How Can I Efficiently Iterate Through a Java 8 Stream with Index Access?

Concise Index-Based Stream Iteration in Java 8

Many programming tasks involve iterating over a data structure while accessing the corresponding index. Java 8 streams provide a wide range of operations for stream processing, but there is no explicit way to obtain the index during iteration.

The provided Java code demonstrates a rather verbose approach using IntStream, boxed(), zip(), filter(), map(), and collect(). While it achieves the desired result, its length and complexity mar its elegance.

Cleanest Method:

The most concise approach 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());

This method constructs a stream of indices from 0 to the length of the array, filters the indices based on the length of the corresponding string, maps the indices to the corresponding strings, and collects the results into a list.

Alternative Method:

Another option is to maintain an ad hoc index counter using a mutable object like 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());

This method preserves the familiarity of for-loop-based index incrementing. However, it should be noted that using this method on a parallel stream can potentially result in incorrect behavior due to the non-deterministic ordering of parallel processing.

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