Collections.singleton() method in Java with example
Last Updated :
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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