Seltsame Fehlermeldung

Hallo,

ich habe erst vor kurzem mit Java angefangen und stehe schon jetzt vor einem fuer mich unloesbaren Problem.

Der folgende Code ist aus einem Buch zum Lernen der Sprache entnommen und sollte eigentlich einen Taschenrechner produzieren.

/*
	Chapter 6: Java Calculator
	Programmer: Benedikt Achatz
	Date: April, 11 2007
	Filename: Calculator.java
	Purpose: This program creates a calculator with menu
*/

import java.awt.*;
import java.awt.event.*;
import java.awt.datatransfer.*;
import java.text.DecimalFormat;
import javax.swing.JOptionPane;

public class Calculator extends Frame implements ActionListener
{
	private Button keys[];
	private Panel keypad;
	private TextField lcd;
	private double op1;
	private boolean first;
	private boolean foundKey;
	private boolean clearText;
	private int last0p;
	private DecimalFormat calcPattern;

	public Calculator()
	{
		//create an instance of the menu
		MenuBar mnuBar = new MenuBar();
		setMenuBar(mnuBar);

		//construct and popilate the File menu
		Menu mnuFile = new Menu("File",true);
		mnuBar.add(mnuFile);
			MenuItem mnuFileExit = new MenuItem("Exit");
			mnuFile.add(mnuFileExit);

		//construct and populate the edit menu
		Menu mnuEdit = new Menu("Edit",true);
		mnuBar.add(mnuEdit);
			MenuItem mnuEditClear = new MenuItem("Clear");
			mnuEdit.add(mnuEditClear);
			mnuEdit.insertSeparator(1);
			MenuItem mnuEditCopy = new MenuItem("Copy");
			mnuEdit.add(mnuEditCopy);
			MenuItem mnuEditPaste = new MenuItem("Paste");
			mnuEdit.add(mnuEditPaste);

		//construct and populate the about menu
		Menu mnuAbout = new Menu("About",true);
			mnuBar.add(mnuAbout);
			MenuItem mnuAboutCalculator = new MenuItem("About Calculator");
			mnuAbout.add(mnuAboutCalculator);

		//add the ActionListener to each menu item
		mnuFileExit.addActionListener(this);
		mnuEditClear.addActionListener(this);
		mnuEditCopy.addActionListener(this);
		mnuEditPaste.addActionListener(this);
		mnuAboutCalculator.addActionListener(this);

		//assign an ActionCommand ti each menu item
		mnuFileExit.setActionCommand("Exit");
		mnuEditClear.setActionCommand("Clear");
		mnuEditCopy.setActionCommand("Copy");
		mnuEditPaste.setActionCommand("Paste");
		mnuAboutCalculator.setActionCommand("About");

		//construct components and initialize beginning
		lcd = new TextField(20);
			lcd.setEditable(false);
		keypad = new Panel();
		keys = new Button[16];
		first = true;
		op1 = 0.0;
		clearText = true;
		last0p = 0;
		calcPattern = new DecimalFormat("########.########");

		//construct and assign captrons to the Buttons
		for(int i=0; i<=9; i++)
			keys[i] = new Button(String.valueOf(i));

		keys[10] = new Button("/");
		keys[11] = new Button("*");
		keys[12] = new Button("-");
		keys[13] = new Button("+");
		keys[12] = new Button("=");
		keys[14] = new Button(".");

		//set Frame and keypad layout to grid layout
		setLayout(new BorderLayout());
		keypad.setLayout(new GridLayout(4,4,10,10));

		for(int i=7; i<=10; i++) //7,8,9, divide
			keypad.add(keys[i]);

		for(int i=4;i<=6;i++) //4,5,6
			keypad.add(keys[i]);

		keypad.add(keys[11]); //multiply

		for(int i=1; i<=4; i++) //1,2,3
			keypad.add(keys[i]);

		keypad.add(keys[12]); //subtract

		keypad.add(keys[0]); //0 key

		for(int i=15; i>=13; i--)
			keypad.add(keys[i]); //decimal point, =, add + keys

		for(int i=0; i<keys.length; i++)
			keys[i].addActionListener(this);

		add(lcd,BorderLayout.NORTH);
		add(keypad,BorderLayout.CENTER);

		addWindowListener(
			new WindowAdapter()
			{
				public void windowClosing(WindowEvent e)
				{
					System.exit(0);
				}
			}
		);

