method only working once

[Mike Lipay]

Ok, so the first time I call the method it works, the second time it throws a NullPointerException.

Here is the run

[new-host-3:~/Documents/Java/Software Registration] mike% java Software
0: Aquatint
Aquatint  1.1
Exception in thread "main" java.lang.NullPointerException
	at Application.getNextApplication(Application.java:53)
	at Software.main(Software.java:10)

Here is the main class

class Software
{
	public static void main (String args[])
	{
	Application app = new Application ();

	app = app.getNextApplication();
	System.out.println (app.getSoftware() + "  " + app.getVersion());

	app = app.getNextApplication();
	System.out.println (app.getSoftware() + "  " + app.getVersion());

	app = app.getNextApplication();
	System.out.println (app.getSoftware() + "  " + app.getVersion());
	}
}

And here is the Application class:

import java.io.*;
import java.util.*;

class Application
{

// Constructors
	public Application ()
	{
	loadApplications ();
	}

	public Application ( String aSoftware, String aVersion, String aManufacturer, String aSerialNumber, String aRegistration, String aPassword, String aEmail, String aWebsite, String aPhone )
	{
	software		= aSoftware;
	version			= aVersion;
	manufacturer	= aManufacturer;
	serialNumber	= aSerialNumber;
	registration	= aRegistration;
	password		= aPassword;
	email			= aEmail;
	website			= aWebsite;
	phone			= aPhone;
	}

	private String software;
	private String version;
	private String manufacturer;
	private String serialNumber;
	private String registration;
	private String password;
	private String email;
	private String website;
	private String phone;

	private int currentRecord = -1;

	Application[] apps = new Application[50];

	public String getSoftware ()		{	return software;		}
	public String getVersion ()			{	return version;			}
	public String getManufacturer ()	{	return manufacturer;	}
	public String getSerialNumber ()	{	return serialNumber;	}
	public String getRegistration ()	{	return registration;	}
	public String getPassword ()		{	return password;		}
	public String getEmail ()			{	return email;			}
	public String getWebsite ()			{	return website;			}
	public String getPhone ()			{	return phone;			}

	public Application getNextApplication ()
	{
	currentRecord++;
	System.out.println(currentRecord + ": " + apps[currentRecord].getSoftware());
	return apps[currentRecord];
	}

[Rahul P Kumar]

when you are putting the first element in array ? you will get that much software records, what you put in array. So, if app[nth] is null, invoking a method will certainly complain.

[Stefan Brandenberger]

Hi Mike

My guess is, that getNextApplication () always returns null. But to verify, I'd like to see the code of method loadApplications () that's called in the parameterless Constructor of Application.

Stefan

[Campbell Ritchie]

Your getNextApplication method is in fact working. It returns whatever it finds, which is null. Now it would appear to be reading the elements of an array. Remember that an array of any reference type is implicitly filled with null references. If you don't explicitly fill the array with real references, you will eventually get one of the nulls back; when you get to the main method and try to something with that null the JVM says

[Mike Lipay]

Here is part of the missing loadApplication, sorry I thought I had copied it.

	public void loadApplications ()
	{
	apps[0] = new Application("Aquatint","1.1","Stick Software","","Mike","crocodiles","http://www.sticksoftware.com/","","");
	apps[1] = new Application("CalendarScript","3.2","","Trans Id 1VL","","","","","");
	apps[2] = new Application("Differences Examiner","3","Alpha-Omega Software","Mike","8685887632085","","http://alphaomega.software.free.fr","[e-mail]","");
	apps[3] = new Application("DropStuff","6","Aladdin Systems","","DRPF","","www.digitalriver.com - order# 2372","","");
	}
}

[Rahul P Kumar]

ok, you are saying

app = app.getNextApplication(); 

first time. getNextApplication() returns an Application object initialized by parameterized constructor and does not initialize your apps array (not even a single value). i value at that time is 0. so apps[0] is null. Avoid reassigning or workaround it.

[Stefan Brandenberger]

On line 5 of class Software you create a new Application with the parameterless Constructor. So in this instance, the Application-array apps holds 4 instances of Application, created in your loadApplications()-method. On line 7 you call getNextApplication() on it, which returns the first entry of the array. Note, that this Application was created with the second constructor (new Application("Aquatint","1.1","Stick Software","","Mike","crocodiles","http://www.sticksoftware.com/","","");). Therefore it's apps-array is empty, because you don't call loadApplications() in that constructor.

So app on line 10 is null. Bingo.

One mistake, you make, is to reuse the variable app again and again. That way you call getNextApplication() always on a new instance of Application. Try this:

class Software  
{  
     public static void main (String args[])  
     {  
     Application app = new Application ();  
   
     Application app2 = app.getNextApplication();  
     System.out.println (app2.getSoftware() + "  " + app2.getVersion());  
   
     app2 = app.getNextApplication();  
     System.out.println (app2.getSoftware() + "  " + app2.getVersion());  
   
     app2 = app.getNextApplication();  
     System.out.println (app2.getSoftware() + "  " + app2.getVersion());  
     }  
}  

...but don't try it more than 4 times .

By the way: what is the intention to hold a list of Applications in Application-class? Shouldn't it rather be a class ApplicationPortfolio or so, to hold the Applications?