问答题

本程序是一个Applet，功能是进行简单的实数四则运算，如图所示。


import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class exam_63 extends JApplet
{
public void init()
{
Container contentPane=getContentPane();
CalculatorPanel panel=new CalculatorPanel();
add(panel) ;
}
}
class CalculatorPanel extends JPanel
{
public CalculatorPanel()
{
setLayout(new BorderLayout());
result=0;
lastCommand="=";
start=true;
display=new JTextField("0");
display.setEditable(false);
add(display, BorderLayout.NORTH);
ActionListener insert=new InsertAction();
ActionListener command=new CommandAction();
panel=new JPanel();
panel.setLayout(new GridLayout(4, 4));
addButton("7", insert);
addButton("8", insert);
addButton("9", insert);
addButton("/", command);
addButton("4", insert);
addButton("5", insert);
addButton("6", insert);
addButton("*", command);
addButton("1", insert);
addButton("2", insert);
addButton("3", insert);
addButton("-", command);
addButton("0", insert);
addButton(".", insert);
addButton("=", command);
addButton("+", 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)
{
display.setText("");
start=false;
}
display.settext(display.getText() + input) ;
}
}
private class CommandAction implements ActionListener
{
public void actionPerformed(ActionEvent evt)
{
String command=evt.getActionCommand();
if (start)
{
if (command.equals("-"))
{
display.setText(command);
start=false;
}
else
lastCommand=command;
}
else
{
calculate(Double.parseDouble(display.getText()));
lastCommand=command;
start=true;
}
}
}
public double calculate(double x)
{
if (lastCommand.equals("+")) result += x;
else if (lastCommand.equals("-")) result -= x;
else if (lastCommand.equals("*")) result *= x;
else if (lastCommand.equals("/")) result /= x;
else if (lastCommand.equals("=")) result=x;
display.setText("" + result);
}
private JTextField display;
private JPanel panel;
private double result;
private String lastCommand;
private boolean start;
}