How to Limit a Java 8 Stream Until a Predicate Fails?

Limiting a Stream Based on a Predicate in Java 8

Java 8 stream operations provide a powerful way to manipulate and filter data. However, it lacks a specific operation to limit a potentially infinite stream until the first element fails a given predicate.

In Java 9, the takeWhile operation can be used to achieve this behavior. For instance, to print all numbers less than 10 using takeWhile, you can write:

IntStream
    .iterate(1, n -> n + 1)
    .takeWhile(n -> n < 10)
    .forEach(System.out::println);

But what about Java 8, where takeWhile is not available? In this case, there are several approaches to implement similar functionality.

One way involves using the filter and findFirst operations. The following code snippet shows how:

IntStream
    .iterate(1, n -> n + 1)
    .filter(n -> n < 10)
    .findFirst()
    .ifPresent(System.out::println);

This solution works by filtering out elements that do not meet the predicate (in this case, numbers greater than or equal to 10) and then finding the first remaining element in the stream. If no matching element is found, findFirst returns an empty optional, which is ignored in this case.

Another approach uses a custom Stream.limitUntil method, which takes a predicate as an argument. Here's an example implementation:

public static <T> Stream<T> limitUntil(Stream<T> stream, Predicate<? super T> predicate) {
    AtomicBoolean stop = new AtomicBoolean(false);
    return stream.takeWhile(t -> {
        boolean stopFlag = stop.get();
        if (!stopFlag) stopFlag = predicate.test(t);
        stop.set(stopFlag);
        return !stopFlag;
    });
}

This method can be used as follows:

IntStream
    .iterate(1, n -> n + 1)
    .limitUntil(n -> n >= 10)
    .forEach(System.out::println);

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