Can you split a stream into two streams?

java

stream-api

performance

best-practices

byNikita Barsukov·Dec 17, 2024

// Split a Stream into two based on a condition
Stream<String> fruits = Stream.of("apple", "banana", "cherry", "date");

Map<Boolean, List<String>> splitMap = 
    fruits.collect(Collectors.partitioningBy(s -> s.length() > 5));

List<String> longerThan5 = splitMap.get(true);  // Splitting bananas and cherries - a fruity dilemma!
List<String> notLongerThan5 = splitMap.get(false); // Short and sweet, like these fruits

Use Collectors.partitioningBy to split a Stream into two Lists. The predicate condition separates elements into respective categories. So, imagine you've got two baskets, and you're sorting fruits based on their length - easy-peasy!

More than binary split? Use `groupingBy`

For scenarios demanding more than a binary partition, use Collectors.groupingBy(). It allows categorization into multiple pockets, like wearing a utility belt for your data. But remember, Batman wouldn't try this on infinite streams. Neither should you.

Run-time considerations - For the love of Speed and Memory

While dealing with streams, you've got to play nice with both space and speed. Think in terms of race cars packed in a parking lot. Tuning our ArrayLists to the full size of the initial collection is like preorder parking spaces. It saves the time, redeems space, and we avoid the dissatisfactory resize operation. But this isn't a one-size-fits-all solution. Stay alert about the memory usage since you're still putting many cars in the lot.

Harness the power of `teeing` for efficient split

Is your Java enjoying the sweetness of version 12 or later? Great! You have the prowess of the teeing collector. Like a skilled lumberjack you are, split the stream efficiently and directly! It's like preparing two dishes from the same ingredients simultaneously, without messing your kitchen.

Duplicate before you decimate

Understand this, once your streaming party is over, it's over. You CAN'T revisit it. So in case, you plan to perform several operations that require filtering, better have two parties. Either duplicate the stream in the first place or have a backup collection before you traverse the stream.

Mind your surroundings - ArrayLists and stream splitting

When preparing for the streaming operations, be aware of your surroundings. Consider initial execution speed and space usage. Predefining your ArrayLists at full size is like arranging your chess pieces before the game starts. But remember! the board size matters too. You don't want to run out of it midway your battle with streams.

Design your own stream splitter

For those of you who like tailoring your own suits, you may want to design a custom collector. You can bath, filter and split a stream all at once with something like a PredicateSplitterConsumer. However, be forewarned: custom implementations require a deep understanding of Stream API usage.

Runtime complexities - Don't burn the candle at both ends

While dealing with your streams, understand and evaluate the duo of time and space complexities. It's the Justice League of system resource management. Sure, having ArrayLists initialized to full size for stream splitting can shave off precious milliseconds, but it can also transform your memory usage from Bruce Banner to Hulk. Plan wisely.

The stream lifecycle - Keep your friends close, but your streams closer

When operating on streams, order, and consumption matter. It's like eating sushi. You don't just grab any piece; you go in a certain order, savoring one piece at a time. Also, operation ordering affects whether you need to cycle through the stream more than once. Like sushi on a conveyor, pick your pieces (operations) smartly!

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