Java Collections singletonMap() Method

The singletonMap() method of Java Collections class is used to get an immutable map, mapping only the specified key to the specified value.

Syntax

Following is the declaration of singletonMap() method:

Parameter

Parameter	Description	Required/Optional
key	It is the key which will be stored in the returned map.	Required
value	It is the value to which the returned map maps the key.	Required

Returns

The singletonMap() method returns an immutable map which contains only the specified key-value pair mapping.

Exceptions

Compatibility Version

Java 1.3 and above

Example 1

import java.util.*;
public class CollectionsSingletonMapExample1 {
	public static void main(String[] args) {
		Map<Integer, Integer> map = Collections.singletonMap(1, 4);
            System.out.println("Output: "+map);
	      }
}

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Output:

Output: {1=4}

Example 2

import java.util.*;
public class CollectionsSingletonMapExample2 {
	public static void main(String[] args) {
		//Create singleton map
	      Map<String,String> map = Collections.singletonMap("key","Value");
	      System.out.println("Singleton map is: "+map);
	      }
}

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Output:

Singleton map is: {key=Value}

Example 3

import java.util.*;
public class CollectionsSingletonMapExample3 {
	public static void main(String[] args) {
		System.out.println("Enter the key and value: ");
		Scanner sc = new Scanner(System.in);
		int key = sc.nextInt();	 
		int value = sc.nextInt();
            System.out.println("Output: "+Collections.singletonMap(key, value));
            sc.close();
	      }
}

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Output:

Enter the key and value: 
55
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.CollectionsSingletonMapExample3.main(CollectionsSingletonMapExample3.java:8)

Example 4

package myPackage;
import java.util.*;
public class CollectionsSingletonMapExample4 {
	public static void main(String[] args) {
		Map<Integer, String> m1 = new HashMap<Integer, String>();
	      m1.put(1, "Sunday");
	      m1.put(2, "Monday");
	      m1.put(3, "Tuesday");
	      m1.put(4, "Wednesday");
	      System.out.println("Before Singleton method-");
	      System.out.println("Contents of the Map Elements");
	      System.out.print( m1);
	      System.out.println("\n");
	      System.out.println("After singleton method-");
	      m1 = Collections.singletonMap(1, "Monday");
	      System.out.println("Contents of the Map Elements");
	      System.out.println(m1);
	      }
}

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Output:

Before Singleton method-
Contents of the Map Elements
{1=Sunday, 2=Monday, 3=Tuesday, 4=Wednesday}

After singleton method-
Contents of the Map Elements
{1=Monday}

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