NullPointerException Headache

I've got a button that when pressed, will call a method in another java class, (a set method), to set a variable in an array. However, as the subject of this thread indicates, I keep getting the NullPointerException error.
I've attached the code that is appropriate for the things being used, and would really love some help.

Java Class 1 - ButtonPanel

Code:

---

class ButtonPanel extends JPanel{
  private Elevator elevator;
  public JButton b[] = new JButton[8];  // 8 Buttons
  public boolean bp[] = new boolean[8]; // the state of each button, pressed or not
  //constructor
  public ButtonPanel(Elevator elevator) {
    this.elevator = elevator;
    this.setLayout(new GridLayout(8,1));
    for(int i = 7 ; i >= 0 ; i--){
        b[i] = new JButton("F" + Integer.toString(i+1));
        b[i].setBackground(Color.LIGHT_GRAY);
        this.add(b[i]);
        b[i].addActionListener(new ButtonListener());
    }
  }


class ButtonListener implements ActionListener{

    public void actionPerformed(ActionEvent e) {
        for(int i = 7 ; i >= 0 ; i--){
            if(e.getSource() == b[i]){
                int j = i + 1;
                System.out.println("Pressed Floor "+ j );
                b[i].setBackground(Color.YELLOW);
                elevator.setFloor(j);
            }
        }
    }

}
}

---

Java Class 2 - Elevator

Code:

---

public void setFloor(int i){
      bp[i] = true;
      System.out.println("Marker");
  }

---

It doesn't seem to matter what's actually inside the setFloor method, as the NullPointerException is thrown inside the actionPerformed method of the button action, when setFloor() is called..

Anyone have any ideas? If you want more code then let me know...

Thanks
Denno

I believe the problem is in your action listener. Do you intend every button click to loop through from 7 - 0? What happens if you remove the loop and use an instance variable and a constructor?

Code:

---

public class ButtonListener implements ActionListener{
  private int j;
  public ButtonListener(int j){
    this.j = j;
  }
  public void actionPerformed(ActionEvent e){
    System.out.println("Pressed floor " + j);
    b[j].setBackgroung(Color.YELLOW);
    elevator.setFloot(j);
  }
}

---

I suspect that when you add the listener to the button it also sets the color of every other button(some of which may not be initialized at the time)

If this doesn't help you, it would be helpful if you gave us the EXACT error message, the line number and exact details help us out a good amount.

Post the EXACT error message and indicate on which line it occurs.

The purpose of the for loop is to loop through each of the numbers in the array, until a match is found.. So it will only change the colour of the one button that is actually pressed, that bit is ok..

As per your request, the exact error message is:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at elevator.ButtonPanel$ButtonListener.actionPerforme d(ButtonPanel.java:35)
at javax.swing.AbstractButton.fireActionPerformed(Abs tractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed (AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed (DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultB uttonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseRe leased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.jav a:6267)
at javax.swing.JComponent.processMouseEvent(JComponen t.java:3267)
at java.awt.Component.processEvent(Component.java:603 2)
at java.awt.Container.processEvent(Container.java:204 1)
at java.awt.Component.dispatchEventImpl(Component.jav a:4630)
at java.awt.Container.dispatchEventImpl(Container.jav a:2099)
at java.awt.Component.dispatchEvent(Component.java:44 60)
at java.awt.LightweightDispatcher.retargetMouseEvent( Container.java:4577)
at java.awt.LightweightDispatcher.processMouseEvent(C ontainer.java:4238)
at java.awt.LightweightDispatcher.dispatchEvent(Conta iner.java:4168)
at java.awt.Container.dispatchEventImpl(Container.jav a:2085)
at java.awt.Window.dispatchEventImpl(Window.java:2478 )
at java.awt.Component.dispatchEvent(Component.java:44 60)
at java.awt.EventQueue.dispatchEvent(EventQueue.java: 599)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:122).

Line 35 corresponds to the line:
elevator.setFloor(j);

I can run the program, and it will run perfectly, until I press the button.. However the program still runs, the animation still occurs (which is that of an elevator going up and then down)..

Denno

The only thing on that line that can be null is elevator. Have you posted the correct code? Have you saved and compiled the correct version of the code? Do you have another constructor which does not initialise the elevator variable?

Another thought, maybe you are passing null to the ButtonPanel constructor.

I have pasted the correct code for the ButtonPanel class, and it compiled and ran and everything (using netbeans).

