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Get started with the Java Collections Framework

Find out how Sun's new offering can help you to make your collections more useful and accessible

By Dan Becker, JavaWorld.com, 11/01/98

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JDK 1.2 introduces a new framework for collections of objects, called the Java Collections Framework. "Oh no," you groan, "not another API, not another framework to learn!" But wait, before you turn away, hear me out: the Collections framework is worth your effort and will benefit your programming in many ways. Three big benefits come immediately to mind:

  • It dramatically increases the readability of your collections by providing a standard set of interfaces to be used by many programmers in many applications.
  • It makes your code more flexible by allowing you to pass and return interfaces instead of concrete classes, generalizing your code rather than locking it down.
  • It offers many specific implementations of the interfaces, allowing you to choose the collection that is most fitting and offers the highest performance for your needs.


And that's just for starters.

Our tour of the framework will begin with an overview of the advantages it provides for storing sets of objects. As you'll soon discover, because your old workhorse friends Hashtable and Vector support the new API, your programs will be uniform and concise -- something you and the developers accessing your code will certainly cheer about.

After our preliminary discussion, we'll dig deeper into the details.

The Java Collections advantage: An overview

Before Collections made its most welcome debut, the standard methods for grouping Java objects were via the array, the Vector, and the Hashtable. All three of these collections have different methods and syntax for accessing members: arrays use the square bracket ([]) symbols, Vector uses the elementAt method, and Hashtable uses get and put methods. These differences have long led programmers down the path to inconsistency in implementing their own collections -- some emulate the Vector access methods and some emulate the Enumeration interface.

To further complicate matters, most of the Vector methods are marked as final; that is, you cannot extend the Vector class to implement a similar sort of collection. We could create a collection class that looked like a Vector and acted like a Vector, but it couldn't be passed to a method that takes a Vector as a parameter.

Finally, none of the collections (array, Vector or Hashtable) implements a standard member access interface. As programmers developed algorithms (like sorts) to manipulate collections, a heated discourse erupted on what object to pass to the algorithm. Should you pass an array or a Vector? Should you implement both interfaces? Talk about duplication and confusion.

Thankfully, the Java Collections Framework remedies these problems and offers a number of advantages over using no framework or using the Vector and Hashtable:

  • A usable set of collection interfaces

    By implementing one of the basic interfaces -- Collection, Set, List, or Map -- you ensure your class conforms to a common API and becomes more regular and easily understood. So, whether you are implementing an SQL database, a color swatch matcher, or a remote chat application, if you implement the Collection interface, the operations on your collection of objects are well-known to your users. The standard interfaces also simplify the passing and returning of collections to and from class methods and allow the methods to work on a wider variety of collections.

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Resources
  • Download the Java Collections Framework as package for Java 1.1 JDK 1.1 at the Sun JavaBeans InfoBus page http://java.sun.com/beans/infobus/index.html
  • Sun's overview and description of the Java Collections Framework for Java 1.2 http://java.sun.com/products/jdk/1.2/docs/guide/collections/index.html
  • Annotated reference of the Java Collections Framework http://java.sun.com/products/jdk/1.2/docs/guide/collections/reference.html
  • A frequently asked questions file for the Java Collections Framework. http://java.sun.com/products/jdk/1.2/docs/guide/collections/designfaq.html
  • Read about ObjectSpace's Java Generic Library -- the main competitor to the Java Collections Framework http://www.objectspace.com/products/jgl/