Writeing / Reading ArrayList

[Mike Lipay]

Ok, the next assignment we received was to take our prior program and convert it from an Array to an ArrayList. For the most part this worked well, but when I tried adding file code (read/write the ArrayList) I ran into problems. The write worked well, by well I mean that it didn't throw an error and it seemed to write the file. But, when I add the code to read the ArrayList the compiler doesn't seem to like reading in an ArrayList as an object. How is this done?

Note: the original assignment was just to write each element out to the file, I am trying to go beyond that and write the ArrayList out as a single object.


Error received:

G:\Java\SoftwareRegistry>javac Application.java
Application.java:250: incompatible types
found   : java.lang.Object
required: java.util.ArrayList<Application>
        apps = oisAppFile.readObject();
                                    ^
1 error

G:\Java\SoftwareRegistry>

Here is the code (problem is in the loadApplications method):

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;

/**
 * Maintain a software registry on a file called SWReg.pbj
 * @author	Mike Lipay
 * @version	1.0
*/
public class Application implements Serializable, Comparable<Application>
{
private static long serialVersionUID = -2701943741777789741L;


/**
 * Blank constructor
*/
	public Application ()
	{
	}


/**
 * Main constructor
 * @param	aSoftware		Name of the software application
 * @param	aVersion		Software version
 * @param	aManufacturer	Name of the person/company who owns/maintains the software
 * @param	aSerialNumber	Unique identifier of this installation of the software
 * @param	aRegistration	Registration number
 * @param	aPassword		Activation password for the application
 * @param	aEmail			Email address for application support
 * @param	aWebsite		Web address for the manufacturer
 * @param	aPhone			Phone number for customer service / technical support
*/
	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;
	ArrayList <Application> apps = new ArrayList <Application> (100);


/**
 * Return the contents of a single field
*/
	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 occurrence of appName in the application array.
 *		The search is case insensitive and the text may appear anywhere with the name
 * @param	appName - Full or partial name of the software
 * @return	Returns the array entry if found, otherwise returns null
*/
	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;
	}


/**
 * Find the first occurrence of appName in the application array.
 *		The search is case insensitive and must be an exact match
 * @param	appName - Full name of the software
 * @return	Returns the array entry if found, otherwise returns null
*/
	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;
	}


/**
 * Return the next entry within the array.
 *		If when the end is reached null is returned and the pointer is set to the beginning
 * @return	Returns the array entry if found, otherwise returns null
*/
	public Application getNextApplication ()
	{
	if (currentRecord > apps.size())	currentRecord = 0;
	return apps.get(currentRecord++);
	}


/**
 * List the name of all applications
*/
	public void listApplications ()
	{
	int i = 0;
	System.out.println();

	for ( Application a : apps )	System.out.println (++i + ": " + a.getSoftware());
	System.out.println();
	return;
	}


/**
 * Change the information on an application entry
 *		The application array is searched for an exact match, the calling routine must make sure
 *		that the application exists in the array before calling this method
 * @param	newName		Name of application
 * @param	newVersion	Version Number
 * @param	newMfgr		Manufacturer
 * @param	newSerialNo	Serial Number
 * @param	newReg		Registration Id
 * @param	newPassword	Password
 * @param	newEmail	Technical support email address
 * @param	newWebsite	Website for the Manufacturer or Application
 * @param	newPhone	Customer or technical support phone number
*/
	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.size(); i++ )
		{
		if ( apps.get(i).software.compareToIgnoreCase(newName) == 0)	
			{
			Application a = new Application (newName, newVersion, newMfgr, newSerialNo, newReg, newPassword, newEmail,
                                                                     newWebsite, newPhone);
			apps.set(i, a);
			return;
			}
		}
	}


/**
 * Delete the application entry
 *		The application array is searched for an exact match, the calling routine must make sure
 *		that the application exists in the array before calling this method
 * @param	appName - Exact name of the application
*/
	public void deleteApplication (String appName)
	{
	for (int i=0; i><apps.size(); i++ )
		if (apps.get(i).software.compareToIgnoreCase(appName) == 0)
			apps.remove(i);
	}


/**
 * Add a new application entry
 *		The application array is searched for an exact match, the calling routine must make sure
 *		that the application does not exist in the array before calling this method
 * @param	newName		Name of application
 * @param	newVersion	Version Number
 * @param	newMfgr		Manufacturer
 * @param	newSerialNo	Serial Number
 * @param	newReg		Registration Id
 * @param	newPassword	Password
 * @param	newEmail	Technical support email address
 * @param	newWebsite	Website for the Manufacturer or Application
 * @param	newPhone	Customer or technical support phone number
*/
	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.size(); i++)
		{
		Application a = apps.get(i);
		if (a.getSoftware().compareToIgnoreCase(newName) == 1)
			{
			apps.add (i, (new Application (newName, newVersion, newMfgr, newSerialNo, newReg, newPassword,
                                                                   newEmail, newWebsite, newPhone)));
			return;
			}
		}
	}