Here is the whole of the Elevator class for consideration:

Code:

---

class Elevator extends JPanel{
  //Declaration of variables
  //private Elevator_Simulation app; //the Elevator Simulation frame
  private boolean up = false; // the elevator is moving up or down
  private int width;  // Elevator width
  private int height; // Elevator height
  private int xco;    // The x coordinate of the elevator's upper left corner
  private int space;  //The height of each floor (space between lines)
  private int yco;    // The y coordinate of the elevator's upper left corner
  private int dy0 = 3;    // Moving interval
  private int topy;  //the y coordinate of the top level
  private int bottomy;// the y coordinate of the bottom level
  private int panelHeight, panelWidth;
  TimeClass tc = new TimeClass();
  private Timer timer = new Timer(100, tc);  //the timer to drive the elevator movement
  public boolean bp[] = new boolean[9];
  private boolean start = true;
  private int delayCount;  //Keep track of the delay whilst the elevator is paused on a floor


  //constructor
  public Elevator() {
  //necessary initialization
      setBackground(Color.green);
      timer.start();
      //For Debugging
      //System.out.println("" + yco);
  }

  public void setFloor(int i){
      bp[i] = true;
      System.out.println("Marker");
  }

  public void unsetFloor(int i){
      bp[i] = false;
  }

  public int findFloor(){
      if(yco < space){
          return 8;
      }else if(yco > space && yco < 2*space){
          return 7;
      }else if(yco > 2*space && yco < 3*space){
          return 6;
      }else if(yco > 3*space && yco < 4*space){
          return 5;
      }else if(yco > 4*space && yco < 5*space){
          return 4;
      }else if(yco > 5*space && yco < 6*space){
          return 3;
      }else if(yco > 6*space && yco < 7*space){
          return 2;
      }else if(yco > 7*space){
          return 1;
      }else{
          return 0;    //will never get to this point
      }
  }
  // Paint elevator area
    @Override
  public void paintComponent(Graphics g) {
        panelHeight = getHeight();
        panelWidth = getWidth();

        super.paintComponent(g);

    //Horizontal Lines
        space = panelHeight / 8;
        g.drawLine(0, 0, panelWidth, 0);
        g.drawLine(0, space, panelWidth, space);
        g.drawLine(0, space*2, panelWidth, space*2);
        g.drawLine(0, space*3, panelWidth, space*3);
        g.drawLine(0, space*4, panelWidth, space*4);
        g.drawLine(0, space*5, panelWidth, space*5);
        g.drawLine(0, space*6, panelWidth, space*6);
        g.drawLine(0, space*7, panelWidth, space*7);
        g.drawLine(0, space*8, panelWidth, space*8);
    //End Horizontal Lines

    //Elevator
        int padding = (int) (panelWidth * 0.05);
        width = panelWidth - (2*padding);
        height = space-1;
        if(start){                                                              //only run this once
            yco = space * 7;                                                    //making the elevator start at floor 1
            start = false;
        }
        g.setColor(Color.LIGHT_GRAY);
        g.fillRect(padding, yco, width, height);
        g.setColor(Color.BLACK);
        g.drawLine(width/2, yco, width/2, yco + height);
    //End Elevator
  }

  //Handle the timer events
  public class TimeClass implements ActionListener {
        public void actionPerformed(ActionEvent tc){
            if(delayCount > 20){
                delayCount--;
            }else{
            if(up){
                yco = yco - dy0;                                                //Make the movement
                if(yco < 0){                                                    //check if the elevator is at the top floor
                    up = false;                                                //reverse the direction
                }
                if(bp[findFloor()]){
                    delayCount = 20;
                    System.out.println("Delay Set");
                }
            }else{
                yco = yco + dy0;                                                //make the movement
                if(yco > panelHeight - height){                                //check if the elevator is at the bottom floor
                    up = true;                                                  //reverse the direction
                }
            }
            repaint();
        }
      }//End actionPerformed
    }//End of TimeClass
}

---

To be honest, I'm not really understand what I'm doing with the elevator part, I have just been looking through as many examples as I could find until I got something to work :S..

Thanks for helping :)

Where is the code that creates the ButtonPanel?

I have found the problem!!

Your second thought saying that I might passing null to ButtonPanel made me look in the top most java class. I was infact passing it elevator correctly, however I hadn't actually created the elevator, I had only made it ready to be created.. Which doesn't make sense... Here is what I mean:

Code:

---

private Elevator elevator;
...
...
control = new ButtonPanel(elevator);
...
...
...
elevator = new Elevator();

---

So as you can see, I didn't actually properly create the object until after I tried to pass it to ButtonPanel :S.

Thanks for your help though! I don't think I would have ever looked there for the answer.

Thanks
Denno

Wow, after all these years my crystal ball finally worked. ;)

While I've got your attention, and I've already posted the code in previous posts, can I ask another question?

