Lesson: Introduction (source: Java.sun.com)

What Is a Collection?

A collection (sometimes called a container) is simply an object that groups multiple elements into a single unit. Collections are used to store, retrieve and manipulate data, and to transmit data from one method to another. Collections typically represent data items that form a natural group, like a poker hand (a collection of cards), a mail folder (a collection of letters), or a telephone directory (a collection of name-to-phone-number mappings).

If you've used Java -- or just about any other programming language -- you're already familiar with collections. Collection implementations in earlier versions of Java included Vector , Hashtable , and array. While earlier versions of Java contained collection implementations, they did not contain a collections framework.

What Is a Collections Framework?

A collections framework is a unified architecture for representing and manipulating collections. All collections frameworks contain three things:

• Interfaces: abstract data types representing collections. Interfaces allow collections to be manipulated independently of the details of their representation. In object-oriented languages like Java, these interfaces generally form a hierarchy.
• Implementations: concrete implementations of the collection interfaces. In essence, these are reusable data structures.
• Algorithms: methods that perform useful computations, like searching and sorting, on objects that implement collection interfaces. These algorithms are said to be polymorphic because the same method can be used on many different implementations of the appropriate collections interface. In essence, algorithms are reusable functionality.

The best known examples of collections frameworks are the C++ Standard Template Library (STL), and Smalltalk's collection classes.

What are the Benefits of a Collections Framework?

• It reduces programming effort: By providing useful data structures and algorithms, a collections framework frees you to concentrate on the important parts of your program, rather than the low-level plumbing required to make it work. By facilitating interoperability among unrelated APIs (as described below), the collections framework frees you from writing oodles of adapter objects or conversion code to connect APIs.
• It increases program speed and quality: The collections framework does this primarily by providing high-performance, high-quality implementations of useful data structures and algorithms. Also, because the various implementations of each interface are interchangeable, programs can be easily tuned by switching collection implementations. Finally, because you're freed from the drudgery of writing your own data structures, you'll have more time to devote to improving the quality and performance of the rest of the program.
• It allows interoperability among unrelated APIs: The collections interfaces will become the "lingua franca" by which APIs pass collections back and forth. If my network administration API furnishes a Collection of node names, and your GUI toolkit expects a Collection of column headings, our APIs will interoperate seamlessly even though they were written independently.
• It reduces the effort to learn and use new APIs: Many APIs naturally take collections on input and output. In the past, each such API had a little "sub-API" devoted to manipulating its collections. There was little consistency among these ad-hoc collections sub-APIs, so you had to learn each one from scratch and it was easy to make mistakes when using them. With the advent of standard collections interfaces, the problem goes away.
• It reduces effort to design new APIs: This is the flip-side of the previous advantage: designers and implementers don't have to reinvent the wheel each time they create an API that relies on collections. They just use the standard collections interfaces.
• It fosters software reuse: New data structures that conform to the standard collection interfaces are by nature reusable. The same goes for new algorithms that operate on objects that implement these interfaces.

Are there any Drawbacks?

Historically, collections frameworks have been quite complex, which gave them a reputation for having a steep learning curve. We believe that Java's new collections framework breaks with this tradition, as you will learn for yourself in the following lessons.

Lesson: 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 manageable, 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 . 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 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 Setis 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 Listis 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, you're already familiar with the general flavor of List.

Map

A Mapis 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, 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 Comparableinterface provides automatic natural order on classes that implement it, while the Comparatorinterface 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 SortedMapis 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.