又发现有关问题了，新人有关问题较多，望各位耐心解答

import java.awt.*;import java.awt.event.*;import javax.swing.*;public class Test {    public static void main(String[] args) {        EventQueue.invokeLater(new Runnable(){            public void run(){                MyFrame frame=new MyFrame();                                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);                            }        });    }}class MyFrame extends JFrame{    public MyFrame(){    Toolkit kit=Toolkit.getDefaultToolkit();    Dimension screenSize=kit.getScreenSize();    setSize(screenSize.width/2,screenSize.height/2);    setLocation(100,100);    setVisible(true);    TextField tf1=new TextField(10);    TextField tf2=new TextField(10);    TextField tf3=new TextField(20);    Label equal=new Label("+");    Panel p=new Panel();    Button b=new Button("=");    p.add(tf1);    p.add(equal);    p.add(tf2);    p.add(b);    p.add(tf3);                    add(p);    Monitor m=new Monitor(this);    b.addActionListener(m);    pack();        }}class Monitor implements ActionListener{    MyFrame mf=null;    public Monitor(MyFrame mf){        this.mf=mf;    }    public void actionPerformed(ActionEvent e) {*        double a=Double.parseDouble(mf.tf1.getText());*        double bb=Double.parseDouble(mf.tf2.getText());*        mf.tf3.setText(""+(a+bb));    }    }

class MyFrame extends JFrame{    public MyFrame(){    Toolkit kit=Toolkit.getDefaultToolkit();    Dimension screenSize=kit.getScreenSize();    setSize(screenSize.width/2,screenSize.height/2);    setLocation(100,100);    setVisible(true);    //TextField tf1=new TextField(10); //这样定义的是局部变量，方法内有效，方法外无法访问    //TextField tf2=new TextField(10);    //TextField tf3=new TextField(20);    Label equal=new Label("+");    Panel p=new Panel();    Button b=new Button("=");    p.add(tf1);    p.add(equal);    p.add(tf2);    p.add(b);    p.add(tf3);                    add(p);    Monitor m=new Monitor(this);    b.addActionListener(m);    pack();        }    TextField tf1=new TextField(10); //这样定义的是成员变量，这里没有public修饰     TextField tf2=new TextField(10); //同一个包的外部类可以访问    TextField tf3=new TextField(20);}