How to Efficiently and Safely Copy a 2D Array in Java?

How to Copy a 2D Array in Java

Question:

You have a 2D array called matrix that you want to copy into a local variable in a method for editing. What's the most efficient and error-proof way to create a complete copy?

Answer:

There are two recommended ways to copy a 2D array in Java:

Using clone():

The clone() method creates a new array with the same size and contents as the original. For 2D arrays, you need to clone each individual row:

<code class="java">int[][] myInt = new int[matrix.length][];
for (int i = 0; i < matrix.length; i++)
    myInt[i] = matrix[i].clone();</code>

Using System.arraycopy():

The System.arraycopy() method can also be used to copy a 2D array. It requires creating a new array and manually copying each element:

<code class="java">int[][] myInt = new int[matrix.length][];
for (int i = 0; i < matrix.length; i++) {
  int[] aMatrix = matrix[i];
  int aLength = aMatrix.length;
  myInt[i] = new int[aLength];
  System.arraycopy(aMatrix, 0, myInt[i], 0, aLength);
}</code>

Advantages:

Both methods are faster and less prone to errors than manually copying the array. System.arraycopy() is particularly efficient because it is implemented in native code.

Note:

In some cases, you may only need a shallow copy of the array, where the modified elements in the copy are reflected in the original array. For a shallow copy, you can simply assign the array directly, e.g., int[][] myInt = matrix;. However, this is not recommended for editing purposes.

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