Homework - Creating a number pad for a calculator

public class CalculatorApp extends JFrame
{


	private LineBuffer _myBuffer;  // a useful instance variable
	
	public CalculatorApp(String title){
		super(title);  
        this.setSize(600, 300);
        this.setDefaultCloseOperation(javax.swing.JFrame.EXIT_ON_CLOSE);
		
	
	JPanel button ; 
	button =  new JPanel(new GridLayout(4,3));
	
	button.add(new JButton("7"));
	button.add(new JButton("8"));
	button.add(new JButton("9"));
	button.add(new JButton("4"));
	button.add(new JButton("5"));
	button.add(new JButton("6"));
	button.add(new JButton("1"));
	button.add(new JButton("2"));
	button.add(new JButton("3"));
	button.add(new JButton("0"));
	button.add(new JButton("C"));
	button.add(new JButton("ENTER"));
	this.add(button, BorderLayout.SOUTH);
	
	JPanel outP;
	
	outP = new JPanel();
	outP.add(new JLabel("0"));
	outP.setBackground(java.awt.Color.WHITE);
	outP.setBorder(BorderFactory.createEtchedBorder());
	this.add(outP, BorderLayout.NORTH);
	
	this.setVisible(true);
	}
	
	public static void main(String[] args) {
		CalculatorApp app = new CalculatorApp ("Calculator");
	
	}
}

public class NumberPanel extends JPanel
{
	
	
	private JButton b1, b2, b3, b4, b5, b6 ,b7, b8, b9, b0, bpi, bEnter; 
	private JLabel empty; 
	private JTextField _textField; 
	
	public NumberPanel()
	{
		super();
		
		
		
		ButtonListener listener = new ButtonListener();
		
		b1 = new JButton ("1");
		b1.addActionListener(listener);
		
		b2 = new JButton("2");
		b2.addActionListener(listener);
		
		b3 = new JButton("3");
		b3.addActionListener(listener);
		
		b4 = new JButton("4");
		b4.addActionListener(listener);
		
		b5 = new JButton("5");
		b5.addActionListener(listener);
		
		b6 = new JButton("6");
		b6.addActionListener(listener);
		
		b7 = new JButton("7");
		b7.addActionListener(listener);
		
		b8 = new JButton("8");
		b8.addActionListener(listener);
		
		b9 = new JButton("9");
		b9.addActionListener(listener);
		
		b0 = new JButton("0");
		b0.addActionListener(listener);
		
		bpi = new JButton("π");
		bpi.addActionListener(listener);
		
		bEnter = new JButton("ENTER");
		bEnter.addActionListener(listener);
		
		JPanel button = new JPanel(new java.awt.GridLayout(4,3));
		
		
		button.add(b7);
		button.add(b8);
		button.add(b9);
		button.add(b4);
		button.add(b5);
		button.add(b6);
		button.add(b1);
		button.add(b2);
		button.add(b3);
		button.add(b0);
		button.add(bpi);
		button.add(bEnter);
		
		this.add(button);
		
		
		
			
		
	}
	
	
	
	
//this is now in my CalculatorApp class

 NumberPanel theNumberPanel = new NumberPanel();
	
	this.add(theNumberPanel, BorderLayout.SOUTH);
	
	
	
	

public class CalculatorApp extends JFrame
{


	private LineBuffer _myBuffer;  // a useful instance variable
	private JTextField _outputField;
	private NumberPanel _theNumberPanel;
	private FunctionButton _thefunctionButtons;


	public CalculatorApp(String title)
	{
		super(title);  
		this.setSize(400, 250);
		this.setDefaultCloseOperation(javax.swing.JFrame.EXIT_ON_CLOSE);



		_theNumberPanel = new NumberPanel();

		this.add(_theNumberPanel, BorderLayout.SOUTH);
		
		_thefunctionButtons = new FunctionButton();
		this.add(_thefunctionButtons, BorderLayout.CENTER);

		_outputField = new JTextField(20);
		_outputField.setHorizontalAlignment(JTextField.RIGHT);
		_outputField.setEditable(false);
		 new Font("SansSerif", Font.BOLD, 16);
		 _outputField.setFont(new Font("SansSerif", Font.BOLD, 16));
		

		JPanel outputPanel;

		outputPanel = new JPanel();
		outputPanel.setSize(600,75);
		outputPanel.add(_outputField);
		
		
		
		this.add(outputPanel, BorderLayout.NORTH);

		this.setVisible(true);
	}

	public static void main(String[] args) 
	{
		CalculatorApp app = new CalculatorApp ("Calculator");

	}
}