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

I would create a new ArrayList append the element and then return that as an unmodifiable list. Something like,

private static <T> List<T> appendOne(List<T> al, T t) {
    List<T> bl = new ArrayList<>(al);
    bl.add(t);
    return Collections.unmodifiableList(bl);
}

And to test it

public static void main(String[] args) {
    List<String> al = appendOne(new ArrayList<>(), "1");
    List<String> bl = appendOne(al, "2");
    System.out.println(bl); 
}

I get (unsurprisingly):

[1, 2]

See this code run at IdeOne.com.