Java: Is int[][] a primitive type? Or does it generate a different instance with clone() method?

Question

I'm working with an int[n][n] board:

0 2 3 
4 1 5 
7 8 6

I want to make a copy called twin, then return the modified copy.

For example:

    int [][] twin = board.clone();

    twin[0][0] = board[0][1];
    twin[0][1] = board[0][0];

    return twin;

What I expected is:

//board
0 2 3 
4 1 5 
7 8 6 

//twin
2 0 3 
4 1 5 
7 8 6

But the result is:

//board
2 2 3 
4 1 5 
7 8 6 

//twin
2 2 3 
4 1 5 
7 8 6

The board and twin were the same all along and the clone was not working as expected. Is it because int[][] is not an object?

Is there a way to clone int[][] and modify the way I was expecting? Should I loop over board and copy the values to twin?

Hey take a gander at how to clone a 2d array: https://stackoverflow.com/questions/1686425/copy-a-2d-array-in-java — Edwin Diaz, Mar 22 '19 at 23:10

score 6 · Accepted Answer · answered Mar 22 '19 at 23:13

First, an array is not a primitive type; it's an object. Second, multidimensional arrays are implemented as arrays of arrays. Also, the clone method for arrays is shallow. When you clone a multi-dimensional array, your new multidimensional array still refers back to the original internal arrays.

board -> [ .           .           . ]
           |           |           |
           v           v           v
          [0, 2, 3], [4, 1, 5], [7, 8, 6]
           ^           ^           ^
           |           |           |
 twin -> [ .           .           . ]

So when you modify the values, you're not copying from two different arrays, you're copying from the same array, which is why you wind up with [2, 2, 3].

If you're going to clone a multidimensional array, you'll need to clone all the way down to the one-dimensional arrays also.

int [][] twin = board.clone();
for (int i = 0; i < board.length; i++) {
    twin[i] = board[i].clone();
}

Now I get [[2, 0, 3], [4, 1, 5], [7, 8, 6]].

Other possibilities include declaring your new multidimensional array without cloning, and using loops and System.arraycopy to copy individual one-dimensional arrays.

score 1 · Answer 2 · answered Mar 23 '19 at 00:16

Quoting JLS Chapter 10. Arrays:

In the Java programming language, arrays are objects

No int[][] is not a primitive.

Quoting JLS 10.7. Array Members:

A clone of a multidimensional array is shallow, which is to say that it creates only a single new array. Subarrays are shared.

The outer array is a new instance, the subarrays are not.

Java: Is int[][] a primitive type? Or does it generate a different instance with clone() method?

2 Answers2