Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

[iuna]

I'm getting this error whenever I "submit" my chosen item from a JComboBox.. what is this?? please help..

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Matchess.myTurn(Matchess.java:66)
    at Matchsticks.actionPerformed(Matchsticks.java:119)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
    at java.awt.Component.processMouseEvent(Component.java:6041)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
    at java.awt.Component.processEvent(Component.java:5806)
    at java.awt.Container.processEvent(Container.java:2058)
    at java.awt.Component.dispatchEventImpl(Component.java:4413)
    at java.awt.Container.dispatchEventImpl(Container.java:2116)
    at java.awt.Component.dispatchEvent(Component.java:4243)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
    at java.awt.Container.dispatchEventImpl(Container.java:2102)
    at java.awt.Window.dispatchEventImpl(Window.java:2440)
    at java.awt.Component.dispatchEvent(Component.java:4243)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)

[Fubarable]

You have a bug in that you are trying to call a method on a null object. Why this is, I haven't the foggiest without seeing code. Consider creating an SSCCE and posting it. Please see the link to find out how to do this.

[iuna]

ok, here's my "parent" class

public class Matchsticks extends JFrame implements ActionListener, ItemListener{

    JComboBox choice = new JComboBox();
    private JButton accept = new JButton("Accept");
    private int selected = 1;
    private int	count = 0;
    private Matchess matches = new Matchess();
    Boardd board = new Boardd(matches);
    JTextField textField = new JTextField(50);
    //...more components
    
    public Matchsticks() {
    	super("Matchsticks");
    	
    	choice.addItem("1");
        choice.addItem("2");
        choice.addItem("3");
    	
    	choice.addItemListener(this);
        submit.addActionListener(this);
    	
        //more panel stuff...
    	}
    
        public int getSelected() {
           return this.selected;
        }

        public void itemStateChanged(ItemEvent e) {
           if (e.getStateChange() == ItemEvent.SELECTED) {
               int result = matches.getCount();
               if (result > 1) {
                   this.selected = e.getStateChange()+ 1;
                   if ((result - selected) < 1) {
                    textField.setText("Wrong choice! ");
                  } else {
                    textField.setText("You picked " + selected);
                  }
               }
           }
       }
        public void actionPerformed(ActionEvent e) {
        if (e.getSource()==start) {
            //...there's supposed to have an existing start button
        }
        if (e.getSource()==accept) {
            boolean edit = counter.isEditable();
            if (edit) {
                //...same with start button
        }
        if (e.getSource()==submit) {
            int newcount = matches.getCount();
            if (newcount > 1) {
                if ((newcount - selected) < 1) {
                    if (newcount == 3) {
                        textField.setText("Take 2 !!!");
                    } else {
                        textField.setText("Take 1 !!!"); 
                    }
                } else {
                    newcount -= selected;
                    matches.setCount(newcount);
                    board.repaint();
                    counter.setText((String.valueOf(newcount).toString()));
                    matches.myTurn(); //this is what the error "Matchess.myTurn(Matchess.java:66)" is pointing at

                }
            }
        }
    }
    
    public static void main(String[] args)
  	{
    //main goes here...
  	}
}

here is the Matchess class

//import stuff...

public class Matchess {
	
	private int count;
	private Matchsticks parent;
	
    public Matchess() {
    }
    
  public void setCount(int value) {
	this.count = value;
  }

  public int getCount() {
	return this.count;
  }

  public int getTurn(int old, int choosen) {
	int getback;
	switch((old+choosen)%4) {
	case 0:
		if (choosen == 1) {
		  getback =  2;
		} else {
		  getback = 1;
		}
		break;
	case 1:
		getback = 4 - choosen;
		break;
	case 2:
		if (choosen == 2) {
		  getback = 3;
		} else {
		  getback = 1;
		}
		break;
	default:
		if (choosen == 1) {
		  getback = 1;
		} else {
		  getback = 3;
		}
		break;
	}
	return getback;
  }		

  public void myTurn() {
	int turn;
	if (count == 1) {
		parent.textField.setText("You WIN !!!"); //suppose a textfield is existing
	} else {
		switch(count%4) {
        	case 1:
			turn = (int)(Math.random()*3+1);
                	break;
        	default:
			turn = getTurn(count, parent.getSelected());
			break;
        	}
		count = count - turn;
		parent.board.repaint();
		if (count == 1) {
		  parent.textField.setText("I took " + turn + " and you LOSE !!!");
		} else {
		  parent.textField.setText("Ok...i took " + turn + ", and you ?");
		}
		parent.counter.setText((String.valueOf(count).toString()));
  	}
  }	
}

there's another class named Boardd but I think that won't be needed here..

I hope I got the SSCCE right...

[Fubarable]

I hope I got the SSCCE right..

not quite. Again, please read the link and it will explain it. It doesn't mean just posting your code. Instead it's an active process where you isolate the problem and nothing but the problem and then post a small self-contained compilable program that demonstrates the problem. Many have said that if you can't create this, you don't understand the problem. I say that if you ask a volunteer to help you, you should be expected to put in the effort to make the volunteer's work as easy as possible.

so, if you still need help, then please post the SSCCE. If not, then no biggie. Again, good luck.

