J2ME 學習筆記（二）：Command、TextBox、TextField、DateField 事件與輸入元件詳解

Command：J2ME 事件模型核心

J2ME 採用命令模式處理用戶交互：定義 Command → 加入 Displayable → 實作 CommandListener → commandAction() 統一分派。

Command 類型與優先級

Command backCmd = new Command("Back", Command.BACK, 2);
Command cancelCmd = new Command("Cancel", Command.CANCEL, 1);
Command exitCmd = new Command("Exit", Command.EXIT, 1);
Command helpCmd = new Command("Help", Command.HELP, 1);
Command itemCmd = new Command("Item", Command.ITEM, 1);
Command okCmd = new Command("OK", Command.OK, 1);
Command screenCmd = new Command("Screen", Command.SCREEN, 1);
Command stopCmd = new Command("Stop", Command.STOP, 1);

• 第 2 參數：語意類型，決定裝置如何呈現（軟鍵、選單）
• 第 3 參數：優先級，數值越小優先級越高

commandAction 統一分派範例

public void commandAction(Command c, Displayable d) {
    if (c == exitCmd) {
        destroyApp(false); notifyDestroyed();
    } else if (c == helpCmd) { showScreen("Help"); }
    else if (c == backCmd) { showScreen("Back"); }
    else if (c == cancelCmd) { showScreen("Cancel"); }
    else if (c == itemCmd) { showScreen("Item"); }
    else if (c == okCmd) { showScreen("OK"); }
    else if (c == screenCmd) { showScreen("Screen"); }
    else if (c == stopCmd) { showScreen("Stop"); }
}

輸入元件對比

TextBox 完整範例

TextBox txtBox = new TextBox("標題", null, 200, TextField.ANY);
// null 表示無初始內容，最大長度 200，ANY 無限制
// 常見限制：EMAILADDR, NUMERIC, PHONENUMBER, URL, DECIMAL

TextField 完整範例

Form form = new Form("註冊");
TextField emailField = new TextField("Email:", "", 50, TextField.EMAILADDR);
TextField pwdField = new TextField("密碼:", "", 20, TextField.PASSWORD);
TextField phoneField = new TextField("電話:", "", 20, TextField.PHONENUMBER);
form.append(emailField);
form.append(pwdField);
form.append(phoneField);

關鍵：TextField 建構子第 4 參數指定輸入模式，裝置會自動彈出對應鍵盤/輸入法。

DateField 完整範例

DateField dateField = new DateField("生日:", DateField.DATE); // 僅日期
DateField dateTimeField = new DateField("約會:", DateField.DATE_TIME); // 日期+時間
DateField timeField = new DateField("鬧鐘:", DateField.TIME); // 僅時間
form.append(dateField);

// 取得值
Date selectedDate = dateField.getDate(); // 回傳 java.util.Date 或 null

完整整合範例：用戶註冊表單

package fancy.test;

import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.util.*;

public class RegistrationForm extends MIDlet implements CommandListener {
    private Display display;
    private Form form;
    private TextField nameField, emailField, phoneField;
    private DateField birthField;
    private Command submitCmd = new Command("提交", Command.OK, 1);
    private Command exitCmd = new Command("離開", Command.EXIT, 1);

    protected void startApp() {
        display = Display.getDisplay(this);
        form = new Form("用戶註冊");
        
        nameField = new TextField("姓名:", "", 30, TextField.ANY);
        emailField = new TextField("Email:", "", 50, TextField.EMAILADDR);
        phoneField = new TextField("電話:", "", 20, TextField.PHONENUMBER);
        birthField = new DateField("生日:", DateField.DATE);
        
        form.append(nameField);
        form.append(emailField);
        form.append(phoneField);
        form.append(birthField);
        
        form.addCommand(submitCmd);
        form.addCommand(exitCmd);
        form.setCommandListener(this);
        display.setCurrent(form);
    }

    public void commandAction(Command c, Displayable d) {
        if (c == exitCmd) {
            destroyApp(false); notifyDestroyed();
        } else if (c == submitCmd) {
            String name = nameField.getString();
            String email = emailField.getString();
            String phone = phoneField.getString();
            Date birth = birthField.getDate();
            
            // 簡單驗證
            if (name.length() == 0) { showAlert("請輸入姓名"); return; }
            if (!email.contains("@")) { showAlert("Email 格式錯誤"); return; }
            
            // 顯示提交結果
            Form result = new Form("註冊成功");
            result.append("姓名: " + name + "\n");
            result.append("Email: " + email + "\n");
            result.append("電話: " + phone + "\n");
            result.append("生日: " + (birth != null ? birth.toString() : "未填") + "\n");
            result.addCommand(exitCmd);
            display.setCurrent(result);
        }
    }
    
    private void showAlert(String msg) {
        Alert alert = new Alert("錯誤", msg, null, AlertType.ERROR);
        alert.setTimeout(Alert.FOREVER);
        display.setCurrent(alert, form);
    }
    // ... pauseApp, destroyApp 標準實作
}

最佳實踐提醒

• Command 語意類型正確：退出用 EXIT、返回用 BACK、確認用 OK，裝置會自動放在對應軟鍵
• 優先級合理設定：主要動作 (OK、Exit) 用 1，次要用 2+
• TextField 輸入模式盡量具體 (EMAILADDR、PHONENUMBER)，提升用戶體驗
• DateField 取得的 Date 可直接用於 RMS 儲存或網路傳輸
• 表單驗證在 commandAction 中進行，複雜驗證可抽離工具類