Learn how to extend the AWT with your own image buttons

Subclass an AWT component so you can build an image button for toolbars, palettes, and other "icon-friendly" environments

By Rich Kadel, JavaWorld.com, 03/01/97

Before you start cursing the AWT (yet again), let me assure you that a solution exists. The most straightforward approach and the technique we'll be examining in this article involves subclasssing the AWT's Canvas class. Although the primary purpose of the Canvas class is drawing (perfect for our purposes), it doesn't provide the mouse handling or any of the typical button-press effects (let alone mouse-over effects) you would expect for a button. We'll address these limitations by working with several classes I've created to support fully flexible, active image buttons.

Getting started with the ImageButton class

   
public class ImageButton extends Canvas {

Determining button states

When the user is not interacting with the button in any way, the button is in its UNARMED state. To follow suit with many applications, such as Internet Explorer and the new Netscape Navigator 4.0 pre-release, we're going to implement an OVER state, which will highlight a button as the user moves the mouse over it. The ARMED state, which we'll represent by showing a depressed button, occurs when the user clicks and holds on a button. If the user releases the mouse while the button is in the ARMED state, the corresponding action will be performed; however, users can return to the UNARMED state by moving the mouse off of the button without releasing the mouse. The last state we'll be supporting is DISABLED, which is implemented through the ImageButton programming interface by calling the AWT disabled function. A disabled button should not respond to any user action. We'll be using a grayed-out effect to indicate a disabled button.

Our image button will support only one state at a time (a button can't very well be ARMED and UNARMED at the same time now, can it?). We'll represent the four states in a single int, with four constants, as shown here:

   
    public static final int UNARMED = 0;
    public static final int ARMED = 1;
    public static final int OVER = 2;
    public static final int DISABLED = 3;

    private int buttonState = UNARMED;

Defining borders

We will have to draw the borders ourselves, so our next step is to decide what type of borders we want and which states they will correspond with. By implementing the border features in a separate class (we'll call this class Border), we can use one or more instances of the class in our ImageButton and switch between them as the states change.