[java]Arrays.copyOf() VS System.arrayCopy()

If we want to copy an array, we can use either System.arraycopy() or Arrays.copyOf(). In this post, I use a simple example to demonstrate the difference between the two.

1. Simple Code Examples

System.arraycopy()

int[] arr = {1,2,3,4,5};   
int[] copied = new int[10];
System.arraycopy(arr, 0, copied, 1, 5);//5 is the length to copy 
System.out.println(Arrays.toString(copied));

Output:

[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 1, 2, 3, 4, 5, 0, 0, 0, 0]

Arrays.copyOf()

int[] copied = Arrays.copyOf(arr, 10); //10 the the length of the new array
System.out.println(Arrays.toString(copied));   copied = Arrays.copyOf(arr, 3);
System.out.println(Arrays.toString(copied));

Output:

[1, 2, 3, 4, 5, 0, 0, 0, 0, 0]
[1, 2, 3]

2. The Major Difference

The difference is that Arrays.copyOf does not only copy elements, it also creates a new array.System.arrayCopy copies into an existing array.

If we read the source code of Arrays.copyOf(), we can see that it uses System.arraycopy().

public static int[] copyOf(int[] original, int newLength) { 
   int[] copy = new int[newLength]; 
   System.arraycopy(original, 0, copy, 0, Math.min(original.length, newLength)); 
   return copy; 
}