Jar J2ME诺基亚s40应用程序初学者无法使用表单找到错误

我正在尝试创建一个简单的诺基亚s40应用程序来添加两个数字（仅供练习），我在代码中找不到任何错误，请帮助

import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Form;
import javax.microedition.lcdui.StringItem;
import javax.microedition.lcdui.TextField;
import javax.microedition.midlet.MIDlet;
import javax.microedition.midlet.MIDletStateChangeException;

public class main extends MIDlet  {

    Display disp;
    Form start;
    TextField a, b;
    StringItem sum;

    main(){
        disp = Display.getDisplay(this);
    }
    protected void destroyApp(boolean arg0) throws MIDletStateChangeException {
        // TODO Auto-generated method stub

    }

    protected void pauseApp() {
        // TODO Auto-g enerated method stub

    }

    protected void startApp() throws MIDletStateChangeException {
        // TODO Auto-generated method stub
        start = new Form("Start");
        a = new TextField("A", "Enter A", 40, TextField.NUMERIC);
        b = new TextField("B", "Enter B", 4, TextField.NUMERIC);
        sum = new StringItem("Sum", "hello");

        start.append(a);
        start.append(b);
        start.append(sum);

        disp.setCurrent(start);
    }

}

然而，当我注释掉这两行时，我的代码开始工作，但没有显示任何内容，请帮助

a = new TextField("A", "Enter A", 40, TextField.NUMERIC);
b = new TextField("B", "Enter B", 4, TextField.NUMERIC);
start.append(a);
start.append(b);

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您的代码引发IllegalArgumentException，因为初始内容字符串（“输入A”）不遵守约束（数字）。你应使用：

a = new TextField("Enter A", "0", 40, TextField.NUMERIC);
b = new TextField("Enter B", "0", 4, TextField.NUMERIC);

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什么是错误消息？谢谢回复，但现在它给出了IllegaAccessException