J2ME 入門實戰（三）：Gauge 組件互動演示

代碼結構

public class GaugeMIDlet extends MIDlet implements CommandListener {
    private Gauge gauge;
    private Form form;
    private Command exitCmd, okCmd;
    private Display display;
    private Alert alert;

    public GaugeMIDlet() {
        form = new Form("GaugeTest");
        exitCmd = new Command("EXIT", Command.EXIT, 1);
        okCmd = new Command("OK", Command.OK, 1);
        gauge = new Gauge("Progress", true, 100, 50); // 互動式、最大100、初始50
        alert = new Alert("Value", null, null, AlertType.INFO);
        form.addCommand(exitCmd); form.addCommand(okCmd);
        form.append(gauge); form.setCommandListener(this);
    }
    protected void startApp() {
        display = Display.getDisplay(this);
        display.setCurrent(form);
    }
    protected void pauseApp() {}
    protected void destroyApp(boolean unconditional) {}

    public void commandAction(Command c, Displayable d) {
        if (c == exitCmd) { destroyApp(false); notifyDestroyed(); }
        else if (c == okCmd) {
            int val = gauge.getValue();
            alert.setString("The Value is " + val);
            display.setCurrent(alert, form);
        }
    }
}

關鍵點

• Gauge(boolean interactive, int max, int initial)：interactive=true 允許用戶左右鍵調整值；false 則為進度條模式。
• CommandListener 處理軟鍵事件，Display.setCurrent(Alert, nextDisplayable) 實現 Alert 彈出後自動返回 Form。
• MIDlet 生命週期三方法：startApp/pauseApp/destroyApp。

運行效果

模擬器中顯示可拖動滑塊的 Gauge，按「OK」彈出當前數值 Alert，按「EXIT」退出程式。