[LANG-1707] Add ArrayUtils.concat methods for concatenating multiple arrays

[ivamly]

Implementation of the LANG-1707 suggestion: added the concat methods to ArrayUtils for convenient concatenation of multiple arrays in a single call.

• Read the contribution guidelines for this project.
• Read the ASF Generative Tooling Guidance if you use Artificial Intelligence (AI).
• I used AI to create any part of, or all of, this pull request. Which AI tool was used to create this pull request, and to what extent did it contribute?
• Run a successful build using the default Maven goal with mvn; that's mvn on the command line by itself.
• Write unit tests that match behavioral changes, where the tests fail if the changes to the runtime are not applied. This may not always be possible, but it is a best practice.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Each commit in the pull request should have a meaningful subject line and body. Note that a maintainer may squash commits during the merge process.

[ivamly]

Hi! This is my first contribution to Apache Commons.
I've started by implementing the concat methods for primitive arrays and their corresponding tests.
I would appreciate verification that I'm on the right track. I'm happy to address any feedback and will gladly add the generic T[] method and its tests once the approach is confirmed.

[ivamly]

I'm not entirely sure I understand the question correctly. My implementation was based on the Jira ticket and its comments, where adding these methods was discussed and approved (as I understood it).

Could you please clarify what I might be missing? I'd be happy to adjust the approach if needed.

[garydgregory]

Hello @ivamly

I see this could be useful now...

• Add Javadoc tags @since 3.21.0 on new public and protected methods
• Remove extra blank lines
• Short-circuit the totalLength computations to throw an IllegalArgumentException once the value exceeds SAFE_MAX_ARRAY_LENGTH; you'll likely need to use a long as the running total or use Math.addExact().
• Javadoc: Sentence should end in a period (@param).

[garydgregory]

Hello @ivamly

Are you still working on this?

[ivamly]

Hello @garydgregory! I'll push the changes today.

[ivamly]

@garydgregory done.

[ivamly]

@garydgregory fix build.

[garydgregory]

Hello @ivamly
Are you sure about the length computation? It looks to me like it allows for values > SAFE_MAX_ARRAY_LENGTH but just less than Integer.MAX_VALUE.

[ivamly]

Really, you right. Work on it.

[garydgregory]

Hello @ivamly

• You are missing tests that check the arithmetic exception path. You can use Mockito or EasyMock to mock Math.addExact() for that.
• The exception try-catch doesn't need to be setup and torn down for every array element, instead of you can do that outside the for loop.
• You can "move" the array element null test "inside" like this: totalLength = Math.addExact(totalLength, getLength(array));
• Then you can refactor all of that copy-pasta, like this:

    public static byte[] concat(final byte[]... arrays) {
        int totalLength = 0;
        for (final byte[] array : arrays) {
            totalLength = addExact(totalLength, array);
        }
        final byte[] result = new byte[totalLength];
        int currentPos = 0;
        for (final byte[] array : arrays) {
            if (array != null && array.length > 0) {
                System.arraycopy(array, 0, result, currentPos, array.length);
                currentPos += array.length;
            }
        }
        return result;
    }

    private static int addExact(final int totalLength, final Object array) {
        try {
            return Math.addExact(totalLength, getLength(array));
        } catch (final ArithmeticException exception) {
            throw new IllegalArgumentException("Total arrays length exceed " + SAFE_MAX_ARRAY_LENGTH);
        }
    }

...

• Finally, when you add new method, make the new methods appear in AB order, like they existing ones already are.

Thank you!

[ivamly]

@garydgregory, thank you for the detailed feedback!

I've implemented the changes: added the helper method and sorted the methods. However, I've run into challenges testing the arithmetic exception path.

The issue: The project uses EasyMock, which doesn't support mocking static methods. Attempting to test this directly by creating an array near Integer.MAX_VALUE consistently causes an OutOfMemoryError, and such a test likely won't run reliably in CI or for other developers

My question: What's the established pattern for testing such scenarios in project?

I'd appreciate your guidance on the best path forward.

[garydgregory]

You can also try Mockito, it's version and BOM POM is inherited from the parent POM.

[ivamly]

Mockito can't mock Math.addExact() due to known limitations with java.lang.* classes (see issue).

[garydgregory]

How about creating an addExcat() bridge method then mock that.

[ivamly]

Mockito's mockStatic() proxies the entire class, and we can't call real methods for testing. Created a inner MathBridge class that encapsulates calls to Math and allows mocking only specific operations.

[garydgregory]

Hello @ivamly
Thank you for your update.
I tested the patch locally and it's green but it's still the case that totalLength computation can be > SAFE_MAX_ARRAY_LENGTH but < Integer.MAX_VALUE.