Usage of Removing any single elements from Collection Objects using Immutable Singleton Set

/*to remove all occurrences of a specified element from a Collection*/
c.removeAll(Collections.singleton(e));

/*to removes all elements that map to a specified value from a Map */
/*suppose you have a Map — job — that maps people to their line of work and suppose you want to eliminate all the lawyers*/

job.values().removeAll(Collections.singleton(LAWYER));

/*use of these constants is as input to methods that take a Collection of values when you don't want to provide any values at all*/

tourist.declarePurchases(Collections.emptySet());