[iuna]

I tried "compressing" my code but it still looked long.. I really tried my best isolating the problem.. And this is compilable.. well here it is..

my "parent" class Matchsticks

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

public class Matchsticks extends JFrame implements ActionListener, ItemListener{

    JComboBox choice = new JComboBox();	
    JButton submit = new JButton("Submit");
    JPanel panel = new JPanel();
    int selected = 1;
    Matchess matches = new Matchess();
    
    public Matchsticks() {
    	super("Matchsticks");
    	
    	choice.addItem("1");
        choice.addItem("2");
        choice.addItem("3");
        
        panel.add(choice);
        panel.add(submit);

        getContentPane().add(panel, BorderLayout.CENTER);

    	choice.addItemListener(this);
    	submit.addActionListener(this);
    	
    	setDefaultCloseOperation(DISPOSE_ON_CLOSE);
    	
    }
    
    public int getSelected() {
        return this.selected;
    }
    
    public void itemStateChanged(ItemEvent e) {
        if (e.getStateChange() == ItemEvent.SELECTED) {
            int result = matches.getCount();
            if (result > 1) {
                selected = choice.getSelectedIndex() + 1;
            }
        }
    public static void main(String[] args){
    	Matchsticks m = new Matchsticks();
    	m.setSize(200,100);
    	m.setVisible(true);
    	m.setResizable(false);
    	}
    }
    
    public void actionPerformed(ActionEvent e) {
    	if (e.getSource()==submit) {
            int newcount = matches.getCount();
                    newcount -= selected;
                    matches.setCount(newcount);
                    matches.myTurn();
                }
            }
}

"subclass" Matchess

public class Matchess {
	
	private int count;
	private Matchsticks parent;
	
    public Matchess() {
    }
    
  public void setCount(int value) {
	this.count = value;
  }

  public int getCount() {
	return this.count;
  }

  public int getTurn(int old, int choosen) {
	int getback;
	switch((old+choosen)%4) {
	case 0:
		if (choosen == 1) {
		  getback =  2;
		} else {
		  getback = 1;
		}
		break;
	case 1:
		getback = 4 - choosen;
		break;
	case 2:
		if (choosen == 2) {
		  getback = 3;
		} else {
		  getback = 1;
		}
		break;
	default:
		if (choosen == 1) {
		  getback = 1;
		} else {
		  getback = 3;
		}
		break;
	}
	return getback;
  }		

  public void myTurn() {
		int turn;
		switch(count%4) {
        	case 1:
			turn = (int)(Math.random()*3+1);
                	break;
        	default:
			turn = getTurn(count, parent.getSelected());
			break;
        	}
		count = count - turn;
		
  	}
}

[Fubarable]

By the way, that "parent" Matchstick variable that is in Matchess,... where in your code do you let it refer to a viable object? What line is line 66 of the Matchess class? The one that refers to parent?

[iuna]

public void actionPerformed(ActionEvent e) {
    	if (e.getSource()==submit) {
            int newcount = matches.getCount();
                    newcount -= selected;
                    matches.setCount(newcount);
                    matches.myTurn(); //this is that line 66 the error is pointing at
                }
            }

[Fubarable]

And as for your SSCCE: it compiles, but doesn't run and thus can't reproduce your error. If my comment above above that "orphan" parent variable doesn't help, you'll need to put more work into a true SSCCE. Again, read the link, it will explain a lot.

And again, where do you set parent = to anything?

[Fubarable]

Look into passing a reference to the Matchsticks object to the Matchess object via the Matchess's constructor. This will make sure that parent is referring to the correct object and is non-Null. Something like so:

in Matchess.java

  public Matchess(Matchsticks parent) //!! add parameter to constructor
  {
    this.parent = parent;
  }

in Matchsticks.java

public class Matchsticks extends JFrame implements ActionListener, ItemListener
{
  JComboBox choice = new JComboBox();
  //...
  Matchess matches; // = new Matchess();  //!! see change

  public Matchsticks()
  {
    super("Matchsticks");
    //....
    
    matches = new Matchess(this);  //!! add this
  }

[iuna]

oh, sorry i forgot my main method..

I have edited my "SSCCE" above..

And again, where do you set parent = to anything?

yes.. because when I tried this:

private Matchsticks parent = new Matchsticks();

i got a StackOverflow error..

[Fubarable]

private Matchsticks parent = new Matchsticks();

will cause a recursive headache. Your new Matchsticks object (which by the way will have no relationship to the one showing on the screen), will create a new Matchess object which will create a new Matchsticks object,...

do what I suggested above. I am 99.9% certain that this is your problem and your solution.

[iuna]

yeah, it worked! thank you so much!

[Fubarable]

You're welcome. I hope you understand what was wrong and how it was fixed. Please ask if you don't. Good luck in your studies.