	}//end of constructor method

	public void actionPerformed(ActionEvent e)
	{
		//test for menu item clicks
		String arg = e.getActionCommand();
		if(arg == "Exit")
			System.exit(0);

		if(arg == "Clear")
		{
			clearText = true;
			first = true;
			op1 = 0.0;
			lcd.setText("");
			lcd.requestFocus();
		}

		if (arg == "Copy")
		{
			Clipboard cb = Toolkit.getDefaultToolkit().getSystemClipboard();
			StringSelection contents = new StringSelection(lcd.getText());
			cb.setContents(contents, null);
		}

		if(arg == "Paste")
		{
			Clipboard cb = Toolkit.getDefaultToolkit().getSystemClipboard();
			Transferable content = cb.getContents(this);
			try
			{
				String s = (String)content.getTransferData(DataFlavor.stringFlavor);
				lcd.setText(calcPattern.format(Double.parseDouble(s)));
			}
			catch (Throwable exc)
			{
				lcd.setText("");
			}
		}

		if(arg == "About")
		{
			String message = "Calculator ver. 1.0\nOpenExibit_Software\nCopyright 2007\nAll rights reserved";
			JOptionPane.showMessageDialog(null,message,"About Calculator", JOptionPane.INFORMATION_MESSAGE);
		}

		//test for button clicks
		foundKey = false;

		//search for the clicked key
		for (int i=0; i<keys.length && !foundKey; i++)
		{
			if(e.getSource() == keys[i])
			{
				foundKey = true;
				switch(i)
				{
					//number and decimal point buttons
					case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7:case 8: case 9: case 15:
						if(clearText)
						{
							lcd.setText("");
							clearText = false;
						}
						lcd.setText(lcd.getText() + keys[i].getLabel());
						break;

					//operator buttons
					case 10: case 11: case 12: case 13: case 14:
						clearText = true;

						if(first)
						{
							if(lcd.getText().length()==0) op1 = 0.0;
							else op1 = Double.parseDouble(lcd.getText());

							first = false;
							clearText = true;
							last0p = i; //save operator
						}
						else //second operand
						{
							switch(last0p)
							{
								case 10: //divide button
									op1 /= Double.parseDouble(lcd.getText());
									break;
								case 11: //multiply button
									op1 *= Double.parseDouble(lcd.getText());
									break;
								case 12:
									op1 -= Double.parseDouble(lcd.getText());
									break;
								case 13:
									op1 += Double.parseDouble(lcd.getText());
									break;
							} //end of switch
							lcd.setText(calcPattern.format(op1));
							clearText = true;

							if(i==14) first = true; //equal button
							else last0p = i; //save last operator
						}//end else
						break;
					}//end of switch(i)
				}//end of if
			}//end of for
		}//end of action performed

		public static void main(String args[])
		{
			//set frame properties
			Calculator f = new Calculator();
			f.setTitle("Calculator Application");
			f.setBounds(200,200,300,300);
			f.setVisible(true);

			//set image properties and add to frame
			Image icon = Toolkit.getDefaultToolkit().getImage("calcImage.gif");
			f.setIconImage(icon);
		}

}

Nun bekomme ich folgenden Fehlermeldung, allerdings nicht beim kompilieren, sonder eher danach, wenn ich versuche das Programm zu starten.

Exception in thread "main" java.lang.NullPointerException
        at java.awt.Container.addImpl(Container.java:1019)
        at java.awt.Container.add(Container.java:351)
        at Calculator.<init>(Calculator.java:112)
        at Calculator.main(Calculator.java:242)
Press any key to continue . . .

Kann mir da irgendjemand helfen?? Vielleicht bin ich auch nur blind und habe einen Fehler uebersehen

Danke schonmal

Benedikt

Gregor

Das Element Nr. 15 in Deinem keys-Array ist null. Das versuchst Du da dem Panel hinzuzufügen. Anscheinend darfst Du dem Panel da nicht null hinzufügen.

danke fuer deine antwort, ich habe es auch versucht diesen button mal temporaer zu entfernen, allerdings funktioniert es einfach trotzdem nicht. vielen danke fuer deine hilfe trotzdem

LeGaN

Naja, soweit ich es sehe, wird keys[15] gar nicht initialisiert. Sehe nur, dass die indexe bis einschließlich 14 initialisiert sein (bis in Zeile 112). Initalisier mal keys[15]!

jup, das wars.
vielen dank an euch !