How to Copy ArrayList in Java

1. Copy ArrayList to Another by Passing It to Another ArrayList’s Constructor
2. Copy ArrayList to Another Using the addAll() Fuction
3. Copy ArrayList Using Java 8 Stream
4. Copy ArrayList to Another Using the clone() Method

After we have introduced how to copy array in Java in another article, we will introduce four methods to copy an ArrayList to another ArrayList in Java in this article. We will use the same elements in every example to copy an ArrayList using different methods.

Copy ArrayList to Another by Passing It to Another ArrayList’s Constructor

An ArrayList in Java can have three types of constructor. We can create an ArrayList object with an empty constructor, with initial size, or a collection in which the ArrayList is created using the collection’s elements.

We will use the third type of constructor; we first create an ArrayList names1 with an empty constructor and then add some random names. We create a new ArrayList names2 to copy the elements of names1 to it and pass names1 to the constructor new ArrayList<>(names1).

At last, we print the whole names2 ArrayList using forEach that prints each element.

import java.util.ArrayList;

public class CopyArrayList {
  public static void main(String[] args) {
    ArrayList<String> names1 = new ArrayList<>();
    names1.add("Alan");
    names1.add("Alex");
    names1.add("Bob");
    names1.add("Bryan");
    names1.add("Cathy");
    names1.add("Drake");

    ArrayList<String> names2 = new ArrayList<>(names1);

    names2.forEach(System.out::println);
  }
}

Output:

Alan
Alex
Bob
Bryan
Cathy
Drake

Copy ArrayList to Another Using the addAll() Fuction

ArrayList comes with a function addAll() that takes a Collection as an argument and adds or appends the given collection’s elements at the end of the ArrayList if there are existing elements. ArrayList implements Collection, which allows us to use the ArrayList names1 as an argument of the addAll() method.

names1 contains a few elements that should be copied to the newly created empty Arraylist names2. And it is done by names2.addAll(names1). The output shows the copied elements of names2.

import java.util.ArrayList;

public class CopyArrayList {
  public static void main(String[] args) {
    ArrayList<String> names1 = new ArrayList<>();
    names1.add("Alan");
    names1.add("Alex");
    names1.add("Bob");
    names1.add("Bryan");
    names1.add("Cathy");
    names1.add("Drake");

    ArrayList<String> names2 = new ArrayList<>();
    names2.addAll(names1);

    names2.forEach(System.out::println);
  }
}

Output:

Alan
Alex
Bob
Bryan
Cathy
Drake

Copy ArrayList Using Java 8 Stream

In this example, we use the new Stream API introduced in Java 8. We create an ArrayList with elements, then call the stream() method with names1 to use the stream methods like the collect() method that collects the stream and folds it into a list using Collectors.toList().

This stream returns a List, which needs to be cast to an ArrayList.

import java.util.ArrayList;
import java.util.stream.Collectors;

public class CopyArrayList {
  public static void main(String[] args) {
    ArrayList<String> names1 = new ArrayList<>();
    names1.add("Alan");
    names1.add("Alex");
    names1.add("Bob");
    names1.add("Bryan");
    names1.add("Cathy");
    names1.add("Drake");

    ArrayList<String> names2 = (ArrayList<String>) names1.stream().collect(Collectors.toList());

    names2.forEach(System.out::println);
  }
}

Output:

Alan
Alex
Bob
Bryan
Cathy
Drake

Copy ArrayList to Another Using the clone() Method

The last method is the clone() method that is a native ArrayList method. It copies the elements and returns a new List, similar to the previous solution. We create an ArrayList with elements and call the clone() method. At last, we cast the returned results to ArrayList<String> to get our desired result.

import java.util.ArrayList;

public class CopyArrayList {
  public static void main(String[] args) {
    ArrayList<String> names1 = new ArrayList<>();
    names1.add("Alan");
    names1.add("Alex");
    names1.add("Bob");
    names1.add("Bryan");
    names1.add("Cathy");
    names1.add("Drake");

    ArrayList<String> names2 = (ArrayList<String>) names1.clone();

    names2.forEach(System.out::println);
  }
}

Output:

Alan
Alex
Bob
Bryan
Cathy
Drake