How to Create a True Copy of a 2D Array in Java?

Copying 2D Arrays in Java

Copying two-dimensional (2D) arrays in Java can be essential for maintaining persistence or working with copies that can be modified without affecting the original array.

The problem encountered in the given Java code stems from the fact that old = current and current = old assign references to the same 2D array instead of creating independent copies. As a result, any changes made to current are also reflected in old.

To create a true copy of a 2D array, it's necessary to create a new array and copy each element into it. This can be achieved using loop-based approaches or by leveraging the streams API introduced in Java 8.

Streams API Method:

<code class="java">int[][] copy = Arrays.stream(matrix).map(int[]::clone).toArray(int[][]::new);</code>

In this example, map(int[]::clone) creates a new copy of each row in the matrix array. The toArray(int[][]::new) method then creates a new 2D array with the copied rows.

Loop-Based Method:

<code class="java">int[][] copy = new int[current.length][current[0].length];
for (int i = 0; i < current.length; i++) {
  for (int j = 0; j < current[i].length; j++) {
    copy[i][j] = current[i][j];
  }
}</code>

In this approach, a new 2D array (copy) with the same dimensions as current is created. Then, a nested loop iterates over each element in current and copies its value into the corresponding position in copy.

Once a copy is made, old can be assigned to the copied array (old = copy) while current can continue to be updated without affecting old.

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