Thread: NullPointerException with JSlider

First of all, here is my code:

MyControlPanel.java

package calculations;
 
import java.awt.*;
import javax.swing.*;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;
 
public class MyControlPanel extends javax.swing.JPanel
{
    private MyShape theShape;
 
    static int min = 0;
    static int max = 100;
    static int initial = 0;
 
    JSlider jSlider;
    JLabel jLabel1;
    JLabel jLabel2;
    JLabel jLabel3;
    JTextField jText1;
    JTextField jText2;
 
    public MyControlPanel() 
    {
        initComponents();
 
        jSlider = new JSlider(JSlider.HORIZONTAL, min, max, initial);
        jSlider.setMajorTickSpacing(10);
        jSlider.setMinorTickSpacing(1);
        jSlider.setPaintLabels(true);
        jSlider.setPaintTicks(true);
        jSlider.setValue(70);
 
        jSlider.addChangeListener(new MyChangeAction());
        jText1 = new JTextField("Displaying JSlider value");
 
        jText1 = new JTextField();
        jText2 = new JTextField();
 
        jText1.setText("");
        jText2.setText("");
 
        jLabel1 = new JLabel();
        jLabel2 = new JLabel();
        jLabel3 = new JLabel();
 
        jLabel1.setText("Shape Dimension");
        jLabel2.setText("Boundary Length = ");
        jLabel3.setText("Area = ");
 
        this.add(jSlider) ;
        this.add(jLabel1) ;
        this.add(jLabel2) ;
        this.add(jText1) ;
        this.add(jLabel3) ;
        this.add(jText2) ;
 
        setLayout(new FlowLayout());
    }
 
    public void setShape(MyShape suppliedShape)
    {       
        theShape = suppliedShape;
    }
 
        public class MyChangeAction implements ChangeListener
        {
            @Override
            public void stateChanged(ChangeEvent e) 
            {
                double sliderValue = jSlider.getValue();
                double area;
                double boundaryLength;
 
                String str = Double.toString(sliderValue);
                jLabel1.setText(str);
 
 
                area = theShape.getArea(sliderValue);
                boundaryLength = theShape.getBoundaryLength(sliderValue);
 
                jText1.setText(Double.toString(Math.round(boundaryLength)));
                jText2.setText(Double.toString(Math.round(area)));  
 
            }            
        }
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {
 
        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
        this.setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 400, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 300, Short.MAX_VALUE)
        );
    }// </editor-fold>                        
    // Variables declaration - do not modify                     
    // End of variables declaration                   
}

MyFrame.java

package calculations;
 
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
 
public class MyFrame extends javax.swing.JFrame implements ActionListener
{   
    public MyShape myShape;
 
    JMenuItem square = new JMenuItem();
    JMenuItem triangle = new JMenuItem();
    JMenuItem circle = new JMenuItem();
 
    MyControlPanel theControlPanel = new MyControlPanel();
 
    public MyFrame() 
    {
        initComponents();
 
        JMenuBar jMenuBar = new JMenuBar();
 
        JMenu Shape = new JMenu();
 
        Shape.setText("Shape");
 
        square.addActionListener(new MySquareAction());
 
        triangle.addActionListener(new MyTriangleAction());
 
        circle.addActionListener(new MyCircleAction());
 
        square.setText("Square");
        triangle.setText("Triangle");
        circle.setText("Circle");
 
        jMenuBar.add(Shape);
 
        Shape.add(square);
        Shape.add(triangle);
        Shape.add(circle);
 
        setJMenuBar(jMenuBar);
 
        MyControlPanel controlPanel = new MyControlPanel();
 
        setLayout(new FlowLayout());
 
        this.add(controlPanel);  
    }
 
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {
 
        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
 
        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 400, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 300, Short.MAX_VALUE)
        );
 
        pack();
    }// </editor-fold>                        
    public static void main(String args[]) 
    {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see [url=http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html]How to Set the Look and Feel (The Java™ Tutorials > Creating a GUI With JFC/Swing > Modifying the Look and Feel)[/url] 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(MyFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(MyFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(MyFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(MyFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>
 
        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() 
        {
            @Override
            public void run() 
            {
                new MyFrame().setVisible(true);
            }
        });
    }
    // Variables declaration - do not modify                     
    // End of variables declaration                   
 
    @Override
    public void actionPerformed(ActionEvent e) 
    {
 
    }
 
public class MySquareAction implements ActionListener
    {
    @Override
        public void actionPerformed(ActionEvent e)
        {
            myShape = new Square();
            theControlPanel.setShape(myShape);
        }  
    }
 
public class MyTriangleAction implements ActionListener
    {
    @Override
        public void actionPerformed(ActionEvent e)
        {
            myShape = new Triangle();
            theControlPanel.setShape(myShape);
        }
    }
 
public class MyCircleAction implements ActionListener
    {
    @Override
        public void actionPerformed(ActionEvent e)
        {
            myShape = new Circle();
            theControlPanel.setShape(myShape);
        } 
    }
}

If you want you can download my whole program here: http://www.filedropper.com/calculations

Sorry If download links aren't allowed, ill remove it straight away if so.

When i run the program and move the slider it is meant to calculate the boundary length and area, of the chosen shape, which is chosen using the menuItems(Circle, Square, Triangle).

