How Can I Concatenate Two Java Lists Without Modifying the Originals and Using Only the JDK?

Joining Lists in Java Without Modification or External Libraries

When merging two lists, it's often desirable to avoid modifying the originals while using only the JDK. Here's a comprehensive analysis of various approaches:

Original Method:

As presented in the question, creating a new list, copying elements from both input lists, and adding them to the new list is a straightforward method:

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

JDK 1.3 Version:

For Java 1.3 compatibility, one can use the Iterator.addAll method:

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

Java 8 Stream Concatenation:

In Java 8 and later, Stream concatenation offers a concise and efficient alternative:

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

Java 16 toList Method:

Java 16 introduces the toList method, which simplifies stream collection:

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

One-Line Solution:

Combining the previous approaches allows for a one-liner:

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

Note:

All these methods satisfy the specified conditions of not modifying the original lists, using only the JDK, and not relying on external libraries. The provided solutions range from classic techniques to modern Java features, offering flexibility based on platform compatibility and coding preferences.

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