Implementing reusable components and multiplatform compatibility

Writing highly reusable UI components in Java ME applications can be challenging, because they can present performance bottlenecks or be too application-specific. However, reusable and fast-performing components are possible if the components are made sufficiently simple. In the fully customized UI of the Paint MIDlet, some parts are so generic that they can be reused practically anywhere:

• The Button class is a good super class for any UI element that can be touched. To see how the class is used in this MIDlet, see the source code for any of the MIDlet classes ending with "Tool".

  Each subclass of Button must override the onSelected method, which is called when the button is pressed (the overriding is forced by making the method abstract). You can also attach listeners to the button and make it a bit more flexible, because it allows n-number of classes to receive a notification when the button is pressed. To make the Button class receive touch events, you only have to add a call to the pointerPressed and pointerReleased methods of the Canvas, as shown in the following example:

  Button button = new Button() {
      // ...
  };
  
  public void paint(Graphics g) {
      button.render(g);
  }
  public void pointerPressed(int x, int y) {
      button.pressed(x,y);
  }
  public void pointerReleased(int x, int y) {
      button.unpressed(x,y);
  }

• The Toolbar class is an easily reusable wrapper for many Buttons, setting them in a line.

• The FileDialog class works with any new Series 40 device that support the TextEditor class. You can override the buttonPressed method to make it do something else when the OK or Cancel button is pressed.

By changing the graphics for the UI elements, you can easily customize the appearance of the UI and reuse the same elements elsewhere.