java.lang.NullPointerException at runtime

[Bhathiya Pathirana]

i get these errors when i'm trying to add data

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at app1$Queue.insert(app1.java:106)
	at app1.actionPerformed(app1.java:272)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
	at javax.swing.AbstractButton$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:6267)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
	at java.awt.Component.processEvent(Component.java:6032)
	at java.awt.Container.processEvent(Container.java:2041)
	at java.awt.Component.dispatchEventImpl(Component.java:4630)
	at java.awt.Container.dispatchEventImpl(Container.java:2099)
	at java.awt.Component.dispatchEvent(Component.java:4460)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
	at java.awt.Container.dispatchEventImpl(Container.java:2085)
	at java.awt.Window.dispatchEventImpl(Window.java:2478)
	at java.awt.Component.dispatchEvent(Component.java:4460)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:122

My coding...

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

public class app1 implements ActionListener {

//Person Class
static class person{
	private String name;
	public person(String name_value)
	{
		name=name_value;
	}
        public String displayPerson()
        {
            return name;
        }
}
//Customer Class
static class customer extends person{
    private String messssage;
    private String email;

    public customer(String name_value, String message_value, String email_value)
    {
        super(name_value);
        messssage=message_value;
        email=email_value;
    }
    public String displayCust()
    {
        int nameLength=super.displayPerson().length();
        int messageLength=messssage.length();
        int emailLength=email.length();

        String Str_nameLength,Str_messageLength,Str_emailLength;

        if(nameLength<10)
        {
            Str_nameLength="0"+Integer.toString(nameLength);
        }
        else
        {
            Str_nameLength=Integer.toString(nameLength);
        }

        if(messageLength<10)
        {
            Str_messageLength="0"+Integer.toString(messageLength);
        }
        else
        {
            Str_messageLength=Integer.toString(messageLength);
        }

        if(emailLength<10)
        {
            Str_emailLength="0"+Integer.toString(emailLength);
        }
        else
        {
            Str_emailLength=Integer.toString(emailLength);
        }
        return Str_nameLength+Str_messageLength+Str_emailLength+super.displayPerson()+messssage+email;
    }
}

    //Declaring Layout Variables
    GridBagLayout gbl1,gbl2,gbl3;
    GridBagConstraints gbc1,gbc2,gbc3;
    JPanel controllersPanel,buttonPanel,mainpanel;
    JFrame frame;

    //Declaring Controller variables
    JLabel custnamelbl,emailbl,messagelbl;
    JTextField custnametxt,emailtxt;
    JTextArea messagetxt;

    //Declaring Buttons
    JButton addmsgbtn,retrievebtn,exitbtn;

    ////////////////////////////////////////////////////////////////
class Queue
{
private int maxSize;
private customer[] queArray;
private int front;
private int rear;
private int nItems;
//-------------------------------------------------------------

public Queue(int s) // constructor
{
maxSize = s;
customer[] queArray = new customer[maxSize];
front = 0;
rear = -1;
nItems = 0;
}
//-------------------------------------------------------------

public void insert(String custname, String email, String message) // put item at rear of queue
{
if(rear == maxSize-1) // deal with wraparound
rear = -1;
queArray[++rear] = new customer(custname,message,email); // increment rear and insert
nItems++; // one more item
}
//-------------------------------------------------------------

public String remove() // take item from front of queue
{
String temp = queArray[front++].displayCust(); // get value and incr front
if(front == maxSize) // deal with wraparound
front = 0;
nItems--; // one less item
return temp;
}
//-------------------------------------------------------------

public String peekFront() // peek at front of queue
{

	return queArray[front].displayCust();
}
//-------------------------------------------------------------

public boolean isEmpty() // true if queue is empty
{
return (nItems==0);
}
//-------------------------------------------------------------

public boolean isFull() // true if queue is full
{
return (nItems==maxSize);
}
//-------------------------------------------------------------

public int size() // number of items in queue
{
return nItems;
}
//-------------------------------------------------------------

} // end class Queue
////////////////////////////////////////////////////////////////

public app1(){

    //Initializing Layout Variables
    gbl1=new GridBagLayout();
    gbl2=new GridBagLayout();
    gbl3=new GridBagLayout();
    gbc1=new GridBagConstraints();
    gbc2=new GridBagConstraints();
    gbc3=new GridBagConstraints();
    controllersPanel=new JPanel();
    buttonPanel=new JPanel();
    mainpanel=new JPanel();

    frame=new JFrame("Data Structures and Algorithms - Task 1");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(350,200);
    frame.setVisible(true);

    //Setting layouts to the panels
    mainpanel.setLayout(gbl1);
    controllersPanel.setLayout(gbl2);
    buttonPanel.setLayout(gbl3);

    //Adding panels to the frame
    gbc1.anchor=GridBagConstraints.NORTHWEST;
    gbc1.gridx=1;
    gbc1.gridy=1;
    gbl1.setConstraints(controllersPanel,gbc1);
    mainpanel.add(controllersPanel);

    gbc1.anchor=GridBagConstraints.NORTHWEST;
    gbc1.gridx=1;
    gbc1.gridy=2;
    gbl1.setConstraints(buttonPanel,gbc1);
    mainpanel.add(buttonPanel);