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



/**
 * Save the registration array to the file SWReg.obj
 * @throws	Throws an exception if a file error occurs
*/
	public void saveApplications () throws Exception
	{
	ObjectOutput oosAppFile	= new ObjectOutputStream(new FileOutputStream("SWReg.obj"));
	oosAppFile.writeObject(apps);
	oosAppFile.close();
	}


/**
 * Reads the registration file and loads it into the registration array
*/
	public void loadApplications () throws Exception
	{
	FileInputStream fisAppFile = new FileInputStream("SWReg.obj");
	ObjectInputStream oisAppFile = new ObjectInputStream(fisAppFile);
	apps = oisAppFile.readObject();
	oisAppFile.close();
	fisAppFile.close();
	}


public static void main (String args[]) throws Exception
	{
	Application software	= new Application ();
	software.loadApplications();
	software.listApplications();
	}
}

>

[Steve Luke]

Mike Lipay wrote:Ok, the next assignment we received was to take our prior program and convert it from an Array to an ArrayList. For the most part this worked well, but when I tried adding file code (read/write the ArrayList) I ran into problems. The write worked well, by well I mean that it didn't throw an error and it seemed to write the file. But, when I add the code to read the ArrayList the compiler doesn't seem to like reading in an ArrayList as an object. How is this done?

Note: the original assignment was just to write each element out to the file, I am trying to go beyond that and write the ArrayList out as a single object.


Error received:

G:\Java\SoftwareRegistry>javac Application.java
Application.java:250: incompatible types
found   : java.lang.Object
required: java.util.ArrayList<Application>
        apps = oisAppFile.readObject();
                                    ^
1 error

G:\Java\SoftwareRegistry>

That error tells you that the required type is java.util.ArrayList - which makes sens because the line is assigning a value to apps, and apps is an ArrayList. The error also tells you that the type returned from the ObjectInputStream#readObject() method is an java.lang.Object. Since the return type isn't an ArrayList you have a problem - the underlying runtime object might actually be an ArrayList but the compiler can't be sure.

Now, in your previous post you solved the same problem on the same line of code using an array instead of an ArrayList. How did you do that?

[Mike Lipay]

Steve Luke wrote:That error tells you that the required type is java.util.ArrayList - which makes sens because the line is assigning a value to apps, and apps is an ArrayList. The error also tells you that the type returned from the ObjectInputStream#readObject() method is an java.lang.Object. Since the return type isn't an ArrayList you have a problem - the underlying runtime object might actually be an ArrayList but the compiler can't be sure.

Now, in your previous post you solved the same problem on the same line of code using an array instead of an ArrayList. How did you do that?

The Array version is:

	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();
	}

I tried duplicating this code, but nothing I tried seemed to be allowed, for example:

	public void loadApplications () throws Exception
	{
	FileInputStream fisAppFile = new FileInputStream("SWReg.obj");
	ObjectInputStream oisAppFile = new ObjectInputStream(fisAppFile);
	ArrayList <Application> appIn = (ArrayList <Application>) oisAppFile.readObject();
	apps = appIn;
	oisAppFile.close();
	fisAppFile.close();
	}

This generated the error:

[new-host-3:MIKESFD/Java/SoftwareRegistry] mike% javac Application.java -Xlint
Application.java:250: warning: [unchecked] unchecked cast
found   : java.lang.Object
required: java.util.ArrayList<Application>
	ArrayList <Application> appIn = (ArrayList <Application>) oisAppFile.readObject();
	                                                                               ^
1 warning

I'm not sure what the correct coding is to cast it properly, I can't find anything in javadoc that covers this, and we haven't covered it in class.

[Steve Luke]

That is a warning only, the class compiles and will run properly, assuming the content of the file actually is an ArrayList<Application>. The warning is telling you that you are 'down-casting' the object from a broader type to a more specific type, and that the compile has no way to know if it will work or not. There are ways to temporarily turn the warning off (such as not using -Xlint on the compile command) but you don't have to worry about them.

Question, when you compiled the Array version, didn't you get the same warning?

[Steve Luke]

Steve Luke wrote:Question, when you compiled the Array version, didn't you get the same warning?

I guess not - looks like it only comes up relative to Generics.

[Mike Lipay]

Without the -Xlint option it comes up with warnings:

Note: Application.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

[Mike Lipay]

I tried a compile and execute, it does work. Is there a better way to accomplish this so that the compile doesn't throw an error?

[Steve Luke]

It isn't an error, it is a warning. Sorry, I was wrong about the -Xlint option, I thought it warning was a standard warning about the cast, I didn't remember it was about the generics cast. You can add the @SupressWarning("unchecked") annotation to the line of code:

@SuppressWarnings("unchecked") ArrayList <Application> appIn =
        (ArrayList <Application>) oisAppFile.readObject();

Or you can just live with the warning since 'you know what you are doing' and know that that line of code won't actually break. I don't know of a compiler command that will hide both the warning and the note.

[Mike Lipay]

Thanks.