Solving Java 8 Streams Shallow Copy Issues with Maps

Learn how to efficiently create a shallow copy of map objects in Java 8 using streams, while avoiding pitfalls that lead to incorrect results.
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This video is based on the question https://stackoverflow.com/q/71790109/ asked by the user 'NightFury' ( https://stackoverflow.com/u/18741163/ ) and on the answer https://stackoverflow.com/a/71790449/ provided by the user 'Shawn' ( https://stackoverflow.com/u/9952196/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving Java 8 Streams Shallow Copy Issues with Maps: A Complete Guide

Java's Stream API is a powerful tool for handling collections, enabling clean, declarative code. However, using it improperly can lead to unexpected behavior - especially when dealing with mutable objects like maps. If you’ve encountered issues when performing cross joins with map objects and noticed that the values in your new key keep getting overridden, you’re not alone. In this post, we’ll address how to perform shallow copies properly using Java 8 Streams while avoiding common pitfalls.

The Problem: Shallow Copies in Streams

Consider the following problem: you have a list of maps and another list of strings, and you want to create combinations of those maps by adding a new key-value pair from the string list. However, using streams, you might find that your values are overwritten due to the shallow copy nature of Java HashMap. This behavior leads to incorrect results when trying to build your output.

Initial Setup

Suppose we have the following inputs:

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Your expected output would look like this:

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However, if you attempted a stream-based solution with flatMap() directly applied, your output would give the correct size but the wrong values, like this:

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This is because you were modifying the existing maps in place.

The Solution: Correctly Using Streams for New Maps

To resolve this, you’ll want to ensure that for each combination of the original maps and the string values, you create new instances of the maps rather than modifying existing ones. Below are the steps to implement a correct solution using streams.

Step-by-Step Implementation

Setup Your Lists: Define your original list of maps and the list of strings.

Use flatMap() Properly: Instead of modifying the existing maps directly, create a new map for each combination using map.

Here’s the corrected code snippet for your main class:

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Output

Running the above code produces the expected output:

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Conclusion

By understanding the nature of shallow copies in Java and how to use streams properly, you can avoid issues related to overwriting values in collections. This approach not only keeps your data intact but also leverages the powerful capabilities of Java's Stream API for cleaner and more efficient code. So next time you're dealing with maps and want to achieve cross joins, remember to create new instances instead of altering the originals. Happy coding!

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