Can you add a list to a list Java?

The Java.util.List is a child interface of Collection. It is an ordered collection of objects in which duplicate values can be stored. Since List preserves the insertion order, it allows positional access and insertion of elements. List Interface is implemented by ArrayList, LinkedList, Vector and Stack classes.

List is an interface, and the instances of List can be created in the following ways:

Below are the following ways to initialize a list:

1. Using List.add[] method

   Since list is an interface, one cant directly instantiate it. However, one can create objects of those classes which have implemented this interface and instantiate them.

   
   
   
   

   Few classes which have implemented the List interface are Stack, ArrayList, LinkedList, Vector etc.

   Syntax:

   List list=new ArrayList[]; List llist=new LinkedList[]; List stack=new Stack[];

   Examples:

   
   

   
   
   

   import java.util.*; import java.util.function.Supplier; public class GFG { public static void main[String args[]] { // For ArrayList List list = new ArrayList[]; list.add[1]; list.add[3]; System.out.println["ArrayList : " + list.toString[]]; // For LinkedList List llist = new LinkedList[]; llist.add[2]; llist.add[4]; System.out.println["LinkedList : " + llist.toString[]]; // For Stack List stack = new Stack[]; stack.add[3]; stack.add[1]; System.out.println["Stack : " + stack.toString[]]; } }
   Output: ArrayList : [1, 3] LinkedList : [2, 4] Stack : [3, 1]

   Double Brace Initialization can also be used to do the above work.

   Syntax:

   List list=new ArrayList[]{{ add[1]; add[2]; add[3]; }};

   Examples:

   
   

   
   
   

   import java.util.*; public class GFG { public static void main[String args[]] { // For ArrayList List list = new ArrayList[] {{ add[1]; add[3]; } }; System.out.println["ArrayList : " + list.toString[]]; // For LinkedList List llist = new LinkedList[] {{ add[2]; add[4]; } }; System.out.println["LinkedList : " + llist.toString[]]; // For Stack List stack = new Stack[] {{ add[3]; add[1]; } }; System.out.println["Stack : " + stack.toString[]]; } }
   Output:
   

   
   ArrayList : [1, 3] LinkedList : [2, 4] Stack : [3, 1]

2. Using Arrays.asList[]

   • Creating Immutable List

     Arrays.asList[] creates an immutable list from an array. Hence it can be used to instantiate a list with an array.

     Syntax:

     List list=Arrays.asList[1, 2, 3];

     Examples:

     
     

     
     
     

     import java.util.Arrays; import java.util.List; public class GFG { public static void main[String args[]] { // Instantiating List using Arrays.asList[] List list = Arrays.asList[1, 2, 3]; // Print the list System.out.println["List : " + list.toString[]]; } }
     Output: List : [1, 2, 3]
   • Creating Mutable List

     Syntax:

     List list=new ArrayList[Arrays.asList[1, 2, 3]];

     Examples:

     
     

     
     
     

     import java.util.ArrayList; import java.util.Arrays; import java.util.List; public class GFG { public static void main[String args[]] { // Creating a mutable list using Arrays.asList[] List list = new ArrayList[ Arrays.asList[1, 2, 3]]; // Print the list System.out.println["List : " + list.toString[]]; list.add[5]; // Print the list System.out.println["Modified list : " + list.toString[]]; } }
     Output: List : [1, 2, 3] Modified list : [1, 2, 3, 5]

3. Using Collections class methods

   There are various methods in Collections class that can be used to instantiate a list. They are:

   1. Using Collections.addAll[]

      Collections class has a static method addAll[] which can be used to initialize a list. Collections.addAll[] take in any number of elements after it is specified with the Collection in which the elements are to be inserted.

