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Interfaces

The core collection interfaces are the interfaces used to manipulate collections, and to pass them from one method to another. The basic purpose of these interfaces is to allow collections to be manipulated independently of the details of their representation. The core collection interfaces are the heart and soul of the collections framework. When you understand how to use these interfaces, you know most of what there is to know about the framework. The core collections interfaces are shown below:
The core collection interfaces form a hierarchy: a Set is a special kind of Collection, and a SortedSet is a special kind of Set, and so forth. Note also that the hierarchy consists of two distinct trees: a Map is not a true Collection.

To keep the number of core collection interfaces managable, the the JDK doesn't provide separate interfaces for each variant of each collection type. (Among the possible variants are immutable, fixed-size, and append-only.) Instead, the modification operations in each interface are designated optional: a given implementation may not support some of these operations. If an unsupported operation is invoked, a collection throws an UnsupportedOperationException(in the API reference documentation) . Implementations are responsible for documenting which of the optional operations they support. All of the JDK's general purpose implementations support all of the optional operations.

The four sections that follow teach you how to use each of the four basic core collection interfaces. In particular, they describe the idioms that allow you to use these interfaces effectively.

Collection

The Collection(in the API reference documentation) interface is the root of the collection hierarchy. A Collection represents a group of objects, known as its elements. Some Collection implementations allow duplicate elements and others do not. Some are ordered and others unordered. The JDK doesn't provide any direct implementations of this interface: It provides implementations of more specific subinterfaces like Set and List. This interface is the least common denominator that all collections implement. Collection is used to pass collections around and manipulate them when maximum generality is desired.

Set

A Set(in the API reference documentation)is a collection that cannot contain duplicate elements. As you might expect, this interface models the mathematical set abstraction. It is used to represent sets like the cards comprising a poker hand, the courses making up a student's schedule, or the processes running on a machine.

List

A List(in the API reference documentation)is 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. The user can access elements by their integer index (position). If you've used Vector(in the API reference documentation), you're already familiar with the general flavor of List.

Map

A Map(in the API reference documentation)is an object that maps keys to values. Maps cannot contain duplicate keys: Each key can map to at most one value. If you've used Hashtable(in the API reference documentation), you're already familiar with the general flavor of Map.

The last two core collection interfaces (SortedSet and SortedMap) are merely sorted versions of Set and Map. In order to understand these interfaces, you have to know how order is maintained among objects. Even if you don't plan to use SortedSet or SortedMap, read the following section if you plan to sort Lists.

Object Ordering

There are two ways to order objects: The Comparable(in the API reference documentation)interface provides automatic natural order on classes that implement it, while the Comparator(in the API reference documentation)interface gives the programmer complete control over object ordering. Note that these are not core collection interfaces, but underlying infrastructure.

Now that you know all about object ordering, here are the last two core collection interfaces:

SortedSet

A SortedSet is a Set that maintains its elements in ascending order. Several additional operations are provided to take advantage of the ordering. The SortedSet interface is used for things like word lists and membership rolls.

SortedMap

A SortedMap(in the API reference documentation)is a Map that maintains its mappings in ascending key order. It is the Map analogue of SortedSet. The SortedMap interface is used for apps like dictionaries and telephone directories.

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