While maps are great data structures that are used constantly in programming, there are times when programmers need to associate more than one value with a given key. While we are free to create our own implementations of maps that have a list or set as a value, Guava makes it much easier. The static factory methods return map instances that give us the familiar semantics of the
put(key,value) operation. The details of checking if a collection exists for the given key and creating one if necessary, then adding the value to that collection, are taken care of for us. Let's dive in and explore this powerful abstraction.
ArrayListMulitmap is a map that uses
ArrayList to store the values for the given key. To create an