    //Adding mainpanel to the frame
    frame.getContentPane().add(mainpanel);

    //Initializing Controller variables
    custnamelbl=new JLabel("Customer name :");
    emailbl=new JLabel("Email :");
    messagelbl=new JLabel("Message :");
    custnametxt=new JTextField(19);
    emailtxt=new JTextField(19);
    messagetxt=new JTextArea(5,19);
    addmsgbtn=new JButton("Add message");
    retrievebtn=new JButton("Retrieve message");
    exitbtn=new JButton("Exit");

    //Adding controllers to the panels
    gbc2.anchor=GridBagConstraints.NORTHWEST;
    gbc2.gridx=1;
    gbc2.gridy=1;
    gbl2.setConstraints(custnamelbl,gbc2);
    controllersPanel.add(custnamelbl);

    gbc2.anchor=GridBagConstraints.NORTHWEST;
    gbc2.gridx=2;
    gbc2.gridy=1;
    gbl2.setConstraints(custnametxt,gbc2);
    controllersPanel.add(custnametxt);

    gbc2.anchor=GridBagConstraints.NORTHWEST;
    gbc2.gridx=1;
    gbc2.gridy=2;
    gbl2.setConstraints(emailbl,gbc2);
    controllersPanel.add(emailbl);

    gbc2.anchor=GridBagConstraints.NORTHWEST;
    gbc2.gridx=2;
    gbc2.gridy=2;
    gbl2.setConstraints(emailtxt,gbc2);
    controllersPanel.add(emailtxt);

    gbc2.anchor=GridBagConstraints.NORTHWEST;
    gbc2.gridx=1;
    gbc2.gridy=3;
    gbl2.setConstraints(messagelbl,gbc2);
    controllersPanel.add(messagelbl);

    gbc2.anchor=GridBagConstraints.NORTHWEST;
    gbc2.gridx=2;
    gbc2.gridy=3;
    gbl2.setConstraints(messagetxt,gbc2);
    controllersPanel.add(messagetxt);


    gbc3.anchor=GridBagConstraints.NORTHWEST;
    gbc3.gridx=1;
    gbc3.gridy=1;
    gbl3.setConstraints(addmsgbtn,gbc3);
    buttonPanel.add(addmsgbtn);

    gbc3.anchor=GridBagConstraints.NORTHWEST;
    gbc3.gridx=2;
    gbc3.gridy=1;
    gbl3.setConstraints(retrievebtn,gbc3);
    buttonPanel.add(retrievebtn);

    gbc3.anchor=GridBagConstraints.NORTHWEST;
    gbc3.gridx=3;
    gbc3.gridy=1;
    gbl3.setConstraints(exitbtn,gbc3);
    buttonPanel.add(exitbtn);

    //Adding action
    addmsgbtn.addActionListener(this);
    retrievebtn.addActionListener(this);
    exitbtn.addActionListener(this);


}
    public void actionPerformed(ActionEvent ae)
    {
        Object obj=ae.getSource();
        Queue theQueue = new Queue(5);
        if(addmsgbtn==obj)
        {

            //Insert
            if(!theQueue.isFull())
            {
                theQueue.insert(custnametxt.getText(), emailtxt.getText(), messagetxt.getText());
            }
            else
            {
                JOptionPane.showMessageDialog(frame, "Object Array is Full!");
            }

        }
        if(retrievebtn==obj)
        {
            //Retrieve
            if(theQueue.isEmpty())
            {
                JOptionPane.showMessageDialog(frame, "Object Array is empty!");
            }
            else
            {
                String temp=theQueue.remove();
                //Decoding result
                int namelength=Integer.parseInt(temp.substring(0,2));
                int messagelength=Integer.parseInt(temp.substring(2,4));
                int emailength=Integer.parseInt(temp.substring(4,6));

                String name,message,email;
                name=temp.substring(6, namelength);
                message=temp.substring(6+namelength, messagelength);
                email=temp.substring(6+namelength+messagelength, emailength);

                custnametxt.setText(name);
                messagetxt.setText(message);
                emailtxt.setText(email);
            }

        }
        if(exitbtn==obj)
        {
            System.exit(0);
        }
    }

    public static void main(String args[])
    {
        new app1();
    }

}

[Henry Wong]

You didn't initialize your queArray instance variable -- in your Queue inner class... hence, it is null.

Henry

[David Newton]

More specifically, this is what's in your constructor:

customer[] queArray = new customer[maxSize];  

The stack trace gives the precise line in your code that starts down the path to the exception. One you have that line number, work backwards in your code.

The code is nearly unreadable because of the lack of proper indentation. Also, class names should begin with a capital letter.

