Why does Array.copyOf() mutate the original array in case of 2D Arrays?

Question

If I create a 2D int array in Java, and then make a copy of it using Arrays.copyOf(), like so -

jshell> int[][] c1 = {{1,2}, {3,4}}
c         


        

Answer 1

Method Arrays.copyOf does not perform a deep copy of an array, nor does System.arraycopy method. You should implement the algorithm of deep copy of the array yourself with the required copy depth. For example:

int[][] arr1 = {{1, 2}, {3, 4}};                 // original array
int[][] arr2 = Arrays.copyOf(arr1, arr1.length); // shallow copy
int[][] arr3 = Arrays.stream(arr1)               // deep copy
        .map(Arrays::stream)
        .map(IntStream::toArray)
        .toArray(int[][]::new);

arr1[0][0] = 7;

System.out.println(Arrays.deepToString(arr1)); // [[7, 2], [3, 4]]
System.out.println(Arrays.deepToString(arr2)); // [[7, 2], [3, 4]]
System.out.println(Arrays.deepToString(arr3)); // [[1, 2], [3, 4]]

And the same is true for an array of objects:

public class Test {
    public static void main(String[] args) {
        SomeObject[] arr1 = {                                 // original array
                new SomeObject(1),
                new SomeObject(2),
                new SomeObject(3),
                new SomeObject(4)};
        SomeObject[] arr2 = Arrays.copyOf(arr1, arr1.length); // shallow copy
        SomeObject[] arr3 = Arrays.stream(arr1)               // deep copy
                .mapToInt(SomeObject::getField)
                .mapToObj(SomeObject::new)
                .toArray(SomeObject[]::new);

        arr1[0].setField(7);

        System.out.println(Arrays.toString(arr1)); // [7, 2, 3, 4]
        System.out.println(Arrays.toString(arr2)); // [7, 2, 3, 4]
        System.out.println(Arrays.toString(arr3)); // [1, 2, 3, 4]
    }

    static class SomeObject {
        int field;

        public SomeObject(int field) { this.field = field; }

        public int getField() { return field; }

        public void setField(int field) { this.field = field; }

        public String toString() { return String.valueOf(field); }
    }
}