::new)
.filter(e -> e.getValue().length() <= e.getKey()).map(Entry::getValue).collect(toList());
System.out.println(nameList);
下面是标准Java的解决方案:
在线解决方案:
Arrays.stream("zero,one,two,three,four".split(","))
.map(new Function<String, Map.Entry<Integer, String>>() {
int index;
@Override
public Map.Entry<Integer, String> apply(String s) {
return Map.entry(index++, s);
}
})
.forEach(System.out::println);
更可读的解决方案与实用方法:
static <T> Function<T, Map.Entry<Integer, T>> mapWithIntIndex() {
return new Function<T, Map.Entry<Integer, T>>() {
int index;
@Override
public Map.Entry<Integer, T> apply(T t) {
return Map.entry(index++, t);
}
};
}
...
Arrays.stream("zero,one,two,three,four".split(","))
.map(mapWithIntIndex())
.forEach(System.out::println);