一个计算器java代码

各位老师和朋友们，我今天看到了一个Java的计算器的代码，对代码做了一些优化，使算法看起来更容易理解，因为我是一人java的初学者，我把原来的代码和修改后的代码发表出来，望各位老师和同学多给指出不当之处，不吝赐教。希望在老师和朋友们的帮助下有更大提高
原来的代码：（从百度贴吧上看到的）
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Calculator //主类Calculator定义开始
{
public static void main(String[] args)
{
CalculatorFrame frame=new CalculatorFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
} //主类Calculator定义结束
class CalculatorFrame extends JFrame //CalculatFrame类定义开始
{
public CalculatorFrame()
{
setTitle("计算器");
CalculatorPanel panel=new CalculatorPanel(); //实例化CalculatorPanel类成panel对象
add(panel); //将panel对象添加CalculatorFrame中
pack();
}
} //CalculatFrame定义结束
class CalculatorPanel extends Panel // CalculatorPanel类定义开始
{
public CalculatorPanel()
{
setLayout(new BorderLayout());
result=0;
lastCommand="=";
start=true;
flag=true;
display=new JButton("0"); //实例化display按钮
display.setEnabled(false); //将按钮设置为不可操作，只能显示字符串
add(display,BorderLayout.NORTH); //将按钮添加到窗口最顶端
ActionListener insert=new InsertAction(); //实例化侦听器器类InsertAction
ActionListener command=new CommandAction(); //实例化侦听器器类CommandAction
panel=new JPanel(); //实例化JPanel类
panel.setLayout(new GridLayout(4,5)); //为panel设置布局管理器:布局为4行5列从左向右添加20个按钮
addButton("7",insert);
addButton("8",insert);
addButton("9",insert);
addButton("/",command);
addButton("清零",command);
addButton("4",insert);
addButton("5",insert);
addButton("6",insert);
addButton("*",command);
addButton("退格",command);
addButton("1",insert);
addButton("2",insert);
addButton("3",insert);
addButton("-",command);
addButton("sqrt",command);
addButton("0",insert);
addButton(".",insert);
addButton("=",command);
addButton("+",command);
addButton("1/x",command);
add(panel,BorderLayout.CENTER);
} //CalculatorPanel类定义结束
private void addButton(String label,ActionListener listener) addButton //方法定义开始
{
JButton button=new JButton(label);
button.addActionListener(listener);
panel.add(button);
} //addButton方法定义结束
private class InsertAction implements ActionListener // InsertAction事件侦听器类定义开始
{
public void actionPerformed(ActionEvent event)
{
String input=event.getActionCommand();
if(start&&flag)
{
display.setText("");
start=false;
}
if(flag)
display.setText(display.getText()+input);
}
} //InsertAction事件侦听器类定义结束
private class CommandAction implements ActionListener // CommandAction事件侦听器类定义开始
{
public void actionPerformed(ActionEvent event)
{
String command=event.getActionCommand();
if(command.equals("清零"))
{
display.setText("0");
start=true;
flag=true;
command="=";
}
else
if(start&&flag)
{
if(command.equals("-"))
{
display.setText(command);
start=false;
}
else
if((command.equals("1/x")||command.equals("sqrt"))&&flag)
calculate(Double.parseDouble(display.getText()),command);
else
if(flag)
lastCommand=command;
}
else
{
if(command.equals("退格")&&flag)
{
String s=display.getText();
char[] s1=s.toCharArray();
if(s.length()>=2)
{
String s2=new String(s1,0,s.length()-1);
display.setText(s2);
}
else
{
display.setText("0");
start=true;
}
}
else if(flag)
{
calculate(Double.parseDouble(display.getText()),command);

