Initializing a List in Java
- Difficulty Level : Medium
- Last Updated : 03 Nov, 2021
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:
List a = new ArrayList[]; List b = new LinkedList[]; List c = new Vector[]; List d = new Stack[];Below are the following ways to initialize a list:
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 ArrayListList list = new ArrayList[];list.add[1];list.add[3];System.out.println["ArrayList : " + list.toString[]];// For LinkedListList llist = new LinkedList[];llist.add[2];llist.add[4];System.out.println["LinkedList : " + llist.toString[]];// For StackList 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 ArrayListList list = new ArrayList[] {{add[1];add[3];} };System.out.println["ArrayList : " + list.toString[]];// For LinkedListList llist = new LinkedList[] {{add[2];add[4];} };System.out.println["LinkedList : " + llist.toString[]];// For StackList stack = new Stack[] {{add[3];add[1];} };System.out.println["Stack : " + stack.toString[]];}}Output:ArrayList : [1, 3] LinkedList : [2, 4] Stack : [3, 1]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 listSystem.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 listSystem.out.println["List : " + list.toString[]];list.add[5];// Print the listSystem.out.println["Modified list : " + list.toString[]];}}Output:List : [1, 2, 3] Modified list : [1, 2, 3, 5]
- Creating Immutable List
Using Collections class methods
There are various methods in Collections class that can be used to instantiate a list. They are:
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:
Examples:
import java.util.*;public class GFG {public static void main[String args[]]{// Create an empty listList list = new ArrayList[];// Instantiating list using Collections.addAll[]Collections.addAll[list, 1, 2, 3, 4];// Print the listSystem.out.println["List : " + list.toString[]];}}Output:List : [1, 2, 3, 4]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 listList list = Collections.unmodifiableList[Arrays.asList[1, 2, 3]];// Print the listSystem.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 listList list = Collections.unmodifiableList[Arrays.asList[1, 2, 3]];// Print the listSystem.out.println["List : " + list.toString[]];// Trying to modify the listSystem.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.UnsupportedOperationExceptionUsing Collections.singletonList[]
Collections.singletonList[] returns an immutable list consisting of one element only.
Syntax:
Example 1:
import java.util.*;public class GFG {public static void main[String args[]]{// Creating the listList list = Collections.singletonList[2];// Print the listSystem.out.println["List : " + list.toString[]];}}Output:List : [2]
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 1List list1 = Stream.of[1, 2, 3].collect[Collectors.toList[]];// Printing the listSystem.out.println["List using Syntax 1: "+ list1.toString[]];// Creating a List using Syntax 2List list2 = Stream.of[3, 2, 1].collect[Collectors.toCollection[ArrayList::new]];// Printing the listSystem.out.println["List using Syntax 2: "+ list2.toString[]];// Creating a List using Syntax 3List list3 = Stream.of[1, 2, 3, 4].collect[Collectors.collectingAndThen[Collectors.toList[],Collections::unmodifiableList]];// Printing the listSystem.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]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 ListSystem.out.println["List : "+ unmodifiableList.toString[]];}}OUTPUT:
[1, 2, 3]