Collections Overview

A collection is a data structureactually, an objectthat can hold references to other objects. Usually, collections contain references to objects that are all of the same type. The collections framework interfaces declare the operations to be performed generically on various types of collections. Figure 19.1 lists some of the interfaces of the collections framework. Several implementations of these interfaces are provided within the framework. Programmers may also provide implementations specific to their own requirements.

Figure 19.1. Some collection framework interfaces.

Interface	Description
Collection	The root interface in the collections hierarchy from which interfaces Set, Queue and List are derived.
Set	A collection that does not contain duplicates.
List	An ordered collection that can contain duplicate elements.
Map	Associates keys to values and cannot contain duplicate keys.
Queue	Typically a first-in, first-out collection that models a waiting line; other orders can be specified.

The collections framework provides high-performance, high-quality implementations of common data structures and enables software reuse. These features minimize the amount of coding programmers need to do to create and manipulate collections. The classes and interfaces of the collections framework are members of package java.util. In the next section, we begin our discussion by examining the collections framework capabilities for array manipulation.

In earlier versions of Java, the classes in the collections framework stored and manipulated Object references. Thus, you could store any object in a collection. One inconvenient aspect of storing Object references occurs when retrieving them from a collection. A program normally has the need to process specific types of objects. As a result, the Object references obtained from a collection typically need to be cast to an appropriate type to allow the program to process the objects correctly.

In J2SE 5.0, the collections framework has been enhanced with the generics capabilities we introduced in Chapter 18. This means that you can specify the exact type that will be stored in a collection. You also receive the benefits of compile-time type checkingthe compiler ensures that you are using appropriate types with your collection and, if not, issues compile-time error messages. Also, once you specify the type stored in a collection, any reference you retrieve from the collection will have the specified type. This eliminates the need for explicit type casts that can throw ClassCastExceptions if the referenced object is not of the appropriate type. Programs that were implemented with prior java versions and that use collections can compile properly because the compiler automatically uses raw types when it encounters collections for which type arguments were not specified.