[英]Assign unique IDs to objects using Java 8 streams
static <T> Map<T, Integer> assignIds(Collection<T> objects);
I want to write a function that takes a collection of unique objects and assigns a different ID number to each. 我想编写一个函数,它接受一组唯一对象,并为每个对象分配一个不同的ID号。 The ID numbers should be assigned sequentially.
ID号应按顺序分配。
I could easily do this with an explicit loop like: 我可以通过显式循环轻松完成此操作:
Map<T, Integer> ids = new HashMap<>();
int id = 0;
for (T object: objects) {
ids.put(object, id++);
}
Is there an elegant way to do this with the new Java 8 Stream API ? 使用新的Java 8 Stream API有一种优雅的方法吗?
You could use a primitive iterator to generate the ids: 您可以使用原始迭代器来生成ID:
static <T> Map<T, Integer> assignIds(Collection<T> objects) {
PrimitiveIterator.OfInt iterator = IntStream.iterate(0, x -> x + 1)
.limit(objects.size())
.iterator();
return objects.stream().collect(Collectors.toMap(obj -> obj, id -> iterator.next()));
}
You might be interested to use the protonpack library which defines some utility methods for Streams (such as zipWithIndex
). 您可能有兴趣使用protonpack库来定义Streams的一些实用方法(例如
zipWithIndex
)。 So it could looks like this: 所以它看起来像这样:
static <T> Map<T, Long> assignIds(Collection<T> objects) {
return StreamUtils.zipWithIndex(objects.stream())
.collect(Collectors.toMap(Indexed::getValue, Indexed::getIndex));
}
Here's one way: 这是一种方式:
static <T> Map<T, Integer> assignIds(Collection<T> objects) {
AtomicInteger ai = new AtomicInteger();
return objects.stream()
.collect(Collectors.toMap(o -> o, o -> ai.getAndIncrement()));
}
The above solution could also make use of parallelStream()
instead of stream()
. 上述解决方案也可以使用
parallelStream()
而不是stream()
。
Here's another that works sequentially: 这是另一个顺序工作:
static <T> Map<T, Integer> assignIds(Collection<T> objects) {
Map<T, Integer> result = new HashMap<>();
objects.stream().forEachOrdered(o -> result.put(o, result.size()));
return result;
}
Building upon ZouZou's answer... 以邹邹的答案为基础......
static <T> Map<T, Integer> assignIds(Collection<T> objects) {
OfInt ids = IntStream.range(0, objects.size()).iterator();
return objects.stream().collect(Collectors.toMap(o -> o, o -> ids.next()));
}
The idiomatic way to do this in for instance Scala would be to use zipWithIndex
. 在例如Scala中执行此操作的惯用方法是使用
zipWithIndex
。 There's no such method in the Java 8 Streams API, not even a zip
method which you could combine with an IntStream
. Java 8 Streams API中没有这样的方法,甚至没有可以与
IntStream
结合使用的zip
方法。
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