Best Way to Copy an Array in Java

General Discussions
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1

What is the best way to copy an array in Java?

I have an array a that keeps getting updated. For example, a = [1,2,3,4,5]. I need to create an exact copy of a and store it in b, so that even if a changes to [6,7,8,9,10], b should still hold [1,2,3,4,5]. I tried using a loop like:

for(int i=0; i<5; i++) {  
    b[i] = a[i];  
}

But it doesn’t seem to work as expected. What is the correct way to copy an array in Java without using complex terms?

2

If you’re looking for a simple, built-in way to copy an array in Java, Arrays.copyOf() is your best friend.

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] a = {1, 2, 3, 4, 5};
        int[] b = Arrays.copyOf(a, a.length);

        // Modify 'a' to check if 'b' remains unchanged
        a[0] = 99;

        System.out.println(Arrays.toString(a)); // [99, 2, 3, 4, 5]
        System.out.println(Arrays.toString(b)); // [1, 2, 3, 4, 5] (unchanged)
    }
}

:white_check_mark: Why use this?

  • Short and efficient—no need to write a loop manually.
  • Creates a completely new array, so changes in a don’t affect b.

:rotating_light: When to avoid?

  • If you need a deep copy (for arrays of objects), this won’t be enough.
3

Arrays.copyOf() is great, but if you’re dealing with large arrays and need better performance, System.arraycopy() is the way to go.

public class Main {
    public static void main(String[] args) {
        int[] a = {1, 2, 3, 4, 5};
        int[] b = new int[a.length];

        System.arraycopy(a, 0, b, 0, a.length);

        a[0] = 99; // Modify original array

        System.out.println(java.util.Arrays.toString(a)); // [99, 2, 3, 4, 5]
        System.out.println(java.util.Arrays.toString(b)); // [1, 2, 3, 4, 5] (unchanged)
    }
}

:white_check_mark: Why use this?

  • Faster than loops, especially for large arrays.
  • More flexible—you can copy only a specific range if needed.

:rotating_light: When to avoid?

  • For small arrays, Arrays.copyOf() is simpler.
4

There’s also the clone() method, which is a quick way to copy an array in Java.

public class Main {
    public static void main(String[] args) {
        int[] a = {1, 2, 3, 4, 5};
        int[] b = a.clone();

        a[0] = 99; // Modify original array

        System.out.println(java.util.Arrays.toString(a)); // [99, 2, 3, 4, 5]
        System.out.println(java.util.Arrays.toString(b)); // [1, 2, 3, 4, 5] (unchanged)
    }
}

:white_check_mark: Why use this?

  • Simple and fast—like Arrays.copyOf().
  • Works great for primitive arrays (e.g., int[], double[]).

:rotating_light: When to avoid?

  • If the array contains objects, clone() only copies references (shallow copy).