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

04 Dec, 2020
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:
Using addAll() methodUsing a copy constructor
Using addAll() method
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);    }}
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:
Create a list to be cloned.Clone the list bypassing the original list as the parameter of the copy constructor of ArrayList.
Create a list to be cloned.
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);     } }
[GeeksForGeeks, Coding, Portal]
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