Java Array Copying Techniques: A Comprehensive Guide

Java Array Copying Techniques

Learn how to duplicate one‑dimensional and two‑dimensional arrays in Java using assignment, loops, System.arraycopy, Arrays.copyOfRange, and more, with clear examples and best‑practice tips.

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1. Copying with the Assignment Operator

Assigning one array variable to another is the simplest method. It copies the reference, not the data.

class Main {
    public static void main(String[] args) {
        int [] numbers = {1, 2, 3, 4, 5, 6};
        int [] positiveNumbers = numbers;    // reference copy
        for (int number: positiveNumbers) {
            System.out.print(number + ", ");
        }
    }
}

Output:

1, 2, 3, 4, 5, 6

Both numbers and positiveNumbers now point to the same array object. Changing one element affects the other:

class Main {
    public static void main(String[] args) {
        int [] numbers = {1, 2, 3, 4, 5, 6};
        int [] positiveNumbers = numbers;
        numbers[0] = -1;
        for (int number: positiveNumbers) {
            System.out.print(number + ", ");
        }
    }
}

Output:

-1, 2, 3, 4, 5, 6

This is a shallow copy. To avoid unintended side effects, use a deep copy technique.

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2. Deep Copy with a Loop

Iterating over each element and assigning it to a new array creates an independent copy.

import java.util.Arrays;
class Main {
    public static void main(String[] args) {
        int [] source = {1, 2, 3, 4, 5, 6};
        int [] destination = new int[source.length];
        for (int i = 0; i < source.length; ++i) {
            destination[i] = source[i];
        }
        System.out.println(Arrays.toString(destination));
    }
}

Output:

[1, 2, 3, 4, 5, 6]

Because source and destination are distinct objects, modifications to one do not affect the other.

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3. Using System.arraycopy()

Java’s System.arraycopy method offers a fast, built‑in way to copy array segments.

arraycopy(Object src, int srcPos, Object dest, int destPos, int length)

• src – source array
• srcPos – starting index in the source
• dest – destination array
• destPos – starting index in the destination
• length – number of elements to copy

Example:

import java.util.Arrays;
class Main {
    public static void main(String[] args) {
        int[] n1 = {2, 3, 12, 4, 12, -2};
        int[] n2 = new int[n1.length];
        System.arraycopy(n1, 0, n2, 0, n1.length);
        System.out.println("n2 = " + Arrays.toString(n2));
        int[] n3 = new int[5];
        System.arraycopy(n1, 2, n3, 1, 2);
        System.out.println("n3 = " + Arrays.toString(n3));
    }
}

Output:

n2 = [2, 3, 12, 4, 12, -2]
n3 = [0, 12, 4, 0, 0]

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4. Copying with Arrays.copyOfRange()

When you need a new array that contains a specific slice, Arrays.copyOfRange is ideal.

import java.util.Arrays;
class ArraysCopy {
    public static void main(String[] args) {
        int[] source = {2, 3, 12, 4, 12, -2};
        int[] destination1 = Arrays.copyOfRange(source, 0, source.length);
        System.out.println("destination1 = " + Arrays.toString(destination1));
        int[] destination2 = Arrays.copyOfRange(source, 2, 5);
        System.out.println("destination2 = " + Arrays.toString(destination2));
    }
}

Output:

destination1 = [2, 3, 12, 4, 12, -2]
destination2 = [12, 4, 12]

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5. Copying 2D Arrays with a Loop

Deep copying a two‑dimensional array requires copying each row individually.

import java.util.Arrays;
class Main {
    public static void main(String[] args) {
        int[][] source = {
              {1, 2, 3, 4},
              {5, 6},
              {0, 2, 42, -4, 5}
        };
        int[][] destination = new int[source.length][];
        for (int i = 0; i < destination.length; ++i) {
            destination[i] = new int[source[i].length];
            for (int j = 0; j < destination[i].length; ++j) {
                destination[i][j] = source[i][j];
            }
        }
        System.out.println(Arrays.deepToString(destination));
    }
}

Output:

[[1, 2, 3, 4], [5, 6], [0, 2, 42, -4, 5]]

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6. 2D Array Copy with System.arraycopy()

Replace the inner loop with arraycopy for cleaner code.

import java.util.Arrays;
class Main {
    public static void main(String[] args) {
        int[][] source = {
              {1, 2, 3, 4},
              {5, 6},
              {0, 2, 42, -4, 5}
        };
        int[][] destination = new int[source.length][];
        for (int i = 0; i < source.length; ++i) {
            destination[i] = new int[source[i].length];
            System.arraycopy(source[i], 0, destination[i], 0, destination[i].length);
        }
        System.out.println(Arrays.deepToString(destination));
    }
}

Output:

[[1, 2, 3, 4], [5, 6], [0, 2, 42, -4, 5]]