How to Duplicate a Java 8 Stream Without Collecting to a List?

How can I efficiently duplicate a Java 8 stream without collecting it into a list?

List<Pair<A, Throwable>> results = doSomething().collect(toList());
Stream<Pair<A, Throwable>> failures = results.stream().flatMap(either -> either.left());
failures.forEach(failure -> ... );
Stream<A> successes = results.stream().flatMap(either -> either.right());
successes.forEach(success -> ... );

In Java 8, streams are designed to be used only once, which is why you encounter the 'stream has already been operated upon or closed' error when you attempt to reuse a stream. However, there are strategies to efficiently process stream data multiple times without converting the entire stream to a collection.

Map<Boolean, List<Either<Pair<A, Throwable>, A>>> partitionedResults = doSomething()
    .collect(Collectors.partitioningBy(either -> either.isLeft()));

List<Either<Pair<A, Throwable>, A>> failures = partitionedResults.get(true);
List<A> successes = partitionedResults.get(false);

failures.forEach(failure -> ...);
successes.forEach(success -> ...);

Causes

• Stream operations are not reusable. Once a terminal operation has been applied to a stream, it cannot be reused.
• Attempting to operate on the same stream for different purposes leads to runtime exceptions.

Solutions

• Instead of trying to duplicate a stream, consider using the 'peek' method to process elements as they pass through the stream pipeline.
• Utilize the 'Collectors.partitioningBy' method to split the results into two groups (successes and failures) in one pass, avoiding separate operations on the same stream.

Mistake: Trying to call a terminal operation multiple times on the same stream instance.

Solution: Always create a new stream instance for each terminal operation.

Mistake: Assuming streams can be reused like collections.

Solution: Understand the lifecycle of a stream. Either collect results or process them in a single stream pipeline.