How to Copy and Add all List Elements to an Empty ArrayList in Java?

We can copy and add List items in Array List using addAll() method. This method accepts a Collection (Ex. List) as an argument and adds the collection items at the end of its calling Collection (Ex. ArrayList). This method returns a boolean value. addAll() return true if the collection successfully adds else it returns false.

We can clone the list by two ways:

1. Using addAll() method
2. Using a copy constructor

Method 1: Using addAll() method

Syntax:

public boolean addAll(Collection c);

Parameter: It accepts a collection to be cloned as its parameter

Return Value: This method returns a boolean value. addAll() return true if the collection successfully adds else it returns false.

Exception: It will throw Null Pointer Exception if the specified Collection is null.

Java

// Java program to clone the list

import java.util.ArrayList;
import java.util.List;

public class GFG {

    public static void main(String[] args)
    {
        // create ArrayList
        ArrayList<String> ArrList = new ArrayList<String>();

        // Adding elements to the ArrayList
        ArrList.add("item 1");
        ArrList.add("item 2");
        ArrList.add("item 3");

        System.out.println("ArrayList = " + ArrList);

        // create List
        List<String> ListItem = new ArrayList<String>();
        ListItem.add("item 4");
        ListItem.add("item 5");

        // add List items in ArrayList
        ArrList.addAll(ListItem);

        System.out.println(
            "After Adding List Item in ArrayList = "+ ArrList);
    }
}

Output

ArrayList = [item 1, item 2, item 3]
After Adding List Item in ArrayList = [item 1, item 2, item 3, item 4, item 5]

Method 2: Using a copy constructor

Using the ArrayList constructor in Java, a new list can be initialized with the elements from another collection.

Syntax:

ArrayList arrlist = new ArrayList(collection c);

Here, c is the collection containing elements to be added to this list.

Approach:

1. Create a list to be cloned.
2. Clone the list bypassing the original list as the parameter of the copy constructor of ArrayList.

Java

// Program to clone a List in Java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

class Example {
    public static void main(String[] args)
    {
        // Create a list
        List<String> original = Arrays.asList(
                "GeeksForGeeks",
                "Coding",
                "Portal");

        // Clone the list
        List<String> cloned_arraylist
            = new ArrayList<String>(original);

        System.out.println(cloned_arraylist);
    }
}

Output

[GeeksForGeeks, Coding, Portal]