Array Copying: System.arraycopy vs Arrays.copyOf

1Concept

Copying arrays with manual for-loops is slow. `System.arraycopy()` is a native C/assembly method that performs a direct hardware memory block transfer (memmove). `Arrays.copyOf()` internally uses `System.arraycopy()`, while allocating a newly sized array.

2Architecture Diagram

[ Source Array ] [10, 20, 30, 40, 50]
                          |
                 System.arraycopy() (Native C memmove)
                          v
[ Target Array ] [ 0, 20, 30, 40,  0]

3Code Example

import java.util.Arrays;

public class ArrayCopyPerformanceDemo {
    public static void main(String[] args) {
        int[] source = {10, 20, 30, 40, 50};
        int[] target = new int[5];

        // Native memory block transfer: copy elements 1, 2, 3 into target at index 1
        System.arraycopy(source, 1, target, 1, 3);
        System.out.println("Target via System.arraycopy: " + Arrays.toString(target));

        // Arrays.copyOf (allocates new array with custom length)
        int[] expanded = Arrays.copyOf(source, 8);
        System.out.println("Expanded via Arrays.copyOf:   " + Arrays.toString(expanded));
    }
}

4Expected Output

Target via System.arraycopy: [0, 20, 30, 40, 0]
Expanded via Arrays.copyOf:   [10, 20, 30, 40, 50, 0, 0, 0]

5Key Takeaways

• ✓System.arraycopy is up to 5x faster than manual element-by-element loops.
• ✓Both methods perform shallow copies: object references are copied, NOT the objects themselves.
• ✓ArrayStoreException is thrown at runtime if target array type is incompatible with source.