问答题 本题中定义了一个简单的计算器，可以进行基本的四则运算。程序中包含16个按钮用来表示0～9、+、-、*、/、=运算符和小数点，程序顶部的文本框用来显示操作数以及结果。
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class java2
public static void main(String[]args)
try
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());

catch(Exception e)
JFrame frame=new CalculatorFrame();
frame.show();


class CalculatorPanel extends JPanel implements ActionListener
private JTextField display;
private JButton btn;
private double arg=0;
private String op="=";
private boolean start=true;
public CalculatorPanel()
setLayout(new BorderLayout());
display=new JTextField("0");
display.setEditable(false);
add(display,"North");
JPanel p=new JPanel();
p.setLayout(new GridLayout(4,4));
String buttons="789/456*123-0.=+";
for(int i=0;i＜buttons.length();i++)
btn=new JButton(buttons.substring(i,i+1));
p.add(btn);
______;

add(p,"Center");

public void actionPerformed(ActionEvent evt)
String s=evt.getActionCommand();
if('0'＜=s.charAt(0)&&s.charAt(0)＜='9'|| s.equals("."))
if(start)display.setText(s);
else display.setText(display.getText()+s);
start=false;

else
if(start)
if(s.equals("-"))
display.setText(s);
start=false;

else op=s;

else
double x=______;
calculate(x);
op=s;
start=true;



public void calculate(double n)
if(op.equals("+"))arg+=n;
else if(op.equals("-"))arg--n;
else if(op.equals("*"))arg*=n;
else if(op.equals("/"))arg/=n;
else if(op.equals("="))arg=n;
display.setText(""+arg);


class CalculatorFrame extends JFrame
public CalculatorFrame()
setTitle("java2");
setSize(220,180);
addWindowListener(new WindowAdapter()
public void windowClosing(WindowEvent e)
System.exit(0);

);
Container contentPane=getContentPane();
contentPane.add(new CalculatorPanel());