My program compiles with no errors, but when I try to use it, the results do not show up and there is an error. Please help!!!

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.text.*;

public class RetailSales extends JFrame
{
    // declarations
    Color black = new Color(0, 0, 0);
    Color white = new Color(255, 255, 255);
    Color light_gray = new Color(192, 192, 192);

    // inputs
    JLabel itemQuantityJLabel;
    JTextField itemQuantityJTextField;

    JLabel itemPriceJLabel;
    JTextField itemPriceJTextField;

    // outputs
    JLabel subTotalResultsJLabel;
    JTextField subTotalResultsJTextField;

    JLabel salesTaxResultsJLabel;
    JTextField salesTaxResultsJTextField;

    JLabel totalAmountResultsJLabel;
    JTextField totalAmountResultsJTextField;


    // controls
    JButton enterJButton;
    JButton clearJButton;
    JButton closeJButton;

    // variables
    int numberOfItems;
    double pricePerItem;
    double subTotal;
   double salesTax;
   double totalAmount;
   double taxRate;

   SalesCalculation salesCalculation;


   DecimalFormat currency;


    public RetailSales()
    {
            createUserInterface();
    }

    public void createUserInterface()
    {
        Container contentPane = getContentPane();
        contentPane.setBackground(white);
        contentPane.setLayout(null);

        /* declarations */

        // inputs
        itemQuantityJLabel = new JLabel();
        itemQuantityJLabel.setBounds(50, 50, 120, 20);
        itemQuantityJLabel.setFont(new Font("Default", Font.PLAIN, 12));
        itemQuantityJLabel.setText("Number Of Items");
        itemQuantityJLabel.setForeground(black);
        itemQuantityJLabel.setHorizontalAlignment(JLabel.RIGHT);
        contentPane.add(itemQuantityJLabel);

        itemQuantityJTextField = new JTextField();
        itemQuantityJTextField.setBounds(190, 50, 75, 20);
        itemQuantityJTextField.setFont(new Font("Default", Font.PLAIN, 12));
        itemQuantityJTextField.setHorizontalAlignment(JTextField.CENTER);
        itemQuantityJTextField.setForeground(black);
        itemQuantityJTextField.setBackground(white);
        itemQuantityJTextField.setEditable(true);
        contentPane.add(itemQuantityJTextField);

        itemPriceJLabel = new JLabel();
        itemPriceJLabel.setBounds(50, 80, 120, 20);
        itemPriceJLabel.setFont(new Font("Default", Font.PLAIN, 12));
        itemPriceJLabel.setText("Price Per Item");
        itemPriceJLabel.setForeground(black);
        itemPriceJLabel.setHorizontalAlignment(JLabel.RIGHT);
        contentPane.add(itemPriceJLabel);

        itemPriceJTextField = new JTextField();
        itemPriceJTextField.setBounds(190, 80, 75, 20);
        itemPriceJTextField.setFont(new Font("Default", Font.PLAIN, 12));
        itemPriceJTextField.setHorizontalAlignment(JTextField.CENTER);
        itemPriceJTextField.setForeground(black);
        itemPriceJTextField.setBackground(white);
        itemPriceJTextField.setEditable(true);
        contentPane.add(itemPriceJTextField);

        // outputs
        subTotalResultsJLabel = new JLabel();
        subTotalResultsJLabel.setBounds(50, 140, 120, 20);
        subTotalResultsJLabel.setFont(new Font("Default", Font.PLAIN, 12));
        subTotalResultsJLabel.setText("Sub Total");
        subTotalResultsJLabel.setForeground(black);
        subTotalResultsJLabel.setHorizontalAlignment(JLabel.RIGHT);
        contentPane.add(subTotalResultsJLabel);

        subTotalResultsJTextField = new JTextField();
        subTotalResultsJTextField.setBounds(190, 140, 75, 20);
        subTotalResultsJTextField.setFont(new Font("Default", Font.PLAIN, 12));
        subTotalResultsJTextField.setHorizontalAlignment(JTextField.CENTER);
        subTotalResultsJTextField.setForeground(black);
        subTotalResultsJTextField.setBackground(white);
        subTotalResultsJTextField.setEditable(false);
        contentPane.add(subTotalResultsJTextField);

        salesTaxResultsJLabel = new JLabel();
        salesTaxResultsJLabel.setBounds(50, 170, 120, 20);
        salesTaxResultsJLabel.setFont(new Font("Default", Font.PLAIN, 12));
        salesTaxResultsJLabel.setText("Sales Tax");
        salesTaxResultsJLabel.setForeground(black);
        salesTaxResultsJLabel.setHorizontalAlignment(JLabel.RIGHT);
        contentPane.add(salesTaxResultsJLabel);

        salesTaxResultsJTextField = new JTextField();
        salesTaxResultsJTextField.setBounds(190, 170, 75, 20);
        salesTaxResultsJTextField.setFont(new Font("Default", Font.PLAIN, 12));
        salesTaxResultsJTextField.setHorizontalAlignment(JTextField.CENTER);
        salesTaxResultsJTextField.setForeground(black);
        salesTaxResultsJTextField.setBackground(white);
        salesTaxResultsJTextField.setEditable(false);
        contentPane.add(salesTaxResultsJTextField);

        totalAmountResultsJLabel = new JLabel();
        totalAmountResultsJLabel.setBounds(50, 200, 120, 20);
        totalAmountResultsJLabel.setFont(new Font("Default", Font.PLAIN, 12));
        totalAmountResultsJLabel.setText("Total Amount");
        totalAmountResultsJLabel.setForeground(black);
        totalAmountResultsJLabel.setHorizontalAlignment(JLabel.RIGHT);
        contentPane.add(totalAmountResultsJLabel);

        totalAmountResultsJTextField = new JTextField();
        totalAmountResultsJTextField.setBounds(190, 200, 75, 20);
        totalAmountResultsJTextField.setFont(new Font("Default", Font.PLAIN, 12));
        totalAmountResultsJTextField.setHorizontalAlignment(JTextField.CENTER);
        totalAmountResultsJTextField.setForeground(black);
        totalAmountResultsJTextField.setBackground(white);
        totalAmountResultsJTextField.setEditable(false);
        contentPane.add(totalAmountResultsJTextField);


        // controls
        enterJButton = new JButton();
        enterJButton.setBounds(20, 320, 100, 20);
        enterJButton.setFont(new Font("Default", Font.PLAIN, 12));
        enterJButton.setText("Enter");
        enterJButton.setForeground(black);
        enterJButton.setBackground(white);
        contentPane.add(enterJButton);
        enterJButton.addActionListener(

            new ActionListener()
            {
                public void actionPerformed(ActionEvent event)
                {
                    enterJButtonActionPerformed(event);
                }
            }
        );

        clearJButton = new JButton();
        clearJButton.setBounds(140, 320, 100, 20);
        clearJButton.setFont(new Font("Default", Font.PLAIN, 12));
        clearJButton.setText("Clear");
        clearJButton.setForeground(black);
        clearJButton.setBackground(white);
        contentPane.add(clearJButton);
        clearJButton.addActionListener(

            new ActionListener()
            {
                public void actionPerformed(ActionEvent event)
                {
                    clearJButtonActionPerformed(event);
                }
            }
        );

        closeJButton = new JButton();
        closeJButton.setBounds(260, 320, 100, 20);
        closeJButton.setFont(new Font("Default", Font.PLAIN, 12));
        closeJButton.setText("Close");
        closeJButton.setForeground(black);
        closeJButton.setBackground(white);
        contentPane.add(closeJButton);
        closeJButton.addActionListener(

            new ActionListener()
            {
                public void actionPerformed(ActionEvent event)
                {
                    closeJButtonActionPerformed(event);
                }
            }
        );

        setTitle("RetailSales");
        setSize( 400, 400 );
        setVisible(true);
    }

    // main method
    public static void main(String[] args)
    {
        RetailSales application = new RetailSales();
        application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public void enterJButtonActionPerformed(ActionEvent event)
    {
        try
        {
            numberOfItems = Integer.parseInt(itemQuantityJTextField.getText());
            numberOfItemsNotZero();
        }
        catch(NumberFormatException exception)
        {
                JOptionPane.showMessageDialog(this,
                "Please enter Number of Items!",
                "Number Format Error", JOptionPane.ERROR_MESSAGE);
                itemQuantityJTextField.setText("");
                itemQuantityJTextField.requestFocusInWindow();
        }
    }

    public void numberOfItemsNotZero()
    {
        if(numberOfItems == 0)
        {
            JOptionPane.showMessageDialog(this,
            "Please enter Number of Items greater than zero!",
            "Number Format Error", JOptionPane.ERROR_MESSAGE);
            itemQuantityJTextField.setText("");
            itemQuantityJTextField.requestFocusInWindow();
        }
        else
        {
            getPricePerItem();
        }
    }

    public void getPricePerItem()
    {
        try
        {
            pricePerItem = Double.parseDouble(itemPriceJTextField.getText());
            pricePerItemNotZero();
        }
        catch(NumberFormatException exception)
        {
                JOptionPane.showMessageDialog(this,
                "Please Price Per Item!",
                "Number Format Error", JOptionPane.ERROR_MESSAGE);
                itemPriceJTextField.setText("");
                itemPriceJTextField.requestFocusInWindow();
        }
    }

    public void pricePerItemNotZero()
    {
        if(pricePerItem == 0)
        {
            JOptionPane.showMessageDialog(this,
            "Please enter second number greater than zero!",
            "Number Format Error", JOptionPane.ERROR_MESSAGE);
            itemPriceJTextField.setText("");
            itemPriceJTextField.requestFocusInWindow();
        }
        else
        {
            getCalculations();
        }
    }

   public void getCalculations()
   {
      getResults();
      displayResults();
   }


    public void getResults()
   {
      salesCalculation = new SalesCalculation(numberOfItems, pricePerItem);
      subTotal = salesCalculation.calculateSubTotal();
      salesTax = salesCalculation.calculateSalesTax();
      totalAmount = salesCalculation.calculateTotalAmount();  
   }

   public void displayResults()
   {
      subTotalResultsJTextField.setText("" + currency.format(subTotal));
      salesTaxResultsJTextField.setText("" + currency.format(salesTax));
      totalAmountResultsJTextField.setText("" + currency.format(totalAmount));  
   }

    public void clearJButtonActionPerformed(ActionEvent event)
    {
        itemQuantityJTextField.setText("");
        itemQuantityJTextField.requestFocusInWindow();
        itemPriceJTextField.setText("");
        subTotalResultsJTextField.setText("");
        salesTaxResultsJTextField.setText("");
        totalAmountResultsJTextField.setText("");
    }

    public void closeJButtonActionPerformed(ActionEvent event)
    {
        RetailSales.this.dispose();
    }
}

class SalesCalculation
{
   int firstNumber;
   double secondNumber;
   double taxRate;

   public SalesCalculation(int numberOfItems, double pricePerItem)
   {
      firstNumber = numberOfItems;
      secondNumber = pricePerItem;
      taxRate = .07;
   }

   public double calculateSubTotal()
   {
      return firstNumber * secondNumber;
   }

   public double calculateSalesTax()
   {
      return firstNumber * secondNumber * taxRate;
   }

   public double calculateTotalAmount()
   {
      return (firstNumber * secondNumber * taxRate) + (firstNumber * secondNumber);
   }
}

Here is the error that comes up after I hit enter:

 ----jGRASP exec: java RetailSales
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at RetailSales.displayResults(RetailSales.java:306)
    at RetailSales.getCalculations(RetailSales.java:292)
    at RetailSales.pricePerItemNotZero(RetailSales.java:285)
    at RetailSales.getPricePerItem(RetailSales.java:261)
    at RetailSales.numberOfItemsNotZero(RetailSales.java:252)
    at RetailSales.enterJButtonActionPerformed(RetailSales.java:228)
    at RetailSales$1.actionPerformed(RetailSales.java:170)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6525)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
    at java.awt.Component.processEvent(Component.java:6290)
    at java.awt.Container.processEvent(Container.java:2234)
    at java.awt.Component.dispatchEventImpl(Component.java:4881)
    at java.awt.Container.dispatchEventImpl(Container.java:2292)
    at java.awt.Component.dispatchEvent(Component.java:4703)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
    at java.awt.Container.dispatchEventImpl(Container.java:2278)
    at java.awt.Window.dispatchEventImpl(Window.java:2739)
    at java.awt.Component.dispatchEvent(Component.java:4703)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:746)
    at java.awt.EventQueue.access$400(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:697)
    at java.awt.EventQueue$3.run(EventQueue.java:691)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
    at java.awt.EventQueue$4.run(EventQueue.java:719)
    at java.awt.EventQueue$4.run(EventQueue.java:717)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:716)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Nevermind! I realize now it was because I had not defined my currency format.

This question has already been answered. Start a new discussion instead.