How to Concatenate Lists in Java Without Modifying the Originals?

Joining Lists in Java Without Altering Originals

In Java, combining multiple lists into a new one is a common task. However, the approach mentioned in the question requires modification of the original lists, which may not always be desirable.

Java 8 Stream Concatenation

Java 8 introduced the Stream API, which provides a more concise and immutable way to manipulate collections. For list concatenation, you can use the concat() method to merge the streams of two lists:

List<String> newList = Stream.concat(listOne.stream(), listTwo.stream())
                             .collect(Collectors.toList());

This method creates a new list without altering the originals, as it operates on streams which are essentially wrappers around the source collections.

Java 16 toList() Improvement

From Java 16 onward, a simpler approach is available using the updated toList() method:

List<String> newList = Stream.concat(listOne.stream(), listTwo.stream()).toList();

This simplifies the code by eliminating the need for a collect() operation, further streamlining the list concatenation process.

Bonus: JDK 1.3 Version

For JDK 1.3, you can use the following technique:

List<String> newList = new ArrayList<String>(listOne.size() + listTwo.size());
newList.addAll(listOne);
newList.addAll(listTwo);

This method creates a new ArrayList with a size that accommodates both original lists, then adds the elements from each list. While it's not as concise as the Stream-based approaches, it still provides an immutable result.

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