Specific Collection type returned by Convenience Factory Method in Java 9

Question

In Java 9 we have convenience factory methods to create and instantiate immutable List, Set and Map.

However, it is unclear about the specific type of the returned object.

For ex:

List list = List.of("item1", "item2", "item3");

In this case which type of list is actually returned? Is it an ArrayList or a LinkedList or some other type of List?

The API documentation just mentions this line, without explicitly mentioning that its a LinkedList:

The order of elements in the list is the same as the order of the provided arguments, or of the elements in the provided array.

Answer 1

The class returned by List.of is one of the package-private static classes and therefore not part of the public API:

package java.util;
...

class ImmutableCollections {
    ...

    // Java 9-10
    static final class List0<E> extends AbstractImmutableList<E> {
        ...
    }

    static final class List1<E> extends AbstractImmutableList<E> {
        ...
    }

    static final class List2<E> extends AbstractImmutableList<E> {
        ...
    }

    static final class ListN<E> extends AbstractImmutableList<E> {
        ...
    }


    // Java 11+
    static final class List12<E> extends AbstractImmutableList<E> {
        ...
    }

    static final class ListN<E> extends AbstractImmutableList<E> {
        ...
    }
}

So this is not an ArrayList (neither a LinkedList). The only things you need to know is that it is immutable and satisfies the List interface contract.