ParallelStream vs Stream Parallel in Java
Summary: Explore the differences between ParallelStream and Stream parallel in Java, including their features, performance, and best use cases.
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ParallelStream vs Stream Parallel in Java
In Java, handling collections and sequences is made easier through the use of Streams. With the advent of Java 8, Stream API was introduced, facilitating new ways of handling data with concise and readable code. One of the primary features of the Stream API is its ability to execute parallel operations, making the best use of multicore processors. This post aims to elucidate the differences between ParallelStream and Stream Parallel and their respective advantages.
The Basics
Stream API
In Java, the Stream interface is part of the java.util.stream package and allows executing a sequence of operations on a set of elements. The Stream API enables both sequential and parallel execution of operations on collections.
ParallelStream
The method parallelStream() is available for collections and is a part of the Stream API. Invoking parallelStream() on a collection such as a List returns a parallel stream of the items in the collection.
Stream's parallel() Method
The Stream interface also includes a parallel() method that converts a sequential stream to a parallel stream. Any stream, whether it started sequentially or from a collection method like stream(), can be toggled to parallel mode using parallel().
Key Differences
Source of the Stream
parallelStream(): Directly called on a collection, such as List, Set. It gives you a parallel stream from the get-go.
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parallel(): Called on an existing stream, converting a sequential stream into a parallel stream.
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Performance
In terms of performance, there is generally no significant difference, as both methods use the common ForkJoinPool under the hood. The choice between them often depends on the initial state of the stream and readability of the code.
Readability and Intent
Using parallelStream() conveys the immediate intent of parallel processing from the collection itself.
Using stream().parallel() can be beneficial for conditional parallelism, where you might decide based on runtime conditions whether to run the stream operations in parallel.
When To Use Which?
parallelStream()
Use when parallel operations are inherently beneficial and when working with collections directly.
It is simpler and more intuitive when the entire stream processing can be parallel.
stream().parallel()
Use when you start with a sequential stream and want to conditionally convert it to parallel.
Useful for transforming sequential streams obtained from various sources (not just collections) into parallel streams mid-way through a pipeline.
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Conclusion
Understanding the distinction between ParallelStream and Stream.parallel() is crucial for optimal and efficient Java stream processing. While the functionalities they offer are quite similar, the choice between the two can impact the readability of your code and convey clear intent resting on how you initially fetch or manipulate the stream. As always, it is good to benchmark both approaches in the context of your specific application to make informed performance-related decisions.
Happy coding!
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