Why System.arraycopy() function is not creating copy but returning reference to the same array?

Question

I have written following piece of code to check the behavior of System.arraycopy and clone functions. I expect these functions to return a copy of the array but all they are doing is returning a reference to the original array, which is evident from the later part of program where I change the values of original. The copy should not change but it also changes. Please help why its behaving in this way?

public class Testing {

    public static int a[][] = new int[2][2];

    public static void setValueOfA() {
        a[0][0] = 1;
        a[0][1] = 1;
        a[1][0] = 1;
        a[1][1] = 1;
    }

    public static int[][] getValueOfA() {
        int[][] t = new int[2][2];

// Case 1: Not working
//        t = (int[][]) a.clone();
// Case 2: Not working
//        System.arraycopy(a, 0, t, 0, 2);
// Case 3: Working
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 2; j++) {
                t[i][j] = a[i][j];
            }
        }
        return t;
    }

    public static void main(String[] args) {
        int[][] temp;

        setValueOfA();
        temp = getValueOfA();
        System.out.println("Value of a");
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 2; j++) {
                System.out.print(a[i][j] + " ");
            }
            System.out.println();
        }
        System.out.println("Value of temp");
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 2; j++) {
                System.out.print(temp[i][j] + " ");
            }
            System.out.println();
        }

        a[0][0] = 2; a[0][1] = 2; a[1][0] = 2; a[1][1] = 2;

        System.out.println("Value of a");
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 2; j++) {
                System.out.print(a[i][j] + " ");
            }
            System.out.println();
        }
        System.out.println("Value of temp");
        for (int i = 0; i < 2; i++) {
            for (int j = 0; j < 2; j++) {
                System.out.print(temp[i][j] + " ");
            }
            System.out.println();
        }
    }
}

System.arraycopy does not create anything and it does not return any reference (i.e. it's void). — weston
– weston, Commented Jan 12, 2015 at 12:01

Eran · Accepted Answer · 2015-01-12 11:54:31Z

2

I believe (haven't tested it) that System.arraycopy performs a shallow copy of the source array to the target array.

Your call to System.arraycopy is equivalent to :

t[0] = a[0];
t[1] = a[1];

Since a[0] and a[1] are themselves arrays, if you later change a[i][j], you are also changing t[i][j] (since a[i] and t[i] refer to the same array).

answered Jan 12, 2015 at 11:54

Eran

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Noor Dawod · Accepted Answer · 2015-01-12 11:52:09Z

0

MUN,

You should use Arrays.copyOf() function. See an example:

http://www.tutorialspoint.com/java/util/arrays_copyof_int.htm

/N.

answered Jan 12, 2015 at 11:52

Noor Dawod

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Sarath Pillai · Accepted Answer · 2020-05-24 19:28:31Z

0

Arrays.copyOf implicitly uses System.arraycopy which does only shallow copy.

To do a deep copy use the traditional way of iterating and assigning.

answered May 24, 2020 at 19:28

Sarath Pillai

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