Java Collections singletonList() Method

The singletonList() method of Java Collections class is used to get an immutable list which contains only the specified object.

Syntax

Following is the declaration of singletonList() method:

Parameter

Parameter	Description	Required/Optional
o	It is the object which will be stored in the returned List.	Required

Returns

The singletonList() method returns an immutable List which contains only the specified object.

Exceptions

Compatibility Version

Java 1.3 and above

Example 1

import java.util.*;
public class CollectionsSingletonListExample1 {
	public static void main(String[] args) {
	    List<Integer> numList = Collections.singletonList(10); 	 
	    System.out.println("Number List elements: " + numList);	 
	    //numList.add(20);       // throws UnsupportedOperationException	 
	    List<String> strList = Collections.singletonList("Hello India"); 
	    System.out.println("String List elements: " + strList);
	    }
}

Test it Now

Output:

Number List elements: [10]
String List elements: [Hello India]

Example 2

import java.util.*;
public class CollectionsSingletonListExample2 {
	public static void main(String[] args) {		       
		//Create an array of string objects
	      String str[] = { "One", "Two", "Three", "One", "Two", "Three", "JavaTpoint" };	      
	      //Create one list
	      List<String> list = new ArrayList<String>(Arrays.asList(str));	      
	      System.out.println("List value before: "+list);	      
	      //Create singleton list
	      list = Collections.singletonList("One");      
	      System.out.println("List value after: "+list);
	      }
}

Test it Now

Output:

List value before: [One, Two, Three, One, Two, Three, JavaTpoint]
List value after: [One]

Example 3

import java.util.*;
public class CollectionsSingletonListExample3 {
	public static void main(String[] args) {
		System.out.print("Enter the value: ");
		Scanner sc = new Scanner(System.in);
		int i= sc.nextInt();		
            System.out.println("Output: "+Collections.singletonList(i));
            sc.close();
	      }
}

Test it Now

Output:

1.	Enter the value: 55
Output: [55]
2.	Enter the value: Java
Exception in thread "main" java.util.InputMismatchException
	at java.base/java.util.Scanner.throwFor(Scanner.java:939)
	at java.base/java.util.Scanner.next(Scanner.java:1594)
	at java.base/java.util.Scanner.nextInt(Scanner.java:2258)
	at java.base/java.util.Scanner.nextInt(Scanner.java:2212)
at myPackage.CollectionsSingletonListExample3.main(CollectionsSingletonListExample3.java:7)

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