新手！编了简单计算器！为什么不行！
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
@SuppressWarnings("serial")
public class Calculator extends JFrame
{


private JTextField text;
StringBuilder str=new StringBuilder("");
double i;
double num=0;
public static void main(String[] args)
{
zys fr=new zys();
fr.setVisible(true);
}
public Calculator()
{
setTitle("计算器");
setBounds(100,100,250,200);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setResizable(false);
JPanel pan = new JPanel();
pan.setLayout(new BorderLayout(0,0));
setContentPane(pan);
JPanel pan1=new JPanel();
pan.add(pan1,BorderLayout.NORTH);
text = new JTextField();
text.setHorizontalAlignment(SwingConstants.RIGHT);
pan1.add(text);
text.setColumns(18);
JPanel pan2=new JPanel();
pan.add(pan2,BorderLayout.CENTER);
pan2.setLayout(new GridLayout(4,4,5,5));
JButton but1=new JButton("7");
pan2.add(but1);
but1.addActionListener(new NumberListener());
JButton but2=new JButton("8");
pan2.add(but2);
but2.addActionListener(new NumberListener());
JButton but3=new JButton("9");
pan2.add(but3);
but3.addActionListener(new NumberListener());
JButton but4=new JButton("+");
pan2.add(but4);
but4.addActionListener(new NumberListener());
JButton but5=new JButton("4");
pan2.add(but5);
but5.addActionListener(new NumberListener());
JButton but6=new JButton("5");
pan2.add(but6);
but6.addActionListener(new NumberListener());
JButton but7=new JButton("6");
pan2.add(but7);
but7.addActionListener(new NumberListener());
JButton but8=new JButton("-");
pan2.add(but8);
but8.addActionListener(new NumberListener());
JButton but9=new JButton("1");
pan2.add(but9);
but9.addActionListener(new NumberListener());
JButton but10=new JButton("2");
pan2.add(but10);
but10.addActionListener(new NumberListener());
JButton but11=new JButton("3");
pan2.add(but11);
but11.addActionListener(new NumberListener());
JButton but12=new JButton("*");
pan2.add(but12);
but12.addActionListener(new NumberListener());
JButton but13=new JButton("0");
pan2.add(but13);
but13.addActionListener(new NumberListener());
JButton but14=new JButton(".");
pan2.add(but14);
but14.addActionListener(new NumberListener());
JButton but15=new JButton("=");
pan2.add(but15);
but15.addActionListener(new NumberListener());
JButton but16=new JButton("/");
pan2.add(but16);
but16.addActionListener(new NumberListener());
}
class NumberListener implements ActionListener
{

public void actionPerformed(ActionEvent e)
{
String stre=new String();
if(e.getActionCommand().equals("+")||e.getActionCommand().equals("-")||e.getActionCommand().equals("*")||e.getActionCommand().equals("/"))

{
stre=e.getActionCommand();
str=new StringBuilder("");
i=num;
}
str.append(String.valueOf(e.getActionCommand()));
text.setText(String.valueOf(str));
num=Double.parseDouble("str");
if(e.getActionCommand().equals("=")){
if (stre.equals("+"))
text.setText(""+(i+num));
else if (stre.equals("-"))
text.setText(""+(i-num));
else if (stre.equals("*"))
text.setText(""+(i*num));
else if (stre.equals("/"))
text.setText(""+(i/num));
}

}
}
}

[解决办法]
我改了几个地方,能运行,楼主参考一下:

Java code

import javax.swing.*;import java.awt.*;import java.awt.event.*;@SuppressWarnings("serial")public class Calculator extends JFrame{        private JTextField text;          StringBuilder str=new StringBuilder("");    String stre=new String();            //设为公用。    double i;          double num=0;    public static void main(String[] args)      {             //zys fr=new zys();        Calculator fr=new Calculator();          fr.setVisible(true);      }      public Calculator()      {        ActionListener al=new NumberListener();        //让所有的按钮公用一个监听对象。          setTitle("计算器");          setBounds(100,100,250,200);          setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);              setResizable(false);              JPanel pan = new JPanel();              pan.setLayout(new BorderLayout(0,0));              setContentPane(pan);              JPanel pan1=new JPanel();              pan.add(pan1,BorderLayout.NORTH);              text = new JTextField();              text.setHorizontalAlignment(SwingConstants.RIGHT);              pan1.add(text);              text.setColumns(18);              JPanel pan2=new JPanel();          pan.add(pan2,BorderLayout.CENTER);          pan2.setLayout(new GridLayout(4,4,5,5));          JButton but1=new JButton("7");          pan2.add(but1);          but1.addActionListener(al);          JButton but2=new JButton("8");          pan2.add(but2);          but2.addActionListener(al);          JButton but3=new JButton("9");          pan2.add(but3);          but3.addActionListener(al);          JButton but4=new JButton("+");          pan2.add(but4);          but4.addActionListener(al);          JButton but5=new JButton("4");          pan2.add(but5);          but5.addActionListener(al);          JButton but6=new JButton("5");          pan2.add(but6);          but6.addActionListener(al);          JButton but7=new JButton("6");          pan2.add(but7);          but7.addActionListener(al);          JButton but8=new JButton("-");          pan2.add(but8);          but8.addActionListener(al);          JButton but9=new JButton("1");          pan2.add(but9);          but9.addActionListener(al);          JButton but10=new JButton("2");          pan2.add(but10);          but10.addActionListener(al);          JButton but11=new JButton("3");          pan2.add(but11);          but11.addActionListener(al);          JButton but12=new JButton("*");          pan2.add(but12);          but12.addActionListener(al);          JButton but13=new JButton("0");          pan2.add(but13);          but13.addActionListener(al);          JButton but14=new JButton(".");          pan2.add(but14);          but14.addActionListener(al);          JButton but15=new JButton("=");          pan2.add(but15);          but15.addActionListener(al);          JButton but16=new JButton("/");          pan2.add(but16);          but16.addActionListener(al);      }      class NumberListener implements ActionListener      {        public void actionPerformed(ActionEvent e)          {              //String stre=new String();            if(e.getActionCommand().equals("+")||e.getActionCommand().equals("-")                    ||e.getActionCommand().equals("*")||                    e.getActionCommand().equals("/"))            {                   stre=e.getActionCommand();                str=new StringBuilder("");                i=num;            }            else if(e.getActionCommand().equals("="))            {                if (stre.equals("+"))                 {                        text.setText(""+(i+num));                    stre="";            //清除原来的值.                    str=new StringBuilder("");                }                else if (stre.equals("-"))                  {                    text.setText(""+(i-num));                    stre="";                    str=new StringBuilder("");                }                else if (stre.equals("*"))                {                    text.setText(""+(i*num));                    stre="";                    str=new StringBuilder("");                }                    else if (stre.equals("/"))                 {                    text.setText(""+(i/num));                    stre="";                    str=new StringBuilder("");                }                }            else    //既不是"+ - * /" 又不是"=" 就是数字。            {                str.append(String.valueOf(e.getActionCommand()));                text.setText(String.valueOf(str));                //num=Double.parseDouble("str");                num=Double.parseDouble(str.toString());            }        }    }}