Singleton Class Variable

i have one class test1 and other class test2. this test1 class should have one class varibale of instance of test2. how this could be done both are single ton class

Is this waht u mean?
class test1{
public static test1 getInstance(){
class test2 {
private test1 test1Obj;
static{
test1Obj=test1.getInstance();
public static test2 getInstance(){
}

Similar Messages

Singleton class with static method

Hi,
I have made a fairly simple singletion class STClass. EX:
Consider tow cases
CASE-1:
private static STClass singleInstance;
private static int i;
private STClass()
public static STClass instance()
     if (singleInstance == null)
          singleInstance = new STClass();
          loadConfigFile("XYZ");
     return singleInstance;
private static void loadConfigFile(String fileName)
     //SOME CODE in which use private variable "i"
CASE-2:
private static STClass singleInstance;
private int i;
private STClass()
     loadConfigFile("XYZ");
public static STClass instance()
     if (singleInstance == null)
          singleInstance = new STClass();
     return singleInstance;
private void loadConfigFile(String fileName)
     //SOME CODE in which use private variable "i"
What is the differnce between two case and which one will be best and why???
Someone told me "If u keep variables and methods as static the the whole purpose of singleton class is defeated"
please justify this statement

It seems to me that this way should be more "correct":
public class STClass {
private static STClass singleInstance;
private int i;
private STClass() {
public static STClass getInstance() {
if (singleInstance == null) {
singleInstance = new STClass();
singleInstance.loadConfigFile("XYZ");
return singleInstance;
private void loadConfigFile(String fileName) {
//SOME CODE in which use private variable "i"
}And it is also compatible with your CASE-2.
Hope this helped,
Regards.I wouldnot agree with this piece of code here because of JMM bug. check this
http://www.javaworld.com/jw-02-2001/jw-0209-double-p2.html
What about proposing your own version, explained ?...

Singleton class issue

I have a singleton class. A method in this class creates an array of structures. This class is shared across different process. So there is a chance that some one unknowigly calls the method to create the array of structures and hence the member variable may now points to another array of structure.
Is there any way in Java to restict invoking of a function only once.
A static variable with a check will solve this issue, but any other method so that calling of method itself becomes invalid to avoid RT usage?.

