Java Collections singleton() Method

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

Syntax

Following is the declaration of singleton() method:

Parameter

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

Returns

The singleton() method returns an immutable set which contains only the specified object.

Exceptions

Compatibility Version

Java 1.3 and above

Example 1

import java.util.*;
public class CollectionsSingletonExample1 {
	public static void main(String[] args) {		       
		List<Integer> list = new ArrayList<>();
            Collections.addAll(list, 1, 2, 4, 2, 7, 2, 4, 4);
            System.out.println(list);
            list.remove(7);
            System.out.println(list);
            list.removeAll(Collections.singleton(4));
            System.out.println(list);
	      }
}

Test it Now

Output:

[1, 2, 4, 2, 7, 2, 4, 4]
[1, 2, 4, 2, 7, 2, 4]
[1, 2, 2, 7, 2]

Example 2

import java.util.*;
public class CollectionsSingletonExample2 {
	public static void main(String[] args) {		       
		List<String> list = new ArrayList<>();
            Collections.addAll(list, "A", "B", "C", "A", "C", "D", "C");
            System.out.println(list);
            list.remove("B");
            System.out.println(list);
            list.removeAll(Collections.singleton("C"));
            System.out.println(list);
	      }
}

Test it Now

Output:

[A, B, C, A, C, D, C]
[A, C, A, C, D, C]
[A, A, D]

Example 3

import java.util.*;
public class CollectionsSingletonExample3 {
	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.singleton(i));
            sc.close();
	      }
}

Test it Now

Output:

Enter the value: A
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.CollectionsSingletonExample3.main(CollectionsSingletonExample3.java:7)

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