Wizard API updated!
Tim Boudreau has released a new version of the Swing Wizard library (version 0.997) that fixes the WizardException bug reported in JavaWorld's recent Open Source Java Project profile. The article's examples have been reworked to test out the new, improved WizardException. Thanks, Tim, for this helpful fix!
Open Source Java Projects: The Wizard API

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Container support for objects in Java 1.0.2

Organizing objects is easy when you put them into containers

By Chuck Mcmanis, JavaWorld.com, 11/01/96

Herbert Spencer wrote, "Science is organized knowledge." The corollary might be that applications are organized objects. Let's take a moment to turn to some aspects of Java that are critical for developing applications rather than applets.

Of those who have heard of Java, the majority has learned about the language through the popular press. The statement that crops up a lot is that Java is for "programming small applications, or applets, that can be embedded on a Web page." While correct, this definition conveys only one aspect of the new language; it doesn't describe the whole picture. Perhaps Java can be better described as a language designed to build systems -- large systems -- of well-understood portable pieces of executable code that can be combined, in whole or in part, to produce a desirable whole.

In this column I will begin to look at the various tools you can use to build in Java. I will demonstrate how these tools can be combined to make a larger application, and how, once you have an application, you can further aggregate the application into still larger systems -- all possible because in Java there is no distinction between a complete application and a simple subroutine.

To provide source code fodder for this and past columns, I chose to build a BASIC interpreter. "Why BASIC?" you might ask, thinking no one uses BASIC anymore. This is not entirely true. BASIC lives on in Visual Basic and in other scripting languages. But more importantly, many people have been exposed to it and can make the following conceptual leap: If "applications" are programmed in BASIC, and BASIC can be written in Java, then applications can be written in Java. BASIC is just another interpreted language; the tools that we will be building can be modified to use any language syntax, thus the core concepts are the focus of these articles. Therefore, what starts as an application becomes a component of other applications -- even applets perhaps.

Generic classes and containers

Building generic classes is particularly relevant when creating applications because reusing classes provides tremendous leverage in reducing both complexity and time to market. In an applet, the value of a generic class is mitigated by the requirement of loading it over the network. The negative impact of loading generic classes over the network is demonstrated by Sun's Java Workshop (JWS). JWS augments the standard version of the abstract windowing toolkit (AWT) by using some very elegant "shadow" classes. The advantage is that applets are easy to develop and are rich in features; the downside is that loading these classes can take a lot of time on a slow network link. While this disadvantage will disappear eventually, what we find is that a system perspective on class development is often required to achieve the best solution.

Since we're starting to look a bit more seriously at application development, we will assume we have already determined that generic classes are a valid solution.

Java, like many general-purpose languages, provides several tools for creating generic classes. Different requirements will necessitate using

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Resources
  • Doug Lea has a set of collection (container) classes that he built here.
  • The folks at ObjectSpace have written some containers, which are included in their product the JGL (Java Generic Library)
  • Introduction to Algorithms, by Thomas Cormen, Charles Leiserson, and Ron Rivest, McGraw-Hill and MIT Press, 1990.