Is there a concise way to iterate over a stream with indices in Java 8?

Solution 1:

The cleanest way is to start from a stream of indices:

String[] names = {"Sam", "Pamela", "Dave", "Pascal", "Erik"};
IntStream.range(0, names.length)
         .filter(i -> names[i].length() <= i)
         .mapToObj(i -> names[i])
         .collect(Collectors.toList());

The resulting list contains "Erik" only.

One alternative which looks more familiar when you are used to for loops would be to maintain an ad hoc counter using a mutable object, for example an AtomicInteger:

String[] names = {"Sam", "Pamela", "Dave", "Pascal", "Erik"};
AtomicInteger index = new AtomicInteger();
List<String> list = Arrays.stream(names)
                          .filter(n -> n.length() <= index.incrementAndGet())
                          .collect(Collectors.toList());

Note that using the latter method on a parallel stream could break as the items would not necesarily be processed "in order".

Solution 2:

The Java 8 streams API lacks the features of getting the index of a stream element as well as the ability to zip streams together. This is unfortunate, as it makes certain applications (like the LINQ challenges) more difficult than they would be otherwise.

There are often workarounds, however. Usually this can be done by "driving" the stream with an integer range, and taking advantage of the fact that the original elements are often in an array or in a collection accessible by index. For example, the Challenge 2 problem can be solved this way:

String[] names = {"Sam", "Pamela", "Dave", "Pascal", "Erik"};

List<String> nameList =
    IntStream.range(0, names.length)
        .filter(i -> names[i].length() <= i)
        .mapToObj(i -> names[i])
        .collect(toList());

As I mentioned above, this takes advantage of the fact that the data source (the names array) is directly indexable. If it weren't, this technique wouldn't work.

I'll admit that this doesn't satisfy the intent of Challenge 2. Nonetheless it does solve the problem reasonably effectively.

EDIT

My previous code example used flatMap to fuse the filter and map operations, but this was cumbersome and provided no advantage. I've updated the example per the comment from Holger.

Solution 3:

Since guava 21, you can use

Streams.mapWithIndex()

Example (from official doc):

Streams.mapWithIndex(
    Stream.of("a", "b", "c"),
    (str, index) -> str + ":" + index)
) // will return Stream.of("a:0", "b:1", "c:2")