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Java Collections singleton() Method

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

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

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

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

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

NA

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);
   }
}

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]

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);
   }
}

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]

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);
   }
}

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 ]]

java_util_collections.htm

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