Why does Arrays.asList return a fixed-size List?

Question

Arrays.asList is a useful and convenient method, but it returns a List whose size is fixed, such that no elements can be added or removed with add or remove (UnsupportedOperationException is thrown).

Is there a good reason for that? It looks like an odd restriction to me.

The documentation does not explain the reason behind it:

Returns a fixed-size list backed by the specified array.

Answer 1

The point is that Arrays.asList is returning a view of the array, and that changes to the array are reflected in the List and vice versa. It's not making a copy, it's just a very simple implementation of the List interface that interprets the specified array as a List. As a result, Arrays.asList is really just reflecting that Java arrays are fixed-size, too. You can't add or remove elements from the backing array, either.

As mentioned in the comments, it's quite easy to get a variable-size list from an array with e.g. new ArrayList<>(Arrays.asList(array)).

Also, for what it's worth: Guava actually regrets providing a "resizable list from varargs arguments" method after discovering that most people who used it actually really wanted an immutable list.