lastCommand=command;
start=true;
}
}
}
} //CommandAction事件侦听器类定义结束
public void calculate(double x,String command) // calculate方法定义开始
{
if(lastCommand.equals("+")) result+=x;
else if(lastCommand.equals("-")) result-=x;
else if(lastCommand.equals("/"))
{
if(x!=0)
result/=x;
else
{
display.setText("除数不能为0");
start=false;
flag=false;
return;
}
}
else if(lastCommand.equals("*")) result*=x;
else if(command.equals("1/x"))
{
if(x!=0)
result=1/x;
else
{
display.setText("除数不能为0");
start=false;
flag=false;
return;
}
}
else if(command.equals("sqrt"))
{
if(x>=0)
result=Math.sqrt(x);
else
{
display.setText("函数输入无效");
start=false;
flag=false;
return;
}
}
else if(lastCommand.equals("=")) result=x;
display.setText(""+result);
} //calculate方法定义结束
private JButton display; //声明按钮
private JPanel panel; //声明面板
private double result; //声明结果变量result
private String lastCommand; //声明字符串 lastCommand
private boolean start;
private boolean flag;
}
改动后的代码：（CommandAction事件侦听器类定义改动了，其他地方没改）
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Calculator
{
public static void main(String[] args)
{
CalculatorFrame frame=new CalculatorFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
}

class CalculatorFrame extends JFrame
{
public CalculatorFrame()
{
setTitle("计算器");
CalculatorPanel panel=new CalculatorPanel();
add(panel);
pack();
}
}

class CalculatorPanel extends Panel
{
public CalculatorPanel()
{
setLayout(new BorderLayout());

result=0;
lastCommand="=";
start=true;
flag=true;

display=new JButton("0");
display.setEnabled(false);
add(display,BorderLayout.NORTH);

ActionListener insert=new InsertAction();
ActionListener command=new CommandAction();

panel=new JPanel();
panel.setLayout(new GridLayout(4,5));

addButton("7",insert);
addButton("8",insert);
addButton("9",insert);
addButton("/",command);
addButton("清零",command);

addButton("4",insert);
addButton("5",insert);
addButton("6",insert);
addButton("*",command);
addButton("退格",command);

addButton("1",insert);
addButton("2",insert);
addButton("3",insert);
addButton("-",command);
addButton("sqrt",command);

addButton("0",insert);
addButton(".",insert);
addButton("=",command);
addButton("+",command);
addButton("1/x",command);

add(panel,BorderLayout.CENTER);
}
private void addButton(String label,ActionListener listener)
{
JButton button=new JButton(label);
button.addActionListener(listener);
panel.add(button);
}
private class InsertAction implements ActionListener
{
public void actionPerformed(ActionEvent event)
{
String input=event.getActionCommand();
if(start&&flag)
{
display.setText("");
start=false;
}
if(flag)
display.setText(display.getText()+input);
}
}
private class CommandAction implements ActionListener
{
public void actionPerformed(ActionEvent event)
{
String command=event.getActionCommand();

if(command.equals("清零"))
{
display.setText("0");
start=true;
flag=true;
lastCommand="=";
}
else
{
if(flag){
if(start){
if(command.equals("-"))
{
display.setText(command);
start=false;
}
else{
if((command.equals("1/x")||command.equals("sqrt")))
{calculate(Double.parseDouble(display.getText()),command);
start=true;}
else

lastCommand=command;
}
}
else{
if(command.equals("退格"))
{
String s=display.getText();
char[] s1=s.toCharArray();
if(s.length()>=2)
{
String s2=new String(s1,0,s.length()-1);
display.setText(s2);
}
else
{
display.setText("0");
start=true;
}

}
else{
calculate(Double.parseDouble(display.getText()),command);

lastCommand=command;
start=true;
}
}
}

}
}
}
public void calculate(double x,String command)
{
if(lastCommand.equals("+")) result+=x;
else if(lastCommand.equals("-")) result-=x;
else if(lastCommand.equals("/"))
{
if(x!=0)
result/=x;
else
{
display.setText("除数不能为0");
start=false;
flag=false;
return;
}
}
else if(lastCommand.equals("*")) result*=x;
else if(command.equals("1/x"))
{
if(x!=0)
result=1/x;
else
{
display.setText("除数不能为0");
start=false;
flag=false;
return;
}

}
else if(command.equals("sqrt"))
{
if(x>=0)
result=Math.sqrt(x);
else
{
display.setText("函数输入无效");
start=false;
flag=false;
return;
}
}
else if(lastCommand.equals("=")) result=x;

display.setText(""+result);
}
private JButton display;
private JPanel panel;
private double result;
private String lastCommand;
private boolean start;
private boolean flag;
}