How to ensure order of processing in java8 streams?

Ensuring order: The code explanations

Honoring order: forEachOrdered

If order is your priority over performance, opt for forEachOrdered() over forEach:

Stream<String> lineStream = Files.lines(Paths.get("file.txt"));
lineStream.forEachOrdered(System.out::println); // Reads a bedtime story to your lines, in order!
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Transform and behold!

Stream transformations using functions like map retain the order in a sequential stream:

orderedList.stream()
           .map(s -> "Number " + s)
           .forEachOrdered(System.out::println); // Announces your numbers like it's a lottery draw
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Skip and limit

Order preservation stays intact while limiting or skipping elements:

orderedList.stream()
           .skip(1)                
           .limit(2)  
           .forEach(System.out::println); // Removes 1, keeps 2 and 3 like a favorite child
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Watch your step with sorted

While sorted ensures order, be aware it could change the original order to match the sorting condition.

Time-saving with unordered streams

When order isn't essential, unordered() can be a performance booster for parallel processing:

unorderedList.parallelStream()
             .unordered()
             .forEach(System.out::println); // Unleashes elements like wild horses, with no particular order!
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This tactic allows the JVM to provide a faster processing ride.

Key takeaways on Stream Operations

Generating order: Stream.iterate

Utilize Stream.iterate for creating ordered streams:

Stream.iterate(0, n -> n + 1)
      .limit(10)
      .forEach(System.out::println); // Meet the orderly family of numbers 0-9!
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On the contrary, Stream.generate offers no order guarantees:

Stream.generate(Math::random)
      .limit(5)
      .forEach(System.out::println); // Roulette time! No order guaranteed
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Unveiling parallel streams

Parallel streams promise ample performance gains but remember, they assure the end list's order, not the order of operation execution:

List<Integer> numbers = IntStream.rangeClosed(1, 1000)
                                 .boxed()
                                 .collect(Collectors.toList());

numbers.parallelStream()
       .mapToInt(i -> heavyComputation(i))
       .boxed()
       .collect(Collectors.toList()); // It's like having all ducks in a row, just not sure who quacked first!
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The terminal operation like collect ensures the sequence is re-established.

Wise Practices

• Check and respect the documentation for each stream function about its order preservation.
• If order isn't a priority, use unordered() combined with parallel processing for a performance upgrade.
• Rest assured as IntStream.range or Files.lines inherently churn out ordered streams.
• Know that typically, stream pipelines can achieve satisfying parallelization while still saving the order.