Immutable array in Java

Question

Is there an immutable alternative to the primitive arrays in Java? Making a primitive array final doesn't actually prevent one from doing something like

final int[] array = new int[] {0, 1, 2, 3};
array[0] = 42;

I want the elements of the array to be unchangeable.

Answer 1

Not with primitive arrays. You'll need to use a List or some other data structure:

List<Integer> items = Collections.unmodifiableList(Arrays.asList(0,1,2,3));

Answer 2

No, this is not possible. However, one could do something like this:

List<Integer> temp = new ArrayList<Integer>();
temp.add(Integer.valueOf(0));
temp.add(Integer.valueOf(2));
temp.add(Integer.valueOf(3));
temp.add(Integer.valueOf(4));
List<Integer> immutable = Collections.unmodifiableList(temp);

This requires using wrappers, and is a List, not an array, but is the closest you will get.

Answer 3

As others have noted, you can't have immutable arrays in Java.

If you absolutely need a method that returns an array that doesn't influence the original array, then you'd need to clone the array each time:

public int[] getFooArray() {
  return fooArray == null ? null : fooArray.clone();
}

Obviously this is rather expensive (as you'll create a full copy each time you call the getter), but if you can't change the interface (to use a List for example) and can't risk the client changing your internals, then it may be necessary.

This technique is called making a defensive copy.

Answer 4

My recommendation is to not use an array or an unmodifiableList but to use Guava's ImmutableList, which exists for this purpose.

ImmutableList<Integer> values = ImmutableList.of(0, 1, 2, 3);

Answer 5

If you need (for performance reason or to save memory) native 'int' instead of 'java.lang.Integer', then you would probably need to write your own wrapper class. There are various IntArray implementations on the net, but none (I found) was immutable: Koders IntArray, Lucene IntArray. There are probably others.