When a button is pressed, I change the colour of it to indicate the press, however when the elevator has stopped on the floor and moved on, I need the colour of the button to change back to where it was, and I'm not sure how to do that.
I don't know how to access the elements in the ButtonPanel class from the Elevator class.. Do you know what I should do?
If it's not a simple answer then it's ok if you don't take it on, but I was hoping there is something simple I need to do, or an example I can follow :).

Thanks,
Denno

How I would do it is to give the elevator class a reference to the button Panel and use getters and setters to access the buttons. However; I don't know for sure since I don't really know all of the code.

How do I go about giving a reference? The setters and getters I understand, but actually making the links between the two files is where I fail...

And if it makes it easier, here is the third and final class that I'm using in my project.

Code:

---

public class Elevator_Simulation extends JFrame{
  public JLabel state; // display the state of the elevator
  private JLabel id;  //your name and group
  public ButtonPanel control; //the button control panel
  private Elevator elevator = new Elevator(); // the elevator area
  public JPanel elevatorInfoPanel;

  //constructor
  public Elevator_Simulation() {
    JFrame frame = new JFrame("Elevator");
    frame.setLayout(new BorderLayout());
    frame.setSize(503,420);
    frame.setMinimumSize(new Dimension(503,420));
    frame.setVisible(true);
   
//exit the program upon closing
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//set the window to start in the centre of the screen
    frame.setLocationRelativeTo(null);
    state = new JLabel("On");
//Build the info panel with name and class
    JPanel buildInfo = new JPanel();
    JLabel info = new JLabel("Name: Luke Denton");
    buildInfo.add(info);
    frame.add(buildInfo, BorderLayout.NORTH);
//Build the buttons panel
    control = new ButtonPanel(elevator);
    frame.add(control, BorderLayout.WEST);
//Build the speed panel
    JPanel speedPanel = new JPanel(new GridLayout(1,1));
    JSlider speed = new JSlider(JSlider.VERTICAL, 2, 20, 8);
    speed.setMajorTickSpacing(2);
    speed.setMinorTickSpacing(1);
    speed.setPaintTicks(true);
    speed.setPaintLabels(true);
    speedPanel.add(speed);
    frame.add(speedPanel, BorderLayout.EAST);
//Build the panel holding the current action of the elevator
    elevatorInfoPanel = new JPanel();
    JLabel elevatorInfo = new JLabel("The elevator is doing jack shit");
    elevatorInfoPanel.add(elevatorInfo);
    frame.add(elevatorInfoPanel, BorderLayout.SOUTH);
//Build the centre part holding the elevator
    //elevator = new Elevator();
    frame.add(elevator, BorderLayout.CENTER);

    frame.pack();
  }

  // Main method
  public static void main(String[] args) {

  }

}

---

Denno

An example I can give is this, I am working on a text editor currently and I wanted to remove the action listeners from the main source file and have a file for each Action, and a file for the actual text editor.

I gave each action a instance variable of type TextEditor and the constructor sets it up. With this I was able to manipulate the text editor stuff from inside the action classes.

Code:

---

public class TextEditor{
  private JButton button = new JButton("Do stuff");
  private JLabel label = new JLabel("H");
  public JLabel getLabel(){
    return label;
  }
  public void go(){
    JFrame frame = new JFrame()
    button.addActionListener(new RandomAction(this));
    frame.add(button);
    frame.add(label);
    frame.setVisible(true);
    frame.pack();
  }
  public static void main(String[] args){
    new TextEditor().go();
  }
}

public class RandomAction extends AbstractAction{
  private TextEditor theTextEditor;
  public RandomAction(TextEditor theTextEditor){
    this.theTextEditor = theTextEditor;
  }
  public void actionPerformed(ActionEvent e){
    Random rand = new Random();
    theTextEditor.getLabel().setText(rand.nextInt(50) + "");
  }
}

---

This allows you to click a button and change the label. The action listener takes a reference to the class TextEditor and uses it for manipulating the items in the text editor.

I'm trying my best to adapt your example to my code, but I'm struggling big time..
I have button listeners already set up, which is the ButtonListener class (in the code in previous posts, I've pasted it with the ButtonPanel class code). This ButtonListener code then tells the Elevator class which floor to stop at.. In the Elevator class there is a timer class which does all the movement stuff.
If it's not too much trouble, do you think you could use my code to show me what to do? I'm really sorry to ask, and I really dislike when people ask on forums for other people to do their homework, but I'm otherwise not going to get it, I don't think :(..


Actually, I've just had a thought.. what if I scrap the ButtonListener class, and instead use the Elevator class to implement the ActionListener.. Would that help?

The reason I gave you an example from something similar to my code is because it's a lot more basic. Ill have to read through your code a bit and see if I can come up with something.

In the method that changes the color of the button when pushed, is it suitable to just sleep for a bit and then change it again?

Thankyou sunde887, I really appreciate it