[Bhathiya Pathirana]

@Henry Wong
i initialized queArray variable

private customer[] queArray= new customer[10];

this time i don't get any errors but it does not get filled up so when i retrieve it is EMPTY

[David Newton]

Please UseAMeaningfulSubjectLine when posting. Explaining your question or remark briefly on the subject line allows us to know what we are getting into, and if we will be able to help.

[Bhathiya Pathirana]

@Henry Wong Subject line is changed Thank for the info

[Henry Wong]

Bhathiya Pathirana wrote:@Henry Wong
i initialized queArray variable

private customer[] queArray= new customer[10];

this time i don't get any errors but it does not get filled up so when i retrieve it is EMPTY

That's not the correct fix. The correct fix is to do it correctly in your constructor. With this change, the array size and the maxsize variable may be out of sync.

Henry

[Henry Wong]

Bhathiya Pathirana wrote:@Henry Wong Subject line is changed Thank for the info

Thanks for letting me know... but it wasn't me who requested it...

Henry

[Bhathiya Pathirana]

This is functioning properly

public Queue(int s) // constructor
{
maxSize = s;
customer[] queArray = new customer[maxSize]; //
front = 0;
rear = -1;
nItems = 0;
}

[Bhathiya Pathirana]

this is the line throwing exception

queArray[++rear] = new customer(custname,message,email); // increment rear and insert

[Henry Wong]

Bhathiya Pathirana wrote:This is functioning properly

public Queue(int s) // constructor
{
maxSize = s;
customer[] queArray = new customer[maxSize]; //
front = 0;
rear = -1;
nItems = 0;
}

Depends on what you mean by functioning properly. If one of the tasks to do is to initialize the queArray *instance* variable, then your constructor doesn't do that.

Henry

[Bhathiya Pathirana]

When i'm debugging

public Queue(int s) // constructor  
{  
maxSize = s;  
customer[] queArray = new customer[maxSize]; //  
front = 0;  
rear = -1;  
nItems = 0;  
}

above lines are successfully executed and the variable values are
queArray==#1020(length=5)
maxSize==5

[Henry Wong]

Bhathiya Pathirana wrote:When i'm debugging

public Queue(int s) // constructor  
{  
maxSize = s;  
customer[] queArray = new customer[maxSize]; //  
front = 0;  
rear = -1;  
nItems = 0;  
}

above lines are successfully executed and the variable values are
queArray==#1020(length=5)
maxSize==5

Yes... but what about the queArray *instance* variable? I believe you that you set the queArray *local* variable correctly.

Henry

[David Newton]

IT'S LOCAL TO THE CONSTRUCTOR. Please KeepItDown, self.

It's local to the constructor.

[Bhathiya Pathirana]

queArray is not null

[Henry Wong]

Bhathiya Pathirana wrote:queArray is not null

Sorry, I can't think of anymore hints to get you to the solution. Just change this (in the constructor) ...

customer[] queArray = new customer[maxSize]; //    

to this...

queArray = new customer[maxSize]; //    

Henry

[Bhathiya Pathirana]

i'm really sorry, it's just i'm a beginner but thank you very much for trying to help me out
i get no error but when i try to retrieve, it is empty

[Henry Wong]

Bhathiya Pathirana wrote:
i get no error but when i try to retrieve, it is empty

Not unexpected. All we did was help you fix one bug. You didn't expect to only have one bug right?

You still have to debug the application to locate the others.

Henry

[Eoin Brosnan]

Bhathiya Pathirana wrote:i'm really sorry, it's just i'm a beginner but thank you very much for trying to help me out
i get no error but when i try to retrieve, it is empty

Hi.
It's a similar error to your first one. You need to learn about the scope of instances of objects/variables.

  customer[] queArray = new customer[maxSize];    

Do you understand why that was wrong?

  class Queue  
{  
  
   private customer[] queArray;  
   private int var = 5;
  
public Queue(int s) // constructor  
{  
   
    customer[] queArray = new customer[maxSize];  //wrong
    int var = 10;
    System.out.println(var); //what do I print? 
}    

Can you see the conflict?

If I were you I would start with a simple example. You are using inheritance, a GUI, inner classes- all very advanced topics. Start off with a very simple example where you can enter your name and get it to return it back to you(System.out.print() will do fine!). After that get it to store a number of names and print them all back to you. Once you can do that you can start worrying about creating Person classes with name/email fields etc.

Good luck!

[Rob Spoor]

Eoin Brosnan wrote:Can you see the conflict?

You mean that compiler error on line 11? But if you leave out the "private" on that line then your example is a good one of local variables shadowing / hiding instance fields.

[Eoin Brosnan]

Rob Prime wrote:

Eoin Brosnan wrote:Can you see the conflict?

You mean that compiler error on line 11? But if you leave out the "private" on that line then your example is a good one of local variables shadowing / hiding instance fields.

Once I turned off my PC last night I felt that I had made that error! Updated.

[Bhathiya Pathirana]

Thank you very much for you all guys being so supportive and kind about me, can anyone please let me have some hints for where i should start learning from? course names or training materials, so i can start having a good foundation..