Скачать или смотреть Using Parallel Stream in Java to Maintain Original Order of an Array List

Learn how to leverage Java's `parallel stream` functionality while ensuring that the order of elements in an array list is preserved.
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How to Use Parallel Stream in Java and Maintain Original Order

When working with a collection of items in Java, especially when dealing with large datasets, you might want to utilize the power of parallel processing to improve performance. However, a common issue developers encounter is maintaining the original order of elements in the list when using parallel streams. In this post, we'll explore how to effectively populate a new list from an existing list while preserving the original order, even when using parallel streams.

The Problem: Is Order Lost in Parallel Streams?

Imagine you have a list of products and you want to generate a corresponding list of ProductModelDTOV2 objects. You decide to use a parallel stream to achieve this, thinking it would provide better performance. However, you notice that the order of elements in the resulting list isn't the same as in the original list. This can be particularly problematic if the order of processing is important for your application's functionality.

Here’s the code snippet that demonstrates this issue:

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The Core Problem

In the code above, the call to productModelDTOV2s.add(ProductModelDTOV2) can be executed in an unordered fashion because the parallel processing does not ensure the sequence of add operations. This means that the resulting productModelDTOV2s list may not reflect the same order as products

The Solution: Separating Concerns

To maintain the original order while still benefiting from parallel processing, you need to separate the part of the code that can be executed in parallel from the part that must maintain order. Here’s how you can achieve that:

Step 1: Use flatMap to Process in Parallel

Change your stream operation to utilize flatMap. This method allows each element to be processed in parallel, and if an exception occurs, it can gracefully handle that without disrupting the overall flow.

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Step 2: Use forEachOrdered to Maintain Order

After the parallel processing, use forEachOrdered to ensure that the original order of items is preserved when adding them to the new list.

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Final Code Implementation

Putting it all together, your final code will look like this:

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Conclusion

By separating the concerns of processing and adding to the collection, you can efficiently use parallel streams in Java without losing the original order of elements. This method not only improves performance but also ensures that the integrity of the data's order is maintained. Happy coding!