      Syntax:

      
      
      
      List list = Collections.EMPTY_LIST; Collections.addAll[list = new ArrayList[], 1, 2, 3, 4];

      Examples:

      
      

      
      
      

      import java.util.*; public class GFG { public static void main[String args[]] { // Create an empty list List list = new ArrayList[]; // Instantiating list using Collections.addAll[] Collections.addAll[list, 1, 2, 3, 4]; // Print the list System.out.println["List : " + list.toString[]]; } }
      Output: List : [1, 2, 3, 4]

   2. Using Collections.unmodifiableList[]

      Collections.unmodifiableList[] returns a list which cant be altered i.e. it can neither add or delete an element. Any attempt to modify the list will result in an UnsupportedOperationExample.

      Syntax:

      List list = Collections .unmodifiableList[Arrays.asList[1, 2, 3]];

      Example 1:

      
      

      
      
      

      import java.util.*; public class GFG { public static void main[String args[]] { // Creating the list List list = Collections.unmodifiableList[ Arrays.asList[1, 2, 3]]; // Print the list System.out.println["List : " + list.toString[]]; } }
      Output: List : [1, 2, 3]

      Example 2:

      
      

      
      
      

      import java.util.*; public class GFG { public static void main[String args[]] { try { // Creating the list List list = Collections.unmodifiableList[ Arrays.asList[1, 2, 3]]; // Print the list System.out.println["List : " + list.toString[]]; // Trying to modify the list System.out.println["Trying to modify the list"]; list.set[0, list.get[0]]; } catch [Exception e] { System.out.println["Exception : " + e]; } } }
      Output: List : [1, 2, 3] Trying to modify the list Exception : java.lang.UnsupportedOperationException

   3. Using Collections.singletonList[]

      Collections.singletonList[] returns an immutable list consisting of one element only.

      Syntax:

      
      
      
      List list = Collections.singletonList[2];

      Example 1:

      
      

      
      
      

      import java.util.*; public class GFG { public static void main[String args[]] { // Creating the list List list = Collections.singletonList[2]; // Print the list System.out.println["List : " + list.toString[]]; } }
      Output: List : [2]

4. Using Java 8 Stream

   With the introduction of Stream and functional programming in Java 8, now one can construct any stream of objects and then collect them as a list.

   Syntax:

   1. List list = Stream.of[1, 2, 3] .collect[Collectors.toList[]]; 2. List list = Stream.of[1, 2, 3] .collect[Collectors.toCollection[ArrayList::new]]; 3. List list = Stream.of[1, 2, 3, 4] .collect[Collectors.collectingAndThen[Collectors.toList[], Collections::unmodifiableList]];

   Examples:

   
   

   
   
   

   import java.util.*; import java.util.stream.Collectors; import java.util.stream.Stream; public class GFG { public static void main[String args[]] { // Creating a List using Syntax 1 List list1 = Stream.of[1, 2, 3] .collect[Collectors.toList[]]; // Printing the list System.out.println["List using Syntax 1: " + list1.toString[]]; // Creating a List using Syntax 2 List list2 = Stream .of[3, 2, 1] .collect[ Collectors .toCollection[ArrayList::new]]; // Printing the list System.out.println["List using Syntax 2: " + list2.toString[]]; // Creating a List using Syntax 3 List list3 = Stream .of[1, 2, 3, 4] .collect[ Collectors .collectingAndThen[ Collectors.toList[], Collections::unmodifiableList]]; // Printing the list System.out.println["List using Syntax 3: " + list3.toString[]]; } }
   Output: List using Syntax 1: [1, 2, 3] List using Syntax 2: [3, 2, 1] List using Syntax 3: [1, 2, 3, 4]

5. Using Java 9 List.of[]

   Java 9 introduced List.of[] method which takes in any number of arguments and constructs a compact and unmodifiable list out of them.

   Syntax:

   List unmodifiableList = List.of[1, 2, 3];

   Examples:

   
   

   
   
   

   import java.util.List; public class GFG { public static void main[String args[]] { // Creating a list using List.of[] List unmodifiableList = List.of[1, 2, 3]; // Printing the List System.out.println["List : " + unmodifiableList.toString[]]; } }

   OUTPUT:

   [1, 2, 3]

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