Hi,
I think, I understood know.
You want to have a singleton holding a defined number of other objects (your arrays).
This objects (your arrays) are semantically singletons itsself, because they should be constructed only once and than reused.
Assuming I interpreted right, see my comments inline below.
I know that who ever participate in this discussion is
pretty sure about singleton class. Some of the code
what I gave was irretating as Martin pointed out. So
let me try to give more transparent code snippet,Thanks, that helped.
>
My aim :- I need a global object which is going to
hold some arrays(data structures). This should be
shared across various threads. (forget about the
synchronousation part here). All these arrays won't be
of same size, so I need methods for each
datastructures to initialise to its required size.That's a little bit part of your problem, see below.
My wayforward :- Create the global object as
singleton. This object has methods for initialising
different data structures.OK, fine.
What is my doubt :- please see the following code
fragment,
public class Singleton {
private Singleton singleton;
private Singleton() {
public static Singleton getInstance() {
if (singleton == null) {
singleton = new Singleton();
return singleton;
//other methods for the class
private someObjType1 myArray1 = null;
private someObjType2 myArray2 = null;
private someObjType3 myArray3 = null; // etc....This "smells" like an candidate for another data structure, so that you don't have a fixed number of array.
F.E.
// Associate class of array elements (someObjTypeX) as keys with arrays of someObjTypeX
private Map arrayMap = new HashMap();>
public void CreateArray1(int size) {
if (myArray1 == null) {
myArray1 = new someObjType1[size];
}Using the map, you create array should look like
public void CreateArray(Class clazzOfArrayElem, int size)
Object arr = arrayMap.get(clazzOfArrayElem)
if(arr == null)
    arr = java.lang.reflect.Array.newInstance(clazzOfArrayElem, size);
    arrayMap.put(clazzOfArrayElem, arr);
}Additionally the "ugliness" of this method results from the problem, that don't know the size of arrays at compile time.
So it seem to be preferable to use a dynamic structure like a Vector instead of array. Then you don't have the need to pass the size in the accessor method.
Then you can use a "cleaner" accessor method like
public Vector getVector(Class clazzOfArrayElem)
Vector vec = arrayMap.get(clazzOfArrayElem)
if(vec == null)
    vec = new Vector();
    arrayMap.put(clazzOfArrayElem, vec);
return vec;
}If you want to expose an array oriented interface f.e. for type safety you can have an accessor like
public Object[] getAsArray(Class clazzOfArrayElem)
Vector vec = arrayMap.get(clazzOfArrayElem)
if(vec == null)
    vec = new Vector();
    arrayMap.put(clazzOfArrayElem, vec);
arr = java.lang.reflect.Array.newInstance(clazzOfArrayElem, 0);
return (Object[])vec.toArray(arr);
// Real typesafe accessors as needed following
public SomeClass1[] getAsArrayOfSomeClass1()
return (SomeClass1[])getAsArray(SomeClass1.class);
/.....This accessor for array interface have the additional advantage, that they return a copy of the data (only the internal arrays, no the object within the arrays!), so that a least a client using your class doesn't have access to your internal data (with the Vector he has).
>
// similarly for other data structures...
So here CreateArray1 method will work fine because of
the check myArray1 == null.
Actual question :- Can I remove this check and
restrick call to CreateArray1 more than one time ?.No. As I understood you want to have something like a "only-use-once-method". There is no such thing in Java I know of.
But I don't see the need to remove this check. Reference comparison should have no real impact to the performance.
this is going to benifit me for some other cases
also.How?
Hope this helps.
Martin

Can not access the Instance Data of a Singleton class from MBean

I am working against the deadline and i am sweating now. From past few days i have been working on a problem and now its the time to shout out.
I have an application (let's call it "APP") and i have a "PerformanceStatistics" MBean written for APP. I also have a Singleton Data class (let's call it "SDATA") which provides some data for the MBean to access and calculate some application runtime stuff. Thus during the application startup and then in the application lifecysle, i will be adding data to the SDATA instance.So, this SDATA instance always has the data.
Now, the problem is that i am not able to access any of the data or data structures from the PerformanceStatistics MBean. if i check the data structures when i am adding the data, all the structures contains data. But when i call this singleton instance from the MBean, am kind of having the empty data.
Can anyone explain or have hints on what's happening ? Any help will be appreciated.
I tried all sorts of DATA class being final and all methods being synchronized, static, ect.,, just to make sure. But no luck till now.
Another unfortunate thing is that, i some times get different "ServicePerformanceData " instances (i.e. when i print the ServicePerformanceData.getInstance() they are different at different times). Not sure whats happening. I am running this application in WebLogic server and using the JConsole.
Please see the detailed problem at @ http://stackoverflow.com/questions/1151117/can-not-access-the-instance-data-of-a-singleton-class-from-mbean
I see related problems but no real solutions. Appreciate if anyone can throw in ideas.
http://www.velocityreviews.com/forums/t135852-rmi-singletons-and-multiple-classloaders-in-weblogic.html
http://www.theserverside.com/discussions/thread.tss?thread_id=12194
http://www.jguru.com/faq/view.jsp?EID=1051835
Thanks,
Krishna

