Building Reusable Java Widgets
If your widget generates events, you have to stay within the bounds of Java's event model. In the “early days”, Java used a cumbersome inheritance-based event model. With the introduction of Java 1.1, the AWT sports a completely modern, delegation-based event model. The concept is simple. When a user interacts with a widget, it generates events. Objects can register an interest in all or some of these events. These interested objects, known as listeners, receive the events and take appropriate action. This process is known as delegation. Listeners are delegated by the widget to handle the events they generate.
When you create event classes, make sure they contain appropriate information and are generated in response to appropriate user actions. It can get confusing when you start to generate your own events. First, you create the event classes. Next, you create a listener interface. This interface defines methods of the listeners that will be called when an event occurs. Finally, you create an event multicaster. Event multicasters have the job of broadcasting events to many listeners. Sounds complicated, but hopefully it will become clear after an example or two. Figure 2 is a diagram of the multicasting process.
The appearance of your widget will not strictly affect its ability to be reused. However, a widget's appearance should be in visual harmony with the rest of your GUI. Here are some things to think about, which, while not hard and fast rules, are important.
Modern widgets produce a 3D appearance by shading. Shading is created by an imaginary light source that is positioned, by convention, at the upper left of the widget. When a button is in the raised state, its upper and left borders are brighter than the face of the button (see Figure 3). Its lower and right borders are darker. This shading makes it appear to be raised. Swap the light and dark areas, and the button will appear depressed (see Figure 4).
If your widget is drawn or has drawn areas on it, keep in mind the 3D effects are what make it as attractive as possible. And remember, drawing occurs in the paint method of your class. The AWT calls the paint method any time your widget needs to be drawn.
There are generally two methods you can use to create reusable widgets: composition and specialization. Before continuing, let's take a moment to discuss each method.
Composition Widgets that are created by composition are sometimes called super-widgets or composite widgets. This type of widget is simply a collection of other widgets that work together to accomplish a specialized task. You should create a composite widget whenever you have a recurring task which requires a number of sub-components working together. Some examples include an order form, file dialog or a color chooser. When you create a composite widget, it is important to hide the events of all its sub-components and generate events at a higher level appropriate to the semantics of the composite widget itself. You will see how this works in the e-mail entry widget and the window-bar widget.
Sometimes a widget is required that is slightly different from a standard AWT widget. Perhaps you need to add some new behavior or look. It is in these situations that you should consider creating a widget by specialization. When you create a widget by specialization, you create a subclass and add or override existing behavior. The power of inheritance gives you all of the behavior of the superclass, so all you need to do is write the new code. An example we will discuss is the VerticalSeparator, a subclass of Canvas. It overrides the paint method to achieve its own special look. The vertical-separator is a good example of this. The collapsing pane is a more subtle example of how to use specialization.
Fast/Flexible Linux OS Recovery
On Demand Now
In this live one-hour webinar, learn how to enhance your existing backup strategies for complete disaster recovery preparedness using Storix System Backup Administrator (SBAdmin), a highly flexible full-system recovery solution for UNIX and Linux systems.
Join Linux Journal's Shawn Powers and David Huffman, President/CEO, Storix, Inc.
Free to Linux Journal readers.Register Now!
- The Qt Company's Qt Start-Up
- Devuan Beta Release
- May 2016 Issue of Linux Journal
- EnterpriseDB's EDB Postgres Advanced Server and EDB Postgres Enterprise Manager
- The US Government and Open-Source Software
- Open-Source Project Secretly Funded by CIA
- The Death of RoboVM
- The Humble Hacker?
- New Container Image Standard Promises More Portable Apps
- BitTorrent Inc.'s Sync
In modern computer systems, privacy and security are mandatory. However, connections from the outside over public networks automatically imply risks. One easily available solution to avoid eavesdroppers’ attempts is SSH. But, its wide adoption during the past 21 years has made it a target for attackers, so hardening your system properly is a must.
Additionally, in highly regulated markets, you must comply with specific operational requirements, proving that you conform to standards and even that you have included new mandatory authentication methods, such as two-factor authentication. In this ebook, I discuss SSH and how to configure and manage it to guarantee that your network is safe, your data is secure and that you comply with relevant regulations.Get the Guide