Question

Maps in Java do not inherit from the interface “Collection” though in the most online “Tutorials” Maps are explained in the same category as Sets, Lists and Queues.

Do the Maps nevertheless belong to the Collection Framework?

Answer 1

The best description of the Collection is at the beginning of Java Collection Tutorial.

A collection — sometimes called a container — is simply an object that groups multiple elements into a single unit. Collections are used to store, retrieve, manipulate, and communicate aggregate data. Typically, they represent data items that form a natural group, such as a poker hand (a collection of cards), a mail folder (a collection of letters), or a telephone directory (a mapping of names to phone numbers).

Furthermore the tutorial lists the core collection interfaces, which all of them follow the paradigm stated above:

The following list describes the core collection interfaces:

Collection — the root of the collection hierarchy. A collection represents a group of objects known as its elements. The Collection interface is the least common denominator that all collections implement and is used to pass collections around and to manipulate them when maximum generality is desired. Some types of collections allow duplicate elements, and others do not. Some are ordered and others are unordered. The Java platform doesn’t provide any direct implementations of this interface but provides implementations of more specific subinterfaces, such as Set and List. Also see The Collection Interface section.

Set — a collection that cannot contain duplicate elements. This interface models the mathematical set abstraction and is used to represent sets, such as the cards comprising a poker hand, the courses making up a student’s schedule, or the processes running on a machine. See also The Set Interface section.

List — an ordered collection (sometimes called a sequence). Lists can contain duplicate elements. The user of a List generally has precise control over where in the list each element is inserted and can access elements by their integer index (position). If you’ve used Vector, you’re familiar with the general flavor of List. Also see The List Interface section.

Queue — a collection used to hold multiple elements prior to processing. Besides basic Collection operations, a Queue provides additional insertion, extraction, and inspection operations.

Queues typically, but do not necessarily, order elements in a FIFO (first-in, first-out) manner. Among the exceptions are priority queues, which order elements according to a supplied comparator or the elements’ natural ordering. Whatever the ordering used, the head of the queue is the element that would be removed by a call to remove or poll. In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds of queues may use different placement rules. Every Queue implementation must specify its ordering properties. Also see The Queue Interface section.

Map — an object that maps keys to values. A Map cannot contain duplicate keys; each key can map to at most one value. If you’ve used Hashtable, you’re already familiar with the basics of Map. Also see The Map Interface section.

So Map is a Collection although it doesn’t really have to implement the Collection interface.