Java Collections singleton() Method

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Description

The Java Collections singleton(T) method is used to return an immutable set containing only the specified object.

Declaration

Following is the declaration for java.util.Collections.singleton() method.

public static <T> Set<T> singleton(T o)

Parameters

o − This is the sole object to be stored in the returned set.

Return Value

The method call returns an immutable set containing only the specified object.

Exception

NA

Getting a Singleton Set of Integer Example

The following example shows the usage of Java Collection singleton(T) method. We've created a List object with some integers, printed the original list. Using singleton(T) method, we've removed elements of the list of given value and then printed the updated list.

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class CollectionsDemo {

   public static void main(String[] args) {
      List<Integer> list = new ArrayList<>(Arrays.asList(1,2,3,4,5));

      System.out.println("Initial collection value: " + list);
      // remove 2 from this collection
      list.removeAll(Collections.singleton(2));
      System.out.println("Final collection value: "+list);
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Initial collection value: [1, 2, 3, 4, 5]
Final collection value: [1, 3, 4, 5]

Getting a Singleton Set of String Example

The following example shows the usage of Java Collection singleton(T) method. We've created a List object with some strings, printed the original list. Using singleton(T) method, we've removed elements of the list of given value and then printed the updated list.

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class CollectionsDemo {

   public static void main(String[] args) {
      List<String> list = new ArrayList<>(Arrays.asList("Welcome","to","Tutorialspoint"));

      System.out.println("Initial collection value: " + list);
      // remove "to" from this collection
      list.removeAll(Collections.singleton("to"));
      System.out.println("Final collection value: "+list);
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Initial collection value: [Welcome, to, Tutorialspoint]
Final collection value: [Welcome, Tutorialspoint]

Getting a Singleton Set of Object Example

The following example shows the usage of Java Collection singleton(T) method. We've created a List object with some Student objects, printed the original list. Using singleton(T) method, we've removed elements of the list of given value and then printed the updated list.

package com.tutorialspoint;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class CollectionsDemo {

   public static void main(String[] args) {
      List<Student> list = new ArrayList<>(Arrays.asList(new Student(1, "Julie"),
         new Student(2, "Robert"), new Student(3, "Adam")));

      System.out.println("Initial collection value: " + list);
      // remove Robert from this collection
      list.removeAll(Collections.singleton(new Student(2, "Robert")));
      System.out.println("Final collection value: "+list);
   }
}
class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
   @Override
   public boolean equals(Object obj) {
      Student s = (Student)obj;
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name);
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Initial collection value: [[ 1, Julie ], [ 2, Robert ], [ 3, Adam ]]
Final collection value: [[ 1, Julie ], [ 3, Adam ]]