Immutable array in Java

Question

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));

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);

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.

There is one way to make an immutable array in Java:

final String[] IMMUTABLE = new String[0];

Arrays with 0 elements (obviously) cannot be mutated.

This can actually come in handy if you are using the List.toArray method to convert a List to an array. Since even an empty array takes up some memory, you can save that memory allocation by creating a constant empty array, and always passing it to the toArray method. That method will allocate a new array if the array you pass doesn't have enough space, but if it does (the list is empty), it will return the array you passed, allowing you to reuse that array any time you call toArray on an empty List.

final static String[] EMPTY_STRING_ARRAY = new String[0];

List<String> emptyList = new ArrayList<String>();
return emptyList.toArray(EMPTY_STRING_ARRAY); // returns EMPTY_STRING_ARRAY

As of Java 9 you can use List.of(...), JavaDoc.

This method returns an immutable List and is very efficient.