Java8 equivalent for ruby's each_with_index - ruby

Question

I wonder, if there's some stream operation that can do as each_with_index in ruby.
Where each_with_index iterates over the value as well as the index of the value.

Answer 1

There is no stream operation specifically for that purpose. But you can mimic the functionality in several ways.
Index variable: The following approach works fine for sequential streams.
int[] index = { 0 };
stream.forEach(item -> System.out.printf("%s %d\n", item, index[0]++));
External iteration: The following approach works fine for parallel streams, as long as the original collection supports random access.
List<String> tokens = ...;
IntStream.range(0, tokens.size()).forEach(
index -> System.out.printf("%s %d\n", tokens.get(index), index));

Answer 2

You can reduce it
<T> void forEachIndexed(Stream<T> stream, BiConsumer<Integer, T> consumer) {
stream.reduce(0, (index, t) -> {
consumer.accept(index, t);
return index + 1;
}, Integer::max);
}
this way:
List<Integer> ints = Arrays.asList(1, 2, 4, 6, 8, 16, 32);
forEachIndexed(ints.stream(), (idx, el) -> {
System.out.println(idx + ": " + el);
});

Answer 3

You can use forEachWithIndex() in Eclipse Collections (formerly GS Collections).
MutableList<Integer> elements = FastList.newList();
IntArrayList indexes = new IntArrayList();
MutableList<Integer> collection = this.newWith(1, 2, 3, 4);
collection.forEachWithIndex((Integer object, int index) -> {
elements.add(object);
indexes.add(index);
});
Assert.assertEquals(FastList.newListWith(1, 2, 3, 4), elements);
Assert.assertEquals(IntArrayList.newListWith(0, 1, 2, 3), indexes);
If you cannot convert your Collection to a GS Collections type, you can use one of the adapters, like ListAdapter.
List<Integer> list = Arrays.asList(1, 2, 3, 4);
ListIterable<Integer> collection = ListAdapter.adapt(list);
collection.forEachWithIndex((object, index) -> {
elements.add(object);
indexes.add(index);
});
Note: I am a committer for Eclipse Collections.

Answer 4

Alternative with stream reduce operation with accumulator (2nd parameter) for side-effect. 3rd parameter could be any function, if you don't need the result from reduce operation.
List<String> tokens = Arrays.asList("A", "B", "C", "D");
tokens.stream().reduce(1, (i, str) -> {
System.out.printf("%s %d\n", str, i);
return i + 1;
}, Integer::max);
PS: Although it is possible, I am personally not satisfied with abuse of reduce function. :)

Answer 5

Easy to do with utility library protonpack: https://github.com/poetix/protonpack
Stream<String> source = Stream.of("Foo", "Bar", "Baz");
List<Indexed<String>> zipped = StreamUtils.zipWithIndex(source).collect(Collectors.toList());
assertThat(zipped, contains(
Indexed.index(0, "Foo"),
Indexed.index(1, "Bar"),
Indexed.index(2, "Baz")));