How to Make a Deep Copy of Java ArrayList?

The Advantage of ArrayList is it can be defined without giving a predefined size. But the disadvantage is it is more expensive to create and maintain. To have the solution for these expenses we can create a deep copy of an ArrayList.

There are two types of copies that can be made the first one is a deep copy and the second one is a shallow copy. In this article, we will learn to make a deep copy of Java ArrayList in Java.

Prerequisite

• Shallow Copy vs Deep Copy
• Java ArrayList

Deep Copy of ArrayList

Unlike shallow copy in deep copy, we entirely make a new ArrayList without referring to the old ArrayList. In this, instead of duplicating the ArrayList it also copies the objects in it. When we make a deep copy we entirely make a new ArrayList at the new address and then copy the elements one by one. Therefore if we make any changes in one list it doesn’t get reflected in the second ArrayList.

Following is the example of code in which we will make a deep copy of an ArrayList in Java.

Java

// Java Program to demonstrate Deep Copy of an ArrayList
import java.util.ArrayList;

public class DeepCopyExample {
    // Method to create a deep copy of an ArrayList
    private static ArrayList<String>
    deepCopy(ArrayList<String> originalList)
    {
        // Create a new ArrayList to store the copied
        // elements
        ArrayList<String> copiedList = new ArrayList<>();

        // Iterate over each element in the original list
        for (String item : originalList) {
            // Creating a new instance of each element
            copiedList.add(new String(item));
        }

        // Return the deep copied list
        return copiedList;
    }

    public static void main(String[] args)
    {
        // Creating the original ArrayList
        ArrayList<String> originalList = new ArrayList<>();
        // Add string "1" to the list
        originalList.add("1");

        // Add string "2" to the list
        originalList.add("2");

        // Add string "3" to the list
        originalList.add("3");

        // Creating a deep copy of the ArrayList
        ArrayList<String> copiedList
            = deepCopy(originalList);

        // Modify the copied list
        // Add string "4" to the copied list
        copiedList.add("4");

        // Print both the original and copied lists

        // Print original list
        System.out.println("Original List: "
                           + originalList);

        // Print copied list
        System.out.println("Copied List: " + copiedList);
    }
}

Output:

Original List: [1,2,3]
Copied List: [1,2,3,4]

Explaination of the above Program:

1. A class method DeepCopy replicates each element of an ArrayList into a new instance, creating a truly independent copy.
2. Starting with “1”, “2”, and “3”, the program generates a deep copy of the list.
3. Element “4” is added only to the copy, leaving the original unchanged.
4. Both lists are displayed, revealing the separation.