help with java method

sound_wave

hey folks im having a probel with this bit of code im tryin to do.

i get an error mesage:



non-static method createAccount() cannot be referenced from a static context
createAccount ();

when i execute this following bit of code:

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

public class Atm extends JFrame
{
	JPanel cardHolder;
	JLabel labelOne, labelTwo;
	
	//declare stream objects

        FileOutputStream outputAccount;
        ObjectOutputStream objSaveAccount; //stream to save an object
        
        FileInputStream inputAccount;
        ObjectInputStream objGetAccount;



	CardLayout card;
	Login panel1;
	Menu panel2;
	withdrawal panel3;
	lodge panel4;
	topUp panel5;



	public Atm()
	{
		super("Welcome to Irish National Bank ATM System");
		Container c = this.getContentPane();
		c.setLayout(new FlowLayout());
		c.setBackground(Color.white);


		cardHolder = new JPanel();
		card = new CardLayout();
		cardHolder.setLayout(card);
		cardHolder.setBackground(Color.white);

		panel1 = new Login(this);
		panel2 = new Menu(this);
		panel3 = new withdrawal(this);
		panel4 = new lodge(this);
		panel5 = new topUp(this);


		cardHolder.add(panel1, "panel1", 0);
		cardHolder.add(panel2, "panel2", 1);
		cardHolder.add(panel3, "panel3", 2);
		cardHolder.add(panel4, "panel4", 3);
		cardHolder.add(panel5, "panel5", 4);
		
		c.add(cardHolder);


		setSize(500,400);
		show();
		
		setVisible(true);

	}

        public void changePanel(String in)
        {
               card.show(cardHolder, in);
        }


	public static void main(String args[])
	{
		Atm app = new Atm();
		createAccount ();



	}
	

	public void openOutputStream()
	{
            try
            {
            	//create file and object output streams
            	outputAccount = new FileOutputStream("Account.txt");
            	objSaveAccount = new ObjectOutputStream(outputAccount);
            }
            
            catch(Exception error)
            {
            	System.err.println("Error opening file");
            	
            }

	}
	
	
	public void closeOutputStream()
	{
		try
		{
			objSaveAccount.close(); //close the object output stream
			outputAccount.close(); //close the file output stream
		}
		
		catch(IOException error)
		{
			System.out.println("Error closing file");
		}
		
	}
	
	
	public void openInputStream()
	{
	   try
	   {
		//create fiel and object input streams
		inputAccount = new FileInputStream("Account.txt");
		objGetAccount = new ObjectInputStream(inputAccount);
		
	   }
	   
	   catch(Exception error)
	   {
	   	System.out.println("Error opening file");
	   }
           
           
         }
         
         public void closeInputStream()
         {
         	try
         	{
         		objGetAccount.close(); //close the object input stream
         		inputAccount.close(); //close the file input stream

         	}
         	
         	catch(IOException error)
         	{
         		System.err.println("Error closing file");
         	}
         	
         }
         
         
        public void createAccount()
        {
         	
         	Account a =  new Account(500,"1234","123456789");

         	try
         	{
                  objSaveAccount.writeObject((Object)a);
         	
                }
                
                catch(IOException error)
                {
                	System.err.println("Error opening file");
                }


          }


}//end class

any help much appreciated

aidan_walsh

If your writing a class with a main method, all other methods within the class must be static. Your only alternative is to move them into the ATM class.

sound_wave

thanks doodle! that got rid of that error..but im gettin another error or errors as the case may be

> Executing: C:\Program Files\ConTEXT\ConExec.exe "C:\j2sdk1.4.1_04\bin\java.exe" Atm

java.lang.NullPointerException
at Atm.createAccount(Atm.java:155)
at Login.<init>(Login.java:98)
at Atm.<init>(Atm.java:43)
at Atm.main(Atm.java:74)
Exception in thread "main"
> Execution finished.


what does this mean?

aidan_walsh

Ok, the problem here is that your instansiating your ObjectOutputStream in openOutputStream() and trying to access it in createAccount(). You can't do this because when you instansiate an object in a method, you can only use it in that method. Its called local scope.

To overcome this, you either need to instansiate the object when you declare it at the head of the class, or just before you use it in the same method.

dicey

Originally posted by sound_wave
hey folks im having a probel with this bit of code im tryin to do.

i get an error mesage:



non-static method createAccount() cannot be referenced from a static context
createAccount ();

when i execute this following bit of code:

	public static void main(String args[])
	{
		Atm app = new Atm();
		createAccount ();
	}

any help much appreciated

The problem is that createAccount is an instance method, not static - you need to change your main to

	public static void main(String args[])
	{
		Atm app = new Atm();
		app.createAccount ();
	}