Java Collections Framework

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.
A collection is the same as the intuitive, mathematical concept of a set. A set is just a group of unique items, meaning that the group contains no duplicates. Java Collections Framework provides a set of interfaces and classes for storing and manipulating groups of data as a single unit, a collection. The framework provides a convenient API to many of the abstract data types maps, sets, lists, trees, arrays, hashtables, and other collections. Because of their object-oriented design, the Java classes in the Collections Framework encapsulate both the data structures and the algorithms associated with these abstractions. With the Java Collections Framework the programmer easily define higher level data abstractions, such as stacks, queues, and thread safe collections.

You should be able to understand the Collections Framework more easily. The Collections Framework is made up of a set of interfaces for working with groups of objects. The different interfaces describe the different types of groups. For the most part, once you understand the interfaces, you understand the framework. While you always need to create specific implementations of the interfaces, access to the actual collection should be restricted to the use of the interface methods, thus allowing you to change the underlying data structure, without altering the rest of your code. The following diagrams shows the framework interface hierarchy.

What Is a Collections Framework?

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

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

Benefits of the Java Collections Framework

The Java Collections Framework provides the following benefits:

  1. Reduces programming effort: By providing useful data structures and algorithms, the Collections Framework frees you to concentrate on the important parts of your program rather than on the low-level "plumbing" required to make it work. By facilitating interoperability among unrelated APIs, the Java Collections Framework frees you from writing adapter objects or conversion code to connect APIs.
  2. Increases program speed and quality: The Collections Framework provides high-performance, high-quality implementations of useful data structures and algorithms. The various implementations of each interface are interchangeable, so programs can be easily tuned by switching collection implementations. Because you're freed from the drudgery of writing your own data structures, you'll have more time to devote to improving programs' quality and performance.
  3. Allows interoperability among unrelated APIs: The collection interfaces are the vernacular by which APIs pass collections back and forth. If my network administration API furnishes a collection of node names and if your GUI toolkit expects a collection of column headings, APIs will interoperate seamlessly, even though they were written independently.
  4. Reduces effort to learn and to use new APIs: Many APIs naturally take collections on input and furnish them as output. In the past, each such API had a small 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 collection interfaces, the problem went away.
  5. 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; instead, they can use standard collection interfaces.
  6. 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.

The Java Collections Framework consists of following classes, interfaces and utility classes.

Collection Classes

Collection Interfaces

  • Collection
  • Set
  • List
  • Queue
  • Deque
  • Map
  • SortedSet
  • SortedMap
  • NavigableSet
  • NavigableMap
  • BlockingQueue
  • BlockingDeque
  • ConcurrentMap
  • ConcurrentNavigableMap