How to Deep Copy Arraylist in Java

In this article, we are going to learn deep copy ArrayList in Java.
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There are mainly two concepts Shallow copy and deep copy.

When an object gets copied via reference not actual values then it is called Shallow copy. In it, changes made to an object will also reflect to another object. It means if we change elements of one object then the other object will also be changed.

In deep copy, the copied object is completely independent and changes made to it do not reflect to the original object.
Let’s understand with the examples.

Example of Deep Copy ArrayList

Here, we are copying one ArrayList elements to ther using addAll() method and see changes made to second list does not modify original list.

import java.util.ArrayList;
public class Main
{
    public static void main(String args[]) 
    {
        ArrayList list =  new ArrayList<>();
        list.add("India");
        list.add("US");
        list.add("China");
        System.out.println(list);
        ArrayList list2 = new ArrayList<>();
        list2.addAll(list);
        System.out.println(list2);
        list2.set(1, "Russia");
        System.out.println(list2);
    }
}

Output:

Note: This method will only work if ArrayList contains primitive data types or immutable collection. In case, ArrayList contains custom objects, then we need to explicitly clone the custom objects.

Deep Copy using Clone() Method

We can also use clone() method to create a copy of ArrayList but this method create swallow copy.

Let’s see with the help of example.

package org.arpit.java2blog.published;

import java.util.Date;
class Student implements Cloneable{
    String name;
    String id;
    Date dateOfBirth;

    public Student(String name, String id,Date dateOfBirth) {
        this.name = name;
        this.id = id;
        this.dateOfBirth=dateOfBirth;
    }

    @Override
    public Student clone() {
        Student clonedStudent = null;
        try {
            clonedStudent = (Student) super.clone();
            clonedStudent.setDateOfBirth((Date)this.dateOfBirth.clone());
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }

        return clonedStudent;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public Date getDateOfBirth() {
        return dateOfBirth;
    }

    public void setDateOfBirth(Date dateOfBirth) {
        this.dateOfBirth = dateOfBirth;
    }
}

Create main class named CloneArrayListMain.java

import java.util.Date;

public class CloneArrayListMain {

    public static void main(String[] args) {
        ArrayList studentList = new ArrayList<>();
        studentList.add(new Student("Rohan","102",new Date(2002, 02, 12)));
        studentList.add(new Student("David","105",new Date(1999, 05, 30)));

        ArrayList clonedStudentList = (ArrayList) studentList.clone();

        // Update cloned list
        clonedStudentList.get(0).setName("John");
        System.out.println("-----------Original List-----------");
        studentList.forEach(e->System.out.println(e.getName()+" "+e.getId()));
        System.out.println("-----------Copied List----------");
        clonedStudentList.forEach(e->System.out.println(e.getName()+" "+e.getId()));
    }
}

Output

As you can see, changes to clonedStudentList also got reflected in studentList.

To create a true deep copy of ArrayList, we should create a new ArrayList and copy all the cloned elements to new ArrayList one by one and we should also clone Student object properly.

To create deep copy of Student class, we can divide its class members to mutable and immutable types.

• Immutable fields( String data types): We can directly use immutable fields in cloned object. Immutable fields include wrapper classes, String and primitive types.
• Mutable fields(Date data type): We should create new object for the mutable attribute and then assign it to cloned object.

Here is the correct clone method for Student class.

@Override
    public Student clone() {
        Student clonedStudent = null;
        try {
            clonedStudent = (Student) super.clone();
            clonedStudent.setDateOfBirth((Date)this.dateOfBirth.clone());
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }

        return clonedStudent;
    }

As you can see, we have used super.clone() to clone the Student object and then set dateOfBirth explicitly to clonedStudent as Date is mutable field.

Here is code to create deep copy of ArrayList by copying cloned elements one by one to new ArrayList.

ArrayList studentList = new ArrayList<>();
studentList.add(new Student("Rohan","102",new Date(2002, 02, 12)));
studentList.add(new Student("David","105",new Date(1999, 05, 30)));

ArrayList clonedStudentList = new ArrayList<>();

for(Student st:studentList)
{
    clonedStudentList.add(st.clone());
}

Complete code to deep copy ArrayList in java

Here is complete java program to create deep copy of ArrayList in java.

public class CloneArrayListMain {

    public static void main(String[] args) {
        ArrayList studentList = new ArrayList<>();
        studentList.add(new Student("Rohan","102",new Date(2002, 02, 12)));
        studentList.add(new Student("David","105",new Date(1999, 05, 30)));

        ArrayList clonedStudentList = new ArrayList<>();

        for(Student st:studentList)
        {
            clonedStudentList.add(st.clone());
        }
        // Update list
        clonedStudentList.get(0).setName("John");
        System.out.println("-----------Original List-----------");
        studentList.forEach(e->System.out.println(e.getName()+" "+e.getId()));
        System.out.println("-----------Copied List----------");
        clonedStudentList.forEach(e->System.out.println(e.getName()+" "+e.getId()));
    }
}

As you can see, changes made to clonedStudentList did not reflect in original ArrayList studentList.

That’s all about how to deep copy ArrayList in java.