Java - Add one element to an immutable list

Question

I need an immutable list where I can get derive a second immutable list preserving all elements of the previous list plus an additional element in Java (without additional libraries).

Note: This question is similar to What is an efficient and elegant way to add a single element to an immutable set? but I need a list and don't have Guava.

What I have tried so far:

var list = List.of(someArrayOfInitialElements);
var newList = Stream.concat(list.stream(), Stream.of(elementToAppend))
        .collect(CollectorsCollectors.toUnmodifiableList());

That would work but creating a stream and copying elements one by one seems inefficient to me. You could basically bulk copy memory given that List.of() stores data in a field-based or array-based data structure.

Is there a more efficient solution than using streams? A better data structure in the Java standard library that I am missing?

Answer 1

The Answer by Frisch is correct, and should be accepted. One further note…

Calling Collections.unmodifiableList produces a collection that is a view onto the original mutable list. So a modification to the original list will "bleed through" to the not-so-immutable second list.

This issue does not apply to the correct code shown in that Answer, because the new ArrayList object deliberately goes out-of-scope. Therefore that new list cannot be accessed for modification. But in other coding scenarios, this issue could be a concern.

List.copyOf

If you want an independent and truly immutable second list, use List.copyOf in Java 10+. This returns an unmodifiable list.

return List.copyOf( bl ) ;

Answer 2

Both answers are great, I would create a bit more generic solution:

private static <T> List<T> append(final List<T> al, final T... ts) {
    final List<T> bl = new ArrayList<>(al);
    for (final T t : ts) {
        bl.add(t);
    }
    return List.copyOf(bl);
}

It can be used exactly like previous answer:

    List<String> al = append(new ArrayList<>(), "1");
    List<String> bl = append(al, "2");
    System.out.println(bl); 

But also slightly more efficient:

    List<String> bl = append(new ArrayList<>(), "1", "2");
    System.out.println(bl);