Calculating taxes

Question

HELP!! The user enters the value of their property and then clicks the button to compute tax.
After I enter the amount and click the button I get the following error.
Exception occurred during event dispatching;
java.lang.NullPointerException
Here is my code.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Debug1Application
public static void main(String args[])
PropTax myCalc = new PropTax();
class PropTax extends JFrame implements ActionListener
// Computes property tax
// as 3.25% of property value
private JTextField propVal;
private JLabel title, countyName, prompt, taxBill;
private JButton calcButton;
public PropTax()
Container c = getContentPane();
c.setLayout(new FlowLayout());
JLabel title = new JLabel("Property Tax Calculator");
JLabel countyName = new JLabel("Opulent County");
JButton calcButton = new JButton("Compute Tax");
JLabel prompt = new JLabel("Enter the value of your property");
JLabel taxBill = new JLabel("Your County Collector");
JTextField propVal = new JTextField(8);
c.add(title);
c.add(countyName);
c.add(prompt);
c.add(taxBill);
c.add(propVal);
c.add(calcButton);
propVal.setText("");
calcButton.addActionListener(this);
setSize(400,300);
setVisible(true);
public void actionPerformed(ActionEvent event)
CalcTax myCalcTax = new CalcTax ();
double value = Double.parseDouble(propVal.getText());
CalcTax.calculate();
taxBill.setText("Property tax is $" + CalcTax.getTax());
class CalcTax
final static double tax = 0.325;
static double pTax;
static double value;
public static void calculate()
pTax = tax * value;
public static double getTax()
return pTax;
}

Answer 1

The NullPointerException occurs because taxBill is null.
Your code defines taxBill as an instance variable of class PropTax in the line:private JLabel title, countyName, prompt, taxBill;Instance variables that are object references are initialized to null.
In the PropTax() constructor, the code defines a local variable also with the name taxBill in the line:JLabel taxBill = new JLabel("Your County Collector");This variable is not the same one as the original definition and its scope is the constructor method. The instance variable taxBill remains null.
In actionPerformed(), the reference to taxBill is to the instance variable and so you get a NullPointerException.
This is easy to fix, just don't declare the second set of variables in the constructor. Change the constructor code to be title = new JLabel("Property Tax Calculator");
countyName = new JLabel("Opulent County");
calcButton = new JButton("Compute Tax");
prompt = new JLabel("Enter the value of your property");
taxBill = new JLabel("Your County Collector");
propVal = new JTextField(8);The changes that slyle pointed out still need to be made to fix the calculation logic.