AWT and Swing – The Graphical User Interface APIs

Packages

Primary AWT packages are java.awt and java.awt.event Other packages include java.awt.color,

java.awt.datatransfer, java.awt.dnd, java.awt.font, java.awt.geom, java.awt.im, java.awt.im.spi, java.awt.image, java.awt.image.renderable, java.awt.print

Use: J2SE (JDK1.0, greatly expanded in JDK1.2 and 1.3)

Primary Swing package is javax.swing

Other packages include javax.swing.border, javax.swing.colorchooser, javax.swing.event, javax.swing.filechooser, javax.swing.plaf, javax.swing.plaf.basic, javax.swing.plaf.metal, javax.swing.plaf.multi, javax.swing.table, javax.swing.text, javax.swing.text.html,

javax.swing.text.html.parser, javax.swing.text.rtf, javax.swing.tree, javax.swing.undo.

Use: J2SE (since JDK1.2, expanded for JDK1.3)

Common Features

Central to both AWT and Swing are the concepts of the component, container, and layout manager. A component is a graphical element of some kind like a button, label or text box. A container is also a kind of graphical element, but is distinguished by its ability to hold other elements. Containers are the organizers of the Java graphics APIs, enabling developers to group graphical elements within the GUI space. Windows are containers, holding components such as buttons and checkboxes inside themselves.

Layout managers are not graphical. They're worker objects, specialists that can determine size and position for components inside a container. A container delegates the task of managing its space to its associated layout manager, relying on it for advice about how and where it should place elements.

Another important feature of AWT and Swing is the event-handling model. In Java graphical applications, user interaction is represented by event objects that are produced by components. For example, if a user were to click on a big red button in a Java GUI labeled “History Eraser”, that button would produce an ActionEvent.

So would this act irrevocably erase our existence? Luckily, not in this universe. Unless there is an associated event handler, the event produced will not trigger any program response. In this example, there has to be a class implementing the ActionListener

interface which has been registered with the button through a call to addActionListener. In that case, the ActionEvent is passed to the associated ActionListener through a call to its actionPerformed method. To date, no one has written an event handler for the History Eraser button, so we're all still safe.

The AWT Architectural Model

The Abstract Window Toolkit (AWT) has been around as long as the language itself. It is built around a simple set of components that allow you to create basic GUIs. When you create an AWT application, its components are guaranteed to have the same look and feel as the platform where JVM runs. In other words, the same code produces a Solaris GUI when run on Solaris, a Macintosh GUI on MacOS and a Windows GUI on a Wintel platform.

There's a very straightforward explanation for this—an AWT application looks like its elements are native to a platform because they really are. AWT bases its functionality on the concept of the peer. Every AWT component has a peer class associated with the operating system, which does most of the real work. The classes that developers use to create graphical components in AWT provide a programming wrapper around the peers. For instance, in AWT a programmer uses the java.awt.Button class to create a button. Associated with that button is an implementor of the java.awt.peer.ButtonPeer interface, which performs most of the real tasks associated with painting the component on-screen.

It follows that there must be some way to keep track of the platform-specific peers within AWT. The Toolkit class provides that capability. Inside Toolkit is the code used to link to the underlying operating system. Developers rarely use this class directly, but it is important to AWT, since it ultimately loads graphical libraries, creates peer objects, and manages other platform-dependent resources such as cursors.

Because the basic AWT components are directly linked to the underlying operating system through their peers, they are also referred to as heavyweight components since they rely directly on the operating system to do things

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