Ok, applauded a little too soon...sort problem

[Mike Lipay]

I thought I had the sort routine worked out, but now it is throwing a NullPointerException whenever I add an item to the array, then try to sort it. The array is loaded with 23 items, I add a 24th, then do the sort to put the array into sequence by Software name, that is when it crashes. The method involved is addApplication.

Exception in thread "main" java.lang.NullPointerException
        at java.util.Arrays.mergeSort(Unknown Source)
        at java.util.Arrays.mergeSort(Unknown Source)
        at java.util.Arrays.mergeSort(Unknown Source)
        at java.util.Arrays.mergeSort(Unknown Source)
        at java.util.Arrays.sort(Unknown Source)
        at Application.addApplication(Application.java:141)
        at Software.addSoftware(Software.java:99)
        at Software.main(Software.java:23)

import java.io.*;
import java.util.*;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

class Application implements Serializable, Comparable<Application>
{
private static long serialVersionUID = -6273965315032407566L;

	public Application ()
	{
	}

	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 = 0;

	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;			}


// find the first application containing the appName
	public Application findApplication (String appName)
	{
	currentRecord		= -1;
	String appName2		= appName.toLowerCase();

	for ( Application a : apps )
		{
		if ( a == null)	return null;

		String software = a.getSoftware();
		software = software.toLowerCase();
		if (software.contains (appName2))	return a;
		}

	return null;
	}


// exactly match the application
	public Application matchApplication (String appName)
	{
	String appName2		= appName.toLowerCase();

	for ( Application a : apps )
		{
		if ( a == null)	return null;

		String software = a.getSoftware();
		software = software.toLowerCase();
		if (software.equals (appName2))	return a;
		}

	return null;
	}


// read the next application
	public Application getNextApplication ()
	{
	return apps[currentRecord++];
	}


// list the applications
	public void listApplications ()
	{
	for ( Application a : apps )
		{
		if ( a == null)	return;

		System.out.println (a.getSoftware());
		}
	}


// change the application data
	public void changeApplication (
                  String newName, String newVersion, String newMfgr, String newSerialNo, String newReg, String newPassword, String newEmail, String newWebsite, String newPhone)
	{
	for (int i=0; i<apps.length; i++ )
		{
		if ( apps[i].software.compareToIgnoreCase(newName) == 0)	
			{
			apps[i].version = newVersion;
			apps[i].manufacturer = newMfgr;
			apps[i].serialNumber = newSerialNo;
			apps[i].registration = newReg;
			apps[i].password = newPassword;
			apps[i].email = newEmail;
			apps[i].website = newWebsite;
			apps[i].phone = newPhone;
			System.out.println ("found and changed");
			return;
			}
		}
	}


// add a new application into the array
	public void addApplication (
                     String newName, String newVersion, String newMfgr, String newSerialNo, String newReg, String newPassword, String newEmail, String newWebsite, String newPhone)
	{
	for (int i=0; i><apps.length; i++ )
		{
		if ( apps[i] == null)	
			{
			apps[i] = new Application (newName, newVersion, newMfgr, newSerialNo, newReg, newPassword, newEmail, newWebsite, newPhone);
			Arrays.sort(apps);
			return;
			}
		}
	}

	public int compareTo(Application other)
	{
	if (other == null)	return -1;
	return (software.compareToIgnoreCase(other.software));
	}



// save the array
	public void saveApplications () throws Exception
	{
	ObjectOutput oosAppFile	= new ObjectOutputStream(new FileOutputStream("SWReg.obj"));
	oosAppFile.writeObject(apps);
	oosAppFile.close();
	}


// load applications into an array
	public void loadApplications () throws Exception
	{
	FileInputStream fisAppFile = new FileInputStream("SWReg.obj");
	ObjectInputStream oisAppFile = new ObjectInputStream(fisAppFile);
	Application[] appIn = (Application[]) oisAppFile.readObject();
	apps = appIn;
	oisAppFile.close();
	fisAppFile.close();
	}
}

>

[Henry Wong]

Basically, you have null elements in the array -- so when a comparison with a null occurs, you can get an NPE, as you have in this case.

Henry

[Mike Lipay]

Ok, but if I declare an array large enough to hold what I have, plus allow for expansion, then I'm going to have null elements. How do I handle this situation? From what I've learned, once I've declared an array to be [24] I can't increase it to [25] to add another entry without recompiling the entire application.

[Janeice DelVecchio]

Is there a reason you're not using a List, ArrayList or LinkedList? Those are dynamically sized so you'll be able to add and delete items without caring about having enough space or comparing null values....

Janeice

[fred rosenberger]

the simplest way is

if (a != null && a.method())

or something like that...

[Henry Wong]

Mike Lipay wrote:Ok, but if I declare an array large enough to hold what I have, plus allow for expansion, then I'm going to have null elements. How do I handle this situation?

You have two options... (1) you can use the version of sort() that allows you to sort a range in the array -- meaning only sort the area in the array that you are actually using. or (2) you can use the version of sort() that takes a comparator. A comparator allows either of the two elements, to be compared, to be null (well, you still have to write it so, but it is possible).

Henry

[Ernest Friedman-Hill]

Use an ArrayList. There's rarely a reason to use a plain array except when you need to pass one to an API that needs one.

[Mike Lipay]

Regarding the lists and array lists...haven't learnt about them yet.

Fred, I have no idea where to put that, and is it a.method or do I replace method with something?

Henry, I thought I did have those instructions, did I do it wrong?

  
// add a new application into the array  
    public void addApplication (
                      String newName, String newVersion, String newMfgr, String newSerialNo, String newReg, String newPassword, String newEmail, String newWebsite, String newPhone)  
    {  
    for (int i=0; i><apps.length; i++ )  
        {  
        if ( apps[i] == null)     
            {  
            apps[i] = new Application (newName, newVersion, newMfgr, newSerialNo, newReg, newPassword, newEmail, newWebsite, newPhone);  
            Arrays.sort(apps);  
            return;  
            }  
        }  
    }  
  
    public int compareTo(Application other)  
    {  
    if (other == null)  return -1;  
    return (software.compareToIgnoreCase(other.software));  
    }  

>

[Henry Wong]

Mike Lipay wrote:
Henry, I thought I did have those instructions, did I do it wrong?

Here is the code snippet that calls the sort...

 
            Arrays.sort(apps);  

This sorts the whole array, by natural ordering. Not part of the array. And not using a comparator.

And BTW, a comparator and comparable are two different things.

Henry

[Mike Lipay]

Ok, right now comparators are beyond me, couldn't make heads or tails out of the doc for it, so I went with supplying the start and end point for the sort. Works. Thanks. (again )