Question

I am having a little trouble understanding the concept of final in Java.

I have a class that follows:

 public class MyClass 
 {
     private int[][] myArray; // intended to be changed
     private final int[][] MYARRAY_ORIGINAL; // intended to be unchangable

     public MyClass(int[][] array) 
     {
        myArray = array;
        MYARRAY_ORIGINAL = array;
     }

 }

I was under the understanding that final would make MYARRAY_ORIGINAL read only. But I have tried editing myArray, and it edits MYARRAY_ORIGINAL as well. My question is, in this context, what exactly does final do? And for extra credit, how can I copy the array passed through the constructor into MYARRAY_ORIGINAL so that I can have 2 arrays, one to edit, and one that will remain preserved?

Answer 1

Your final MYARRAY_ORIGINAL is indeed read only: you can’t assign a new value to the MYARRAY_ORIGINAL reference in other side than class constructor or attribute declaration:

public void someMethod() {
    //it won't compile
    MYARRAY_ORIGINAL = new int[X][];
}

The values inside the array are not final. Those values can change anytime in the code.

public void anotherMethod() {
    MYARRAY_ORIGINAL[0][0] = 25;
    //later in the code...
    MYARRAY_ORIGINAL[0][0] = 30; //it works!
}

If you indeed need a List of final elements, in other words, a List whose elements can’t be modified, you can use Collections.unmodifiableList:

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

The last piece of code was taken from here: Immutable array in Java