When i move the slider, I get a this error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at calculations.MyControlPanel$MyChangeAction.stateChanged(MyControlPanel.java:79)
	at javax.swing.JSlider.fireStateChanged(JSlider.java:432)
	at javax.swing.JSlider$ModelListener.stateChanged(JSlider.java:350)
	at javax.swing.DefaultBoundedRangeModel.fireStateChanged(DefaultBoundedRangeModel.java:364)
	at javax.swing.DefaultBoundedRangeModel.setRangeProperties(DefaultBoundedRangeModel.java:302)
	at javax.swing.DefaultBoundedRangeModel.setValueIsAdjusting(DefaultBoundedRangeModel.java:231)
	at javax.swing.JSlider.setValueIsAdjusting(JSlider.java:652)
	at javax.swing.plaf.basic.BasicSliderUI$TrackListener.mousePressed(BasicSliderUI.java:1632)
	at javax.swing.plaf.synth.SynthSliderUI$SynthTrackListener.mousePressed(SynthSliderUI.java:924)
	at java.awt.Component.processMouseEvent(Component.java:6502)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
	at java.awt.Component.processEvent(Component.java:6270)
	at java.awt.Container.processEvent(Container.java:2229)
	at java.awt.Component.dispatchEventImpl(Component.java:4861)
	at java.awt.Container.dispatchEventImpl(Container.java:2287)
	at java.awt.Component.dispatchEvent(Component.java:4687)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4489)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
	at java.awt.Container.dispatchEventImpl(Container.java:2273)
	at java.awt.Window.dispatchEventImpl(Window.java:2719)
	at java.awt.Component.dispatchEvent(Component.java:4687)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
	at java.awt.EventQueue.access$200(EventQueue.java:103)
	at java.awt.EventQueue$3.run(EventQueue.java:694)
	at java.awt.EventQueue$3.run(EventQueue.java:692)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
	at java.awt.EventQueue$4.run(EventQueue.java:708)
	at java.awt.EventQueue$4.run(EventQueue.java:706)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Can anyone help me with this ? Probably easier if you download my program..

Thanks in advance!

line 79 is this:

area = theShape.getArea(sliderValue);

I don't know how to do that, hence why i came here for help.

i have this method in my MyControlPanel class:

public void setShape(MyShape suppliedShape)
    {       
        theShape = suppliedShape;
    }

which is meant to get the shape from the actionListeners in the MyFrame class for the menuItems, for example:

public class MySquareAction implements ActionListener
    {
    @Override
        public void actionPerformed(ActionEvent e)
        {
            myShape = new Square();
            theControlPanel.setShape(myShape);
        }  
    }

When i run the program with the print statement, it comes back 'true'

It is called in my JSlider ChangeListener, it is being used so that whenever say the square is selected from the jMenu (MyFrame class), it sends the selected shape to the MyControlPanel class so that the correct calculation can be chosen and carried out of the sliders value.

public class MyChangeAction implements ChangeListener
        {
            @Override
            public void stateChanged(ChangeEvent e) 
            {
                double sliderValue = jSlider.getValue();
                double area;
                double boundaryLength;
 
                System.out.println("1. The shape is: "+theShape);
 
                String str = Double.toString(sliderValue);
                jLabel1.setText(str);
 
                System.out.println("2. The shape is: "+theShape);
 
                area = theShape.getArea(sliderValue);
                boundaryLength = theShape.getBoundaryLength(sliderValue);
 
                jText1.setText(Double.toString(Math.round(boundaryLength)));
                jText2.setText(Double.toString(Math.round(area)));  
            }            
        }

to me it would seem that the setShape method isn't working, although I'm not sure even where to start to try and find the error

I tried clicking each of the shapes, and then moving the slider, i get the null error.

I tried not clicking the menu and moving the slider, and the same thing happens.


I tried using:

System.out.println("The shape is: "+myShape);

in the menuItem actionListeners, and when I click each shape on the menu I get:

The shape is: calculations.Square@1ed290f2
The shape is: calculations.Triangle@e279c82
The shape is: calculations.Circle@485b0752

is this what myShape should contain? Surely it should just be, 'Square', 'Triangle' or 'Circle' though I'm not sure

but my actionlistener has these lines of code :

            myShape = new Circle();
            theControlPanel.setShape(myShape);
            System.out.println("The shape is: "+myShape);

so theShape is myShape

seeing as the setShape() method takes myShape and assigns it to theShape:

public void setShape(MyShape suppliedShape)//suppliedShape is the shape which is sent from the actionListener for the menuItems
    {       
        theShape = suppliedShape;
    }

MyShape.java

package calculations;
 
public class MyShape 
{
       public double getArea(double length)       
       {
           return 0;
       }//end of getArea
 
       public double getBoundaryLength(double length)
       {
           return 0;
       }//end of getBoundaryLength
}//end of class

Square.java

package calculations;
 
public class Square extends MyShape 
{   
      public double getArea(double length)    
       {
           return length * length;
       }//end of getArea
 
       public double getBoundaryLength(double length)
       {
           return length + length + length + length;
       }//end of getBoundaryLength
}

I posted a download link that allows you to download my full program, if you want to run it.

Thanks!! I was calling MyControlPanel twice.. -__-

Well thank you so much! Can't believe I didn't realize I had called it twice.

Now I have to get it so that the shape is drawn on the screen, and changes size depending on the value selected by the slider/shape chosen. I haven't done anything like this before.

Alright thanks a lot, I'll most likely be back for help -__-

--- Update ---

One question, would I make a new class for the painting, or would i implement it within the MyFrame or MyControlPanel class ?

I tried making a new class, though I've been having a lot of trouble,

I made a new thread here, your help would be much appreciated, If you want i will post all my code, or any code you would like to see:
http://www.javaprogrammingforums.com...tml#post132811