Is there any way to make an ordinary array immutable in Java?

Question

Googled for it, found plenty of code. But any of them gave me what I want. I want to make an ordinary array Immutable. I tried this:

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class test {

    public static void main(String[] args) {

        final Integer array[];

        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);

        array = immutable.toArray(new Integer[immutable.size()]);

        for(int i=0; i<array.length; i++)
            System.out.println(array[i]);

        array[0] = 5;

        for(int i=0; i<array.length; i++)
            System.out.println(array[i]);

    }
}

But it doesnt work, I CAN assign a 5 into array[0] ... Is there any way to make this array immutable?

http://stackoverflow.com/questions/3700971/immutable-array-in-java — Evgeni Dimitrov, Jun 15 '13 at 11:12
Making you array variable final only prevents assigning a new array to it. As far as I know there no way to make it's values final. — André Stannek, Jun 15 '13 at 11:12
because my teacher said that this array cant be a collection :/ — yak, Jun 15 '13 at 11:13
Well in that case you write your own wrapper, but let me tell you it would still be a collection:) . be it array or arraylist end of the day you use to for storing data. — ajay.patel, Jun 15 '13 at 11:15
@yak your teacher has some fundamental misunderstandings about arrays, _or_ you did not tell us everything... But in any case SO is not a "homework solving" site — fge, Jun 15 '13 at 11:17
@zerocool it may be a collection of elements, but not a `Collection` as defined by the JDK — fge, Jun 15 '13 at 11:21
Agreed, but yak says that his teacher says that this Array Cant Be Collection, just wanted to clarify that even array is collection. I understand that its not same as java.util.Collection. — ajay.patel, Jun 15 '13 at 11:25
A small point of pedantry: An immutable list is not the same thing as an unmodifiable list. An immutable list *will not change*. An unmodifiable list is one that *you can't change through that list's regular access methods*. But `Collections.unmodifiableList` merely wraps the original list rather than creating a copy, so as long as you keep the original reference to `temp` around, you can call `temp.add` and watch the "immutable" list change during its lifetime. (On the other hand, `toArray` does create a copy, so even if the list really were immutable, the new array is always modifiable.) — Boann, Jun 15 '13 at 11:44

fge · Accepted Answer · 2013-06-15T11:30:15.480

If you want to use it as an array, you can't.

You have to create a wrapper for it, so that you throw an exception on, say, .set(), but no amount of wrapping around will allow you to throw an exception on:

array[0] = somethingElse;

Of course, immutability of elements is another matter entirely!

NOTE: the standard exception to throw for unsupported operations is aptly named UnsupportedOperationException; as it is unchecked you don't need to declare it in your method's throws clause.

score 3 · Answer 2 · answered Jun 15 '13 at 11:15

3

It's impossible with primitive arrays.

You will have to use the Collections.unmodifiableList() as you already did in your code.

answered Jun 15 '13 at 11:15

darijan

9,725
25
38

score 1 · Answer 3 · edited Sep 20 '17 at 08:37

1

You can also use Guava's ImmutableList, a high-performance, immutable, random-access List implementation that does not permit null elements. Unlike Collections.unmodifiableList(java.util.List), which is a view of a separate collection that can still change, an instance of ImmutableList contains its own private data and will never change.

edited Sep 20 '17 at 08:37

Dustin Nieffenegger

618
7
22

answered Jun 15 '13 at 11:37

1218985

7,531
2
25
31

However, OP says the "end result" must not be a `Collection`... Otherwise, yeah, Guava's `Immutable*` rock. – fge Jun 15 '13 at 11:42

Is there any way to make an ordinary array immutable in Java?

3 Answers3

Linked