When change values of cloned array, changes the source array values

Question

Please see this below code, i have two 2D array, when i change values of the goalState array, the values of startState changes:

public static void main(String[] args) throws CloneNotSupportedException {
    int[][] startState = new int[][]{{1, 2, 3}, {8, 0, 4}, {7, 6, 5}};
    int[][] goalState = new int[][]{};

    goalState = startState.clone();

    goalState[0][1] = 12;

    System.out.println(Arrays.deepToString(startState));
    System.out.println(Arrays.deepToString(goalState));
}

output:

[[1, 12, 3], [8, 0, 4], [7, 6, 5]]
[[1, 12, 3], [8, 0, 4], [7, 6, 5]]

you could find the answer here stackoverflow.com/questions/14149733/… — jmen7070
– jmen7070, Commented May 14, 2020 at 16:30

Vikas · Accepted Answer · 2020-05-14 16:25:15Z

1

You need to clone as below,

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

With java-8+,

int[][] goalState = Arrays.stream(startState)
                          .map(int[]::clone)
                          .toArray(int[][]::new);

answered May 14, 2020 at 16:25

Vikas

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1 Comment

Gus Over a year ago

Why this works: clone() is essentially a shallow copy (stackoverflow.com/questions/1175620/…), so you need to "go deeper" and clone each object separately if you don't want changes to both

GK7 · Accepted Answer · 2020-05-14 17:29:56Z

So, After analysing heap dump of your code , it seems on doing startState.clone(), then all the references of 1 d array (like reference of {1,2,3}) are copied from startState to goalState

GoalState memory object: That's why any change you do in goalState will be reflected in startState and vice -versa.

So , in order to fix this all you need to do is:

    int[][] startState = new int[][]{{1, 2, 3}, {8, 0, 4}, {7, 6, 5}};
    int[][] goalState = new int[startState.length][startState[0].length];

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

Note, this will only work in case of primitive type array ,as In primitive type 1d array only value of int is copied, and that's why we can use this feature in cloning of 2d array also

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