小小计算器
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.lang.*;
import java.applet.Applet;
import javax.swing.*;
import javax.swing.border.*;

public class Calculator extends JApplet implements ActionListener
{
private final String[] KEYS={"7","8","9","/","sqrt",
"4","5","6","*","%",
"1","2","3","-","1/x",
"0","+/-",".","+","="
};
private final String[] COMMAND={"Backspace","CE","C"};
private final String[] M={" ","MC","MR","MS","M+"};
private JButton keys[]=new JButton[KEYS.length];
private JButton commands[]=new JButton[COMMAND.length];
private JButton m[]=new JButton[M.length];
private JTextField display =new JTextField("0");


// public JTextField setHorizontalAlignment(int alignment);
// private JTextField display.setHorizontalAlignment(JTextField.RIGHT)=new JTextField("0");

private void setup()
{
display.setHorizontalAlignment(JTextField.RIGHT);
JPanel calckeys=new JPanel();
JPanel command=new JPanel();
JPanel calms=new JPanel();
calckeys.setLayout(new GridLayout(4,5,3,3));
command.setLayout(new GridLayout(1,3,3,3));
calms.setLayout(new GridLayout(5,1,3,3));


for(int i=0;i<KEYS.length;i++)
{
keys[i]=new JButton(KEYS[i]);
calckeys.add(keys[i]);
keys[i].setForeground(Color.blue);
}
keys[3].setForeground(Color.red);
keys[4].setForeground(Color.red);
keys[8].setForeground(Color.red);
keys[9].setForeground(Color.red);
keys[13].setForeground(Color.red);
keys[14].setForeground(Color.red);
keys[18].setForeground(Color.red);
keys[19].setForeground(Color.red);


for(int i=0;i<COMMAND.length;i++)
{
commands[i]=new JButton(COMMAND[i]);
command.add(commands[i]);
commands[i].setForeground(Color.red);
}
for(int i=0;i<M.length;i++)
{
m[i]=new JButton(M[i]);
calms.add(m[i]);
m[i].setForeground(Color.red);
}
JPanel panel1=new JPanel();
panel1.setLayout(new BorderLayout(3,3));
panel1.add("North",command);
panel1.add("Center",calckeys);
JPanel top=new JPanel();
top.setLayout(new BorderLayout());
display.setBackground(Color.WHITE);
top.add("Center",display);
getContentPane().setLayout(new BorderLayout(3,5));
getContentPane().add("North",top);
getContentPane().add("Center",panel1);
getContentPane().add("West",calms);
}

public void init()
{

setup();

for(int i=0;i<KEYS.length;i++)
{
keys[i].addActionListener(this);
}
for(int i=0;i<COMMAND.length;i++)
{
commands[i].addActionListener(this);
}
for(int i=0;i<M.length;i++)
{
m[i].addActionListener(this);
}
display.setEditable(false);
}

public void actionPerformed(ActionEvent e)
{
String label=e.getActionCommand();
// double zero=e.getActionCommand();
if(label=="C")
handleC();
else if(label=="Backspace")
handleBackspace();
else if(label=="CE")
display.setText("0");
else if("0123456789.".indexOf(label)>=0)
{handleNumber(label);
// handlezero(zero);
}

else
handleOperator(label);
}

private boolean firstDigit=true;

private void handleNumber(String key)
{
if(firstDigit)
display.setText(key);
else if((key.equals("."))&&(display.getText().indexOf(".")<0))
display.setText(display.getText()+".");
else if(!key.equals("."))
display.setText(display.getText()+key);
firstDigit=false;
}
//private void handlezero(String key)
//{
// if(!((double)display.setText(key))
// display.setText(0);
// }
private double number=0.0;
private String operator="=";

private void handleOperator(String key)
{
if(operator.equals("/"))
{
if(getNumberFromDisplay()==0.0)
display.setText("除数不能为零");
else
{
number/=getNumberFromDisplay();
long t1;
double t2;
t1=(long)number;
t2=number-t1;
if(t2==0)
display.setText(String.valueOf(t1));
else
display.setText(String.valueOf(number));
}
}
else if(operator.equals("1/x"))
{
if(number==0.0)
display.setText("零没有倒数");
else
{
number=1/number;
long t1;
double t2;
t1=(long)number;
t2=number-t1;
if(t2==0)
display.setText(String.valueOf(t1));
else
display.setText(String.valueOf(number));
}
}
else if(operator.equals("+"))
number+=getNumberFromDisplay();
else if(operator.equals("-"))
number-=getNumberFromDisplay();
else if(operator.equals("*"))
number*=getNumberFromDisplay();
else if(operator.equals("sqrt"))
{
number=Math.sqrt(number);

}
else if(operator.equals("%"))
number=number/100;
else if(operator.equals("+/-"))
number=number*(-1);
else if(operator.equals("="))
number=getNumberFromDisplay();
long t1;
double t2;
t1=(long)number;
t2=number-t1;
if(t2==0)
display.setText(String.valueOf(t1));
else
display.setText(String.valueOf(number));
operator=key;
firstDigit=true;
}

private double getNumberFromDisplay()
{
return Double.valueOf(display.getText()).doubleValue();
}

private void handleC()
{
display.setText("0");
firstDigit=true;
operator="=";
}
private void handleBackspace()
{
String text=display.getText();
int i=text.length();
if(i>0)
{
text=text.substring(0,i-1);
if(text.length()==0)
{
display.setText("0");
firstDigit=true;
operator="=";
}
else
display.setText(text);
}

}
public static void main(String args[])
{
JFrame f=new JFrame();
Calculator Calculator1=new Calculator();
Calculator1.init();
f.getContentPane().add("Center",Calculator1);
f.setVisible(true);
f.setBounds(300,200,380,245);
f.setBackground(Color.LIGHT_GRAY);
f.validate();
f.setResizable(false);
f.addWindowListener(new WindowAdapter()
{
public void windowClosing(WindowEvent e)
{
System.exit(0);
}
});
f.setTitle("计算器");
}
}