How to Implement Java 8's Equivalent of Java 9's `takeWhile` for Streams?

Java 8 Stream: Limiting by Predicate

In scenarios with potentially infinite streams, we often encounter the need to limit the stream until the first element fails a specific predicate. Java 9 introduced the useful takeWhile operation, enabling this functionality effortlessly.

Unfortunately, Java 8 lacks such an operation. Thus, we must explore alternative approaches to achieve this behavior.

Implementing takeWhile in Java 8

To implement takeWhile in Java 8, we can leverage the following strategy:

1. Create an iterator from the stream using iterator().
2. Iterate through the iterator until the predicate fails.
3. Collect the filtered elements into a new stream using generate().

Below is an example implementation:

import java.util.Iterator;
import java.util.stream.Stream;

public class StreamTakeWhile {

    public static <T> Stream<T> takeWhile(Stream<T> stream, java.util.function.Predicate<T> predicate) {
        Iterator<T> iterator = stream.iterator();
        return Stream.generate(() -> {
            if (iterator.hasNext() && predicate.test(iterator.next())) {
                return iterator.next();
            }
            return null;
        });
    }

    public static void main(String[] args) {
        StreamTakeWhile.takeWhile(Stream.iterate(1, n -> n + 1), n -> n < 10)
                .forEach(System.out::println);
    }
}

Using Java 9 takeWhile

With Java 9, you can directly utilize the takeWhile operation as shown below:

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

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