Collections.singleton() method in Java with example - GeeksforGeeks

02 Jul, 2021
java.util.Collections.singleton() method is a java.util.Collections class method. It creates a immutable set over a single specified element. An application of this method is to remove an element from Collections like List and Set.
Syntax: 
public static  Set singleton(T obj)
and
public static  List singletonList(T obj)
Parameters: obj : the sole object to be stored in
                  the returned list or set.
Return: an immutable list/set containing only the 
       specified object.
Example:
myList : {"Geeks", "code", "Practice", " Error",  "Java", 
          "Class", "Error", "Practice", "Java" }

To remove all "Error" elements from our list at once, we use 
singleton() method 
myList.removeAll(Collections.singleton("Error"));

After using singleton() and removeAll, we get following.
{"Geeks", "code", "Practice", "Java", "Class", "Practice", "Java" }
Java
// Java program to demonstrate// working of singleton()import java.util.*; class GFG {    public static void main(String args[])    {        String[] geekslist = { "1", "2", "4", "2", "1", "2",                               "3", "1", "3", "4", "3", "3" };         // Creating a list and removing        // elements without use of singleton()        List geekslist1 = new ArrayList(Arrays.asList(geekslist));        System.out.println("Original geeklist1: " + geekslist1);         geekslist1.remove("1");        System.out.println("geekslist1 after removal of 1 without"                           + " singleton " + geekslist1);        geekslist1.remove("1");        System.out.println("geekslist1 after removal of 1 without"                           + " singleton " + geekslist1);        geekslist1.remove("2");        System.out.println("geekslist1 after removal of 2 without"                           + " singleton " + geekslist1);         /* Creating another list and removing    its elements using singleton() method */        List geekslist2 = new ArrayList(Arrays.asList(geekslist));        System.out.println("\nOriginal geeklist2: " + geekslist2);         // Selectively delete "1" from        // all it's occurrences        geekslist2.removeAll(Collections.singleton("1"));        System.out.println("geekslist2 after removal of 1 with "                           + "singleton:" + geekslist2);         // Selectively delete "4" from        // all it's occurrences        geekslist2.removeAll(Collections.singleton("4"));        System.out.println("geekslist2 after removal of 4 with "                           + "singleton:" + geekslist2);         // Selectively delete "3" from        // all it's occurrences        geekslist2.removeAll(Collections.singleton("3"));        System.out.println("geekslist2 after removal of 3 with"                           + " singleton: " + geekslist2);    }}
Output: 
Original geeklist1                 [1, 2, 4, 2, 1, 2, 3, 1, 3, 4, 3, 3]
geekslist1 after removal of 1 without singleton [2, 4, 2, 1, 2, 3, 1, 3, 4, 3, 3]
geekslist1 after removal of 1 without singleton [2, 4, 2, 2, 3, 1, 3, 4, 3, 3]
geekslist1 after removal of 2 without singleton [4, 2, 2, 3, 1, 3, 4, 3, 3]

Original geeklist2                 [1, 2, 4, 2, 1, 2, 3, 1, 3, 4, 3, 3]
geekslist2 after removal of 1 with singleton     [2, 4, 2, 2, 3, 3, 4, 3, 3]
geekslist2 after removal of 4 with singleton     [2, 2, 2, 3, 3, 3, 3]
geekslist2 after removal of 3 with singleton     [2, 2, 2]
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