How to short-circuit reduce on Stream?

问题

Suppose I have a stream of boolean values and the reduce operation that I am writing is || (OR). Can I write it in a way such that the evaluation of at least some of the elements is abandoned if a true value is encountered?

I am looking for some amount of optimization (perhaps if it is a parallel stream), not necessarily full optimization although the latter would be awesome.

回答1:

I suspect you want this type of construct.

// stop when any element evaluates to true
boolean any = stream.anyMatch(t -> t);

You can check this with peek

Stream.of(1, 2, 3, 4).peek(System.out::println).anyMatch(i -> i == 2);

prints

1
2

For a parallel example

AtomicInteger count = new AtomicInteger();
IntStream.range(0, 1000).parallel().peek(t -> count.incrementAndGet()).anyMatch(i -> i == 2);
System.out.println("count: " + count);

prints a number like

count: 223

The exact number varies.

For a referencePipeline, the anyMatch calls

@Override
public final boolean anyMatch(Predicate<? super P_OUT> predicate) {
    return evaluate(MatchOps.makeRef(predicate, MatchOps.MatchKind.ANY));
}

which calls this

public static <T> TerminalOp<T, Boolean> makeRef(Predicate<? super T> predicate,
        MatchKind matchKind) {
    Objects.requireNonNull(predicate);
    Objects.requireNonNull(matchKind);
    class MatchSink extends BooleanTerminalSink<T> {
        MatchSink() {
            super(matchKind);
        }

        @Override
        public void accept(T t) {
            if (!stop && predicate.test(t) == matchKind.stopOnPredicateMatches) {
                stop = true;
                value = matchKind.shortCircuitResult;
            }
        }
    }

    return new MatchOp<>(StreamShape.REFERENCE, matchKind, MatchSink::new);
}

where you can start to see the short circuiting code.

来源：https://stackoverflow.com/questions/25113748/how-to-short-circuit-reduce-on-stream

标签

java

java-8

java-stream