Utilities for working with Java 8 streams
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README.md

streams

Utilities for working with Java 8 streams. KeyedStream makes working with streams of Map entries readable.

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KeyedStream

Consider using streamex's EntryStream instead of this class.

import com.palantir.common.streams.KeyedStream;

A KeyedStream<K, V> is syntactic sugar around a Stream<Map.Entry<K, V>>, with methods for filtering/updating keys, values and whole entries. You can create a KeyedStream from a Map, or from a Stream or Iterable, giving initially identical keys and values.

KeyedStream.of(Stream.of(1, 2, 3))  // keys and values are initially identical
    .map(v -> v * 2)                // keys remain unchanged, values are doubled
    .mapKeys(Integer::toString)     // values remain unchanged
    .collectToMap();                // returns a Map<String, Integer>

Each map function also accepts a BiFunction, making it easy to modify keys based on values, and vice versa:

KeyedStream.stream(map)
    .map((k, v) -> new FooType(k, v))  // keys remain unchanged
    .collectToMap();

MoreStreams

Utility methods for streams. Currently supported are inCompletionOrder and blockingStreamWithParallelism. It is tricky to handle streams of futures. Running

foos.stream().map(executorService::submit).map(Futures::getUnchecked).collect(toList());

will only execute one task at a time, losing the benefit of concurrency.

On the other hand, collecting to a List in the meantime can lead to other issues - not only it is inconvenient to stream twice, but there are plenty of issues that can appear, especially if the returned objects are large - there is no back-pressure mechanism.

Instead, when a ListenableFuture can be returned by a function, consider calling

MoreStreams.inCompletionOrder(foos.stream().map(service::getBarAsync), maxParallelism)
    .map(Futures::getUnchecked)
    .collect(toList());

which will provide a new stream which looks ahead up to maxParallelism items in the provided stream of Guava futures, ensuring that only maxParallelism futures exist at any one time.

In some cases, it may not be beneficial to push down the async computation. Here, one can provide their own executor, calling

MoreStreams.inCompletionOrder(foos.stream(), service::getBar, executor, maxParallelism).collect(toList());

The difference between inCompletionOrder and blockingStreamWithParallelism is that the blockStreamWithParallelism methods keep the futures in the order they were provided, whilst the inCompletionOrder methods return futures in the order in which they complete.