J2ME 學習筆記（四）：MIDP API 核心套件與行動銀行雙畫面實作

前言

詳細介紹 MIDP 四大核心套件功能，並以「SaveMyMoney 行動銀行」雙畫面範例示範：畫面一顯示歡迎訊息與跑馬燈，畫面二動態顯示當前日期、時間、總記憶體、可用記憶體。

一、MIDP 四大核心套件

二、實作範例：雙畫面資訊顯示

需求

• 畫面一：歡迎訊息 + 跑馬燈「Welcome to the World of Mobile Banking!」，右鍵「Info」進入畫面二，左鍵「Exit」離開。
• 畫面二：顯示當前月/日/年、時:分:秒、總記憶體、可用記憶體，左鍵「Back」返回，右鍵「Exit」離開。

核心程式碼

public class MB extends MIDlet implements CommandListener {
    private Display display;
    private Form form1, form2;
    private Ticker ticker1;
    private static final Command okCmd = new Command("Info", Command.OK, 1);
    private static final Command backCmd = new Command("Back", Command.BACK, 0);
    private static final Command exitCmd = new Command("Exit", Command.STOP, 2);

    public void startApp() throws MIDletStateChangeException {
        display = Display.getDisplay(this);
        ticker1 = new Ticker("Welcome to the World of Mobile Banking!");

        form1 = new Form("SaveMyMoney");
        form1.append(new StringItem("Welcome to SaveMyMoney Bank!", ""));
        form1.addCommand(exitCmd);
        form1.addCommand(okCmd);
        form1.setCommandListener(this);
        form1.setTicker(ticker1);

        form2 = new Form("INFORMATION");
        Calendar cal = Calendar.getInstance();
        String month[] = {"January","February","March","April","May","June",
                          "July","August","September","October","November","December"};
        String dateStr = cal.get(Calendar.DAY_OF_MONTH) + " " + month[cal.get(Calendar.MONTH)] + " " + cal.get(Calendar.YEAR);
        String timeStr = cal.get(Calendar.HOUR_OF_DAY) + ":" + cal.get(Calendar.MINUTE) + ":" + cal.get(Calendar.SECOND);
        Runtime rt = Runtime.getRuntime();
        String totalMem = Long.toString(rt.totalMemory());
        String freeMem = Long.toString(rt.freeMemory());

        form2.append(new StringItem(" ", "DATE: " + dateStr));
        form2.append(new StringItem(" ", "TIME: " + timeStr));
        form2.append(new StringItem(" ", "Total Memory: " + totalMem));
        form2.append(new StringItem(" ", "Free Memory: " + freeMem));
        form2.addCommand(exitCmd);
        form2.addCommand(backCmd);
        form2.setCommandListener(this);

        display.setCurrent(form1);
    }

    public void pauseApp() {}
    public void destroyApp(boolean uc) { notifyDestroyed(); }

    public void commandAction(Command c, Displayable d) {
        String lbl = c.getLabel();
        if ("Exit".equals(lbl)) destroyApp(true);
        else if ("Back".equals(lbl)) display.setCurrent(form1);
        else display.setCurrent(form2); // Info
    }
}

關鍵技術點

• Calendar 取得日期時間，配合月份陣列本地化。
• Runtime.getRuntime() 監控記憶體使用狀況。
• Command 語意類型 (OK/BACK/STOP) 決定軟鍵映射優先權。
• 畫面切換集中於 commandAction，狀態由表單實例保持。