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

Question

Is there a concise way to iterate over a stream whilst having access to the index in the stream?

String[] names = {\"Sam\",\"Pamela\", \"Dave\", \"Pascal\",          


        

Answer 1

If you don't mind using a third-party library, Eclipse Collections has zipWithIndex and forEachWithIndex available for use across many types. Here's a set of solutions to this challenge for both JDK types and Eclipse Collections types using zipWithIndex.

String[] names = { "Sam", "Pamela", "Dave", "Pascal", "Erik" };
ImmutableList expected = Lists.immutable.with("Erik");
Predicate> predicate =
    pair -> pair.getOne().length() <= pair.getTwo() + 1;

// JDK Types
List strings1 = ArrayIterate.zipWithIndex(names)
    .collectIf(predicate, Pair::getOne);
Assert.assertEquals(expected, strings1);

List list = Arrays.asList(names);
List strings2 = ListAdapter.adapt(list)
    .zipWithIndex()
    .collectIf(predicate, Pair::getOne);
Assert.assertEquals(expected, strings2);

// Eclipse Collections types
MutableList mutableNames = Lists.mutable.with(names);
MutableList strings3 = mutableNames.zipWithIndex()
    .collectIf(predicate, Pair::getOne);
Assert.assertEquals(expected, strings3);

ImmutableList immutableNames = Lists.immutable.with(names);
ImmutableList strings4 = immutableNames.zipWithIndex()
    .collectIf(predicate, Pair::getOne);
Assert.assertEquals(expected, strings4);

MutableList strings5 = mutableNames.asLazy()
    .zipWithIndex()
    .collectIf(predicate, Pair::getOne, Lists.mutable.empty());
Assert.assertEquals(expected, strings5);

Here's a solution using forEachWithIndex instead.

MutableList mutableNames =
    Lists.mutable.with("Sam", "Pamela", "Dave", "Pascal", "Erik");
ImmutableList expected = Lists.immutable.with("Erik");

List actual = Lists.mutable.empty();
mutableNames.forEachWithIndex((name, index) -> {
        if (name.length() <= index + 1)
            actual.add(name);
    });
Assert.assertEquals(expected, actual);

If you change the lambdas to anonymous inner classes above, then all of these code examples will work in Java 5 - 7 as well.

Note: I am a committer for Eclipse Collections