import java.awt.Container;

import java.awt.event.ActionEvent;

import java.awt.event.ActionListener;

import java.awt.event.WindowAdapter;

import java.awt.event.WindowEvent;

import javax.swing.JButton;

import javax.swing.JFrame;

import javax.swing.JTextField;

public class Calculator extends JFrame implements ActionListener

{

private JTextField displayText = new JTextField(30);

private JButton[] button = new JButton[19];

private String[] keys = { "7","8","9","/","4","5","6","*","1","2","3","-","0","C","=","+","M","R","E"};

private String numStr1 = "";

private String numStr2 = "";

private String memory = "";

private char op;

private boolean firstInput = true;

public Calculator()

{

setTitle("Calculator");

setSize(230,230);

Container pane = getContentPane();

pane.setLayout(null);;

displayText.setSize(200,30);;

displayText.setLocation(10,10);

pane.add(displayText);

int x,y;

x=10;

y=40;

for(int ind=0;ind<19;ind++)

{

button[ind] = new JButton(keys[ind]);

button[ind].addActionListener(this);

button[ind].setSize(50, 30);

button[ind].setLocation(x,y);

pane.add(button[ind]);

x = x + 50;

if((ind +1) % 4 == 0)

{

x=10;

y=y+30;

}

}

this.addWindowListener(new WindowAdapter()

{

public void windowClosing(WindowEvent e)

{

System.exit(0);

}

}

);

//java.awt.event.ActionListener.actionPerformed()

setVisible(true);

}

public void actionPreformed(ActionEvent ae)

{

String resultStr;

String str = String.valueOf(ae.getActionCommand());

char ch = str.charAt(0);

switch(ch)

{

case'0':

case'1':

case'2':

case'3':

case'4':

case'5':

case'6':

case'7':

case'8':

case'9':

if(firstInput)

{

numStr1 = numStr1 +ch;

displayText.setText(numStr1);

}

else

{

numStr2 = numStr2 + ch;

displayText.setText(numStr2);

}

break;

case'+':

case'-':

case'*':

case'/':

op = ch;

firstInput = false;

break;

case'=':

resultStr = evaluate();

displayText.setText(resultStr);

numStr1 = resultStr;

numStr2 = "";

firstInput = false;

break;

case'C':

displayText.setText("");

numStr1 = "";

numStr2 = "";

firstInput = true;

case 'M':

memory = displayText.getText();

break;

case'R':

displayText.setText(memory);

numStr1 = memory;

break;

case 'E':

System.exit(0);

}

}

private String evaluate()

{

final char beep = '\u0007';

try

{

int num1 = Integer.parseInt(numStr1);

int num2 = Integer.parseInt(numStr2);

int result = 0;

switch(op)

{

case '+':

result = num1 + num2;

break;

case '-':

result = num1 - num2;

break;

case '*':

result = num1 * num2;

break;

case '/':

result = num1 / num2;

}

return String.valueOf(result);

}

catch(ArithmeticException ae)

{

System.out.print(beep);

return "E R R O R: " + ae.getMessage();

}

catch(NumberFormatException nfe)

{

System.out.print(beep);

if(numStr1.equals(""))

return "E R R O R: Invalid First Number";

else

return "E R R O R: Invalid Second Number";

}

catch(Exception e)

{

System.out.println(beep);

return " E R R O R";

}

}

public static void main(String[] args)

{

Calculator C = new Calculator();

}

}

Can you help me fix my JAVA program?

I beleive i need to implement java.awt.event.ActionListener.actionPerformed()