Reversing an array in Java

Reversing an array is a popular interview question especially in languages like C. Some days ago I faced the problem in a Java legacy project I was maintaining. As a Java guy I did not want to fiddle with a for-loop and indexes. So I looked for another solution. Besides showing the one-liner I want to give some insights which some might not be aware of. Here is my solution:

public static <T> T[] reverse(T[] array) {
    Collections.reverse(Arrays.asList(array));
    return array;
}

1. This approach works, because the ArrayList implementation Arrays.asList() returns a specially tailored ArrayList which writes the changes right through to the backing array. It is not a java.util.ArrayList!
2. The above means that the array is changed in-place and no array copying is involved. The approach thus has good performance it that matters.
3. My solution directly changes the given array. To make it free of side-effects you could create a new array and copy the original one into it before converting to a list and reversing.

Side note regarding Arrays.asList()

Arrays.asList() is nice for bridging to old code and APIs. It has to be used with caution though: If you read the API documentation and keep 1.) in mind you will notice, that the returned list is fixed size. So calls to add() and remove() will fail with an UnsupportedOperationException! Passing such lists around in your system using the java.util.List interface may lead to unexpected behaviour since most people expect lists to be of variable sized in Java. In our use case we do not return the resulting list to the caller but only use it temporarily and locally. So this is no problem here.

Conclusion

Reversing an array can be a nice interview question for Java candidates too as you can discuss about in-place reversing, memory and time complexity and different approaches. It may show their knowledge of the collection API and the utility classes. If the above approach is discovered there is also something to discuss as depicted in this article.