How do I prevent the modification of a private field in a class?

Question

If you can use a List instead of an array, Collections provides an unmodifiable list:

public List<String> getList() {
    return Collections.unmodifiableList(list);
}

You must return a copy of your array.

public String[] getArr() {
  return arr == null ? null : Arrays.copyOf(arr, arr.length);
}

Modifier private protects only field itself from being accessed from other classes, but not the object references by this field. If you need to protect referenced object, just do not give it out. Change

public String [] getArr ()
{
    return arr;
}

to:

public String [] getArr ()
{
    return arr.clone ();
}

or to

public int getArrLength ()
{
    return arr.length;
}

public String getArrElementAt (int index)
{
    return arr [index];
}

The Collections.unmodifiableList has already been mentioned - the Arrays.asList() strangely not! My solution would also be to use the list from the outside and wrap the array as follows:

String[] arr = new String[]{"1", "2"}; 
public List<String> getList() {
    return Collections.unmodifiableList(Arrays.asList(arr));
}

The problem with copying the array is: if you're doing it every time you access the code and the array is big, you'll create a lot of work for the garbage collector for sure. So the copy is a simple but really bad approach - I'd say "cheap", but memory-expensive! Especially when you're having more than just 2 elements.

If you look at the source code of Arrays.asList and Collections.unmodifiableList there is actually not much created. The first just wraps the array without copying it, the second just wraps the list, making changes to it unavailable.