The Collections class in the java.util package provides convenient methods to create singleton collection objects. A singleton collection object is a List, Map or Set that contains only a single element. Here’s how you can create a singleton set:
Set set = Collections.singleton( "The Element" );
There are 2 more methods which allow you to create a singleton list or map:
List list = Collections.singletonList( "The Element" ); Map map = Collections.singletonMap( "Key" , "Value" );
The collection object returned in all these methods is an immutable object. Also, all 3 of them are static methods in the Collections class.