Java AWT Contexts

This discussion looks at several of the facilities provided by the Java AWT for creating user interfaces, placing UI objects in those interfaces, and drawing on them. These issues will be approached from the standpoint of contexts in which objects can be placed and in which text and graphical shapes drawn.

The two contexts in which a user interface may be constructed are Panels and Frames.

Panels don't have borders and are displayed within the window provided by a browser or other application (e.g., the Appletviewer). Panelis also the superclass of Applet, so all applets inherently include a Panel.

Frames are a subclass of Window and are displayed within a separate window, with its own border, close spot, etc.

Java AWT Class Hierarchy. Branch in which Panel and Frame appear.

The discussions that follow are organized as a tutorial. Issues are discussed incrementally and cumulative through seven steps. The discussion begins with Panels and finishes with Frames. In several places, concepts that will be considered in more detail in later lessons but are needed here are included with minimal explanation.

Step 1

Step 1 creates several user interface components and adds them to the applet's inherent panel. Components include a TextArea and a Button.

Step 2

Step 2 prints text and draws several graphical shapes on the panel.

Step 3

Step 3 draws on the applet's panel and adds UI components to it. Thus, it combines code from Steps 1 & 2.

Step 4

A better approach would be to use two panels: one for the UI components and the other for drawing. This is done in Step 4.

Step 5

To provide more space for the second panel, Step 5 begins exploring the use of Layout Managers and the nesting of panels within panels.

Step 6

Step 6 illustrates subclassing, or extending, the Panel class in order to control which of two panels is drawn on.

Step 7

The six previous steps have worked within the context of the browser's window. There are times, however, when it is desirable for the applet to have its own window. This can be done by using a Frame as the outer container, rather than a Panel.

Discussion

The discussion has focused on basic tools for creating user interface components and drawing on the screen. A number of other UI components are available. Some of these will be discussed in subsequent lessons.

A further limitation is that while several interface components (i. e., a TextAreaand a Button) were created in the preceding steps, nothing was done with them. In the next lesson, you will see how to determine when a button has been pressed or how to get the text entered into a TextArea.

References

References useful for this discussion include the following::