I am working against the deadline and i am sweating now. From past few days i have been working on a problem and now its the time to shout out.
I have an application (let's call it "APP") and i have a "PerformanceStatistics" MBean written for APP. I also have a Singleton Data class (let's call it "SDATA") which provides some data for the MBean to access and calculate some application runtime stuff. Thus during the application startup and then in the application lifecysle, i will be adding data to the SDATA instance.So, this SDATA instance always has the data.
Now, the problem is that i am not able to access any of the data or data structures from the PerformanceStatistics MBean. if i check the data structures when i am adding the data, all the structures contains data. But when i call this singleton instance from the MBean, am kind of having the empty data.
Can anyone explain or have hints on what's happening ? Any help will be appreciated.
I tried all sorts of DATA class being final and all methods being synchronized, static, ect.,, just to make sure. But no luck till now.
Another unfortunate thing is that, i some times get different "ServicePerformanceData " instances (i.e. when i print the ServicePerformanceData.getInstance() they are different at different times). Not sure whats happening. I am running this application in WebLogic server and using the JConsole.
Please see the detailed problem at @ http://stackoverflow.com/questions/1151117/can-not-access-the-instance-data-of-a-singleton-class-from-mbean
I see related problems but no real solutions. Appreciate if anyone can throw in ideas.
http://www.velocityreviews.com/forums/t135852-rmi-singletons-and-multiple-classloaders-in-weblogic.html
http://www.theserverside.com/discussions/thread.tss?thread_id=12194
http://www.jguru.com/faq/view.jsp?EID=1051835
Thanks,
Krishna

Assign a value to class variable

I want to define a class variable in class. And I want all subclasses of that class ot have a different value for that class variable. How can I do that?
public class BaseClass {
public static String tableName = "";
}Now if I have a ClassA and I want to assign a value like this :
public class ClassA extends BaseClass {
tableName = "location";
} I got an error message.
I can move it in a static initializer block but then it will only work when the class is loaded. In my case its possible i want to get this value without loading the class.
Ditto if i move it to constructor.
Any input? Thanks

Are you saying that if i have 2 classes ClassA and
ClassB inherited from BaseClass, then both are
sharing the same copy of 'tableName' staticvariable?
If yes then I should go with an instance variable.No, I am saying that you can easily declare a
tablName in A and another tableName in B.
A.tableName will be shared between all the instances
s of A. B.tableName will be shared between all the
instances of B. And BaseClass.tableName is
irelevant. I think you try to use
BaseClass.tableName as some kind of template for
sub-classes, but this does not happen: you need to
declare tableName again and again in each subclass.
IThanks for clarifying. Each class needs to have a variable "tableName" and it needs to have one copy of this variable for all of that class's objects. 2 classes will not have the same value of this tableName variable.
Thats why I was defining it as static variable. And I define it in BaseClass so that I dont have to define it again in each subclass.
Is there any better way? Thanks

Unbound Class Variable Error

Hi All,
I am doing a webdynpro application. while deploying, it gives me an error
"Unbound class Variable:'KMC_LIBS/bc.srf.framework_api.jar'.
What is this error?
Regards,
Divya

Hi,
A Classpath Variable is just a variable which points to a directory in your file system.
Useful when 1st developer has external jars on path1 and 2nd developer has external jars on path2 (the two paths are different) and the developers want to prevent classpath issues.
In NWDS go to:
Window -> Preferences -> Java -> Classpath Variables and check if 'KMC_LIBS' points to a directory.
Also, you can delete this entry from your project's classpath and add the file 'bc.srf.framework_api.jar'' manually (without classpath variable).
Regards,
Omri

Create an instance of my class variable

Hello all,
I'm a newbie to iPhone/iPad programming and am having some trouble, I believe the issue is that I'm not creating an instance of my class variable. I've got a class that has a setter and a getter, I know about properties but am using setters and getters for the time being. I've created this class in a View-based application, when I build the program in XCode 3.2.6 everything builds fine. When I step through the code with the debugger there are no errors but the setters are not storing any values and thus the getters are always returning 0 (it's returning an int value). I've produced some of the code involved and I was hoping someone could point out to me where my issue is, and if I'm correct about not instantiating my variable, where I should do that. Thanks so much in advance.
<p>
Selection.h
@interface Selection : NSObject {
int _choice;
//Getters
-(int) Choice;
//Setters
-(void) setChoice:(int) input;
Selection.m
#import "Selection.h"
@implementation Selection
//Getters
-(int)Choice {
return _choice;
//Setter
-(void)setChoice:(int)input{
          _choice = input;
RockPaperScissorsViewController.m
#import "RockPaperScissorsViewController.h"
@implementation RockPaperScissorsViewController
@synthesize rockButton, paperButton, scissorsButton, label;
//@synthesize humanChoice, computerChoice;
-(void)SetLabel:(NSString *)selection{
          label.text = selection;
-(IBAction)rockButtonSelected {
//          [self SetLabel:@"Rock"];
          [humanChoice setChoice:1];
</p>
So in the above code it's the [humanChoice setChoice:1] that is not working. When I step through that portion with the debugger I get no errors but when I call humanChoice.Choice later on in the code I get a zero.
-NifflerX

It worked, thank you so much. I put
humanChoice = [[Selection alloc]init];
In viewDidLoad and it worked like a charm. Thank you again.
-NifflerX

Help required in understanding the rationale behind singleton class

Can anyone help me in understanding singleton classes?why do we need them?Architecturally when should we go for a singleton class?

answer lies in GoF book.

To ragnic and other about Singleton class

Hi ragnic. Thanks for your reply. I posted the code wrong. Heres' my correct one.
I have a GUI first loaded information and the information is stored in a databse, I have some EJB classes..and my singleton class ABC has some method to access to the EJB..
so my first GUI will gather info using singleton class and later if I click on
a button in my first GUI class, will pop up another frame of another class , this class also need to class setPassword in my Singleton..
are my followign codes correctly??
iS my Class ABC a SINgleton class? thanks
Is my class ABC use as single correctly. And It is called from other classes is also correct?
I'm new to java and like to learn about Singleton class.
But I really dont' understand it clearly after reading many examples of it.
I have a project to convert my class abc to a singleton.
But I dont know how to do it.
In my class(soon will become a singleton) will have few methods that later I need to use it from another class A and class B.
I have a GUI application that first load my class A..and my class will call
class abc(singleton) to get some information from it.
and then in class A has a button, if I click on that button I will call SIngleton class again to update my password, in the singleton class has method calls updatePassword. But I dont know how to call a singleton from other class.
I have my code for them below:
1)public class ABC //attempt using a singleton
private static ABC theABC = null;
private ABC(){}
public synchronized static ABC getABC()
if(theABC == null)
theABC= new ABC();
return the ABC;
public void updateUserInfo(SimpleUser user)
throws UserNotFoundException, DelegateException
try
UserCollectionHome userCollectionHome = (UserCollectionHome)
EJBHomeFactory.getFactory().lookupHome("vista/UserCollection",
UserCollectionHome.class);
UserHome userHome = (UserHome)
EJBHomeFactory.getFactory().lookupHome("vista/User",UserHome.class);
UserCollection uc = userCollectionHome.create();
uc.updateUserInfo(user, userHome);
} catch(HomeFactoryException hfe) {
hfe.printStackTrace();
throw new DelegateException(hfe);
} catch(RemoteException re) {
re.printStackTrace();
throw new DelegateException(re);
} catch(CreateException ce) {
ce.printStackTrace();
throw new DelegateException(ce);
} catch(FinderException fe) {
fe.printStackTrace();
throw new UserNotFoundException();
public SimpleUser getID(String id)
throws UserNotFoundException, DelegateException
try
UserCollectionHome userCollectionHome = (UserCollectionHome)
EJBHomeFactory.getFactory().lookupHome("vista/UserCollection",
UserCollectionHome.class);
UserHome userHome = (UserHome)
EJBHomeFactory.getFactory().lookupHome("vista/User",UserHome.class);
UserCollection uc = userCollectionHome.create();
SimpleUser su = uc.getID(id, userHome);
return su;
} catch(HomeFactoryException hfe) {
throw new DelegateException(hfe);
} catch(RemoteException re) {
throw new DelegateException(re);
} catch(CreateException ce) {
throw new DelegateException(ce);
} catch(FinderException fe) {
throw new UserNotFoundException();
public void setPassword(String lname,String pw)
throws UserNotFoundException, DelegateException
try
UserCollectionHome userCollectionHome = (UserCollectionHome)
EJBHomeFactory.getFactory().lookupHome("vista/UserCollection",
UserCollectionHome.class);
UserHome userHome = (UserHome)
EJBHomeFactory.getFactory().lookupHome("vista/User",UserHome.class);
UserCollection uc = userCollectionHome.create();
uc.setPassword(lname,pw, userHome);//assume that all lname are differents.
} catch(HomeFactoryException hfe) {
hfe.printStackTrace();
throw new DelegateException(hfe);
} catch(RemoteException re) {
re.printStackTrace();
throw new DelegateException(re);
} catch(CreateException ce) {
ce.printStackTrace();
throw new DelegateException(ce);
} catch(FinderException fe) {
fe.printStackTrace();
throw new UserNotFoundException();
}//Do I have my class as a Singleton correctly???
2)//Here is my First Frame that will call a Singleton to gather user information
public A(Frame owner)
super(owner, "User Personal Information",true);
initScreen();
loadPersonalInfo();
* This method instantiates all the GUI widgets and places them into panels and
* onto the frame.
private void initScreen()
txtFname = new JTextField(20);
txtLname=new JTextField(20);
btnsave =new JButton("Save");
btnChange= new JButton("Click here to change PW");//when you click this button there will be a frame pop up for you to enter informaton..this iwll call class B
JPanel pnlMain=new JPanel();
JPanel pnlFname= new JPanel();
pnlFname.setLayout(new BoxLayout(pnlFname, BoxLayout.X_AXIS));
pnlFname.setBorder(BorderFactory.createEmptyBorder(0,87,0,90));
pnlFname.add(new JLabel("First Name:"));
pnlFname.add(Box.createRigidArea(new Dimension(5,0)));
pnlFname.add(txtFname);
JPanel pnlLname= new JPanel();
pnlLname.setLayout(new BoxLayout(pnlLname, BoxLayout.X_AXIS));
pnlLname.setBorder(BorderFactory.createEmptyBorder(0,87,0,90));
pnlLname.add(new JLabel("Last Name:"));
pnlLname.add(Box.createRigidArea(new Dimension(5,0)));
pnlLname.add(txtLname);
pnlMain.add(pnlFname);
pnlMain.add(pnlLname);
pnlMain.add(btnsave);
pnlMain.add(btnChange");
btnSave = new JButton("Save");
btnSave.setActionCommand("SAVE");
btnSave.addActionListener(this);
btnCancel = new JButton("Cancel");
btnCancel.setActionCommand("CANCEL");
btnCancel.addActionListener(this);
JPanel pnlBottom = new JPanel();
pnlBottom.setLayout(new BoxLayout(pnlBottom, BoxLayout.X_AXIS));
pnlBottom.setBorder(BorderFactory.createEmptyBorder(25,55,0,0));
pnlBottom.add(btnSave);
pnlBottom.add(Box.createRigidArea(new Dimension(25,0)));
pnlBottom.add(btnCancel);
pnlMain.add(pnlBottom);
this.setContentPane( pnlMain);
setSize(500,500);
GraphicUtilities.center(this);
theABC=ABC.getABC();
//Do I call my ABC singleton class correctly??
private void loadPersonalInfo()
String ID= System.getProperty("user.name");
SimpleUser user = null;
try {
user = ABC.getID(ID);
//I tried to use method in ABC singleton class. IS this correctly call?
} catch(UserNotFoundException nfe)
JOptionPane