Trying to create and run my first class

Question

I am creating a simple class that should create a basic looking calculator type UI.
When I try to build the program it builds successfully.
However I get some null pointer exeption messages, and the GUI never appears.
I am hoping that my problem is something simple.
Any advice is appreciated.
This is the class file:
It will be followed by the main file.
* CalculatorInterface.java
* Created on: October 2, 2006, 10:43 AM
* By: @author Administrator
* Purpose: To create a Calculator GUI
package Calculator;
import java.text.*;
import java.util.*;
import javax.swing.*;
import java.awt.*;
public class CalculatorInterface extends JFrame{
    private static final int WIDTH = 400;   //the width of the container
    private static final int HEIGHT = 400;  //the height of the container
    private JTextField numberTextField1, numberTextField2; //declare the text fields
    private JButton addB, subtractB, multiplyB, divideB; //declare the math buttons
    private JButton exitB; //declare the exit button
    private JLabel resultLabel;//declare result label
    private JPanel numbersPanel, buttonsPanel, resultsPanel;//declare panels
    private Container pane;//declare the container
    private GridLayout grid2by2, grid1by2;//declare the grids that will format buttonsPanel and numbersPanel
    private FlowLayout flowCenter;//declare the format for results panel
    private Toolkit aToolKit;//declare toolkit object used to get screen dimensions
    private Dimension screen;//declare variable used to hold screen dimensions
    private int xPositionOfFrame;//declare variable to hold the x position of frame
    private int yPositionOfFrame;//declare variable to hold the y position of frame
    private Formatter formatter;//declare formatter object to be applied to results
    /** Creates a new instance of CalculatorInterface */
    public CalculatorInterface() {
        setTitle( "Calculator" );//set title of frame
        setSize( WIDTH, HEIGHT );//set size of frame
        //instantiate JTextField objects
        numberTextField1 = new JTextField(5);
        numberTextField2 = new JTextField(5);
        //instantiate JButton Objects
        addB        = new JButton("+");
        subtractB   = new JButton("-");
        multiplyB   = new JButton("*");
        divideB     = new JButton("/");
        pane = getContentPane(); //instantiate the pane
        //instantiate the grid layouts
        grid1by2 = new GridLayout( 1, 2 );//grid for text fields
        grid2by2 = new GridLayout( 2, 2 );//grid for math buttons
        flowCenter = new FlowLayout();//to be applied to results pane
        //create the 3 panels
        buttonsPanel = new JPanel();
        numbersPanel = new JPanel();
        resultsPanel = new JPanel();
        //set layouts for panels
        buttonsPanel.setLayout( grid2by2 );
        numbersPanel.setLayout( grid1by2 );
        resultsPanel.setLayout( flowCenter );
        //add button fields to buttons panel
        buttonsPanel.add( addB );
        buttonsPanel.add( multiplyB );
        buttonsPanel.add( subtractB );
        buttonsPanel.add( divideB );
        //add text fields to numbers panel
        numbersPanel.add( numberTextField1 );
        numbersPanel.add( numberTextField2 );
        //add result label and exit button to results panel
        resultsPanel.add( resultLabel );
        resultsPanel.add( exitB );
        //add panels to content pane
        pane.add( numbersPanel, BorderLayout.WEST );
        pane.add( buttonsPanel, BorderLayout.EAST );
        pane.add( resultsPanel, BorderLayout.SOUTH );
}This is the main file.
* Main.java
* Created: October 2, 2006, 10:41 AM
* Author: Klinton Kerber
* Purpose:To demonstrate knowledge gained in chapter 5
*  by building a calculator that can add, subtract, divide,
*  and multiply two numbers.
package Calculator;
import javax.swing.*;
public class Main {
    /** Creates a new instance of Main */
    public Main() {
     * @param args the command line arguments
    public static void main(String[] args) {
        JFrame aCalculatorGUI = new CalculatorInterface(); //create the GUI
        aCalculatorGUI.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        aCalculatorGUI.setVisible( true );
}Thanks for the time.

Answer 1

You read a stack trace from bottom up to get a chrnological list of events that led to the problem.
The last line says that you were executing the main method in
the Calculator.Main class. On line 26 of the file Main.java, the
constructor of Calculator.CalculatorInterface was called (see line
above the last one). <init> stands for constructor. On line 95 of
CalculatorInterface.java, you called Container.add. As you can see,
other calls follow. They are internal to the Java libraries, therefore
not so relevant. You now know that the problem is on line 95 of
CalculatorInterface.java. Why would a library method "crash"?
Probably because you gave it a wrong parameter as explained in the
other answer.