text field calculation applet error

[aleX_X]

Here is what I need to do: have three editable text fields labelled operand1, operator, and operand2, and a fourth read-only text field labelled result. Have a button labelled Calculate. WHen the button is clicked the appropriate operator is applied to the two operands and the resulting value placed in the result text field. If the operator is invalid, send a message to System.err<.code>

Here is my code

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class calculator extends Applet implements ActionListener {
	public void init() {
		Label lbOp1 = new Label("operand1");
		op1 = new TextField("0");

		calculate = new Button("=");
		calculate.addActionListener(this);
		clearEm = new Button("Clear");
		clearEm.addActionListener(this);
		
		Label lbOper = new Label("operator");
		oper = new TextField("+");

		Label lbOp2 = new Label("operand2");
		op2 = new TextField("0");

		Label lbResult = new Label("result");
		
		add(lbOp1);
		add(op1);

		add(lbOper);
		add(oper);

		add(lbOp2);
		add(op2);
		
		add(lbResult);
		add(result);
		
		add(calculate);
		add(clearEm);
	}

	public void actionPerformed(ActionEvent e) {
		if(e.getSource()==calculate)
		{
			one = op1.getText();
			two = op2.getText();
			num1 = Integer.parseInt(one);
			num2 = Integer.parseInt(two);
			strOper = oper.getText();
			if(strOper.equals("+"))
			{	
				answer = num1 + num2;
				math = Integer.toString(answer);
				result.setText(math);
			}
			else if(strOper.equals("-"))
			{	
				answer = num1 - num2;
				math = Integer.toString(answer);
				result.setText(math);
			}
			else if(strOper.equals("*"))
			{	
				answer = num1 * num2;
				math = Integer.toString(answer);
				result.setText(math);
			}
			else if(strOper.equals("/"))
			{	
				answer = num1 / num2;
				math = Integer.toString(answer);
				result.setText(math);
			}
			else
				System.err.println("invalid operator");
		}
		if(e.getSource()==clearEm)
		{
			op1.setText("");
			op2.setText("");
			oper.setText("");
		}
	}

	TextField op1, op2, oper, result;
	Button calculate, clearEm;
	String one, two, strOper, math;
	int num1, num2, answer;
}

The app won't show and the console shows this error

java.lang.NullPointerException
	at java.awt.Container.addImpl(Unknown Source)
	at java.awt.Container.add(Unknown Source)
	at calculator.init(calculator.java:33)
	at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)
Exception: java.lang.NullPointerException

[hanvyj]

It would really help if you could tell us which line the error occurs on, null pointers are quite simple to debug though, it means you are trying to use a variable that is "null", what fields are before you set it a value.

If you forget to call init(), then you will get a load of null pointer exceptions because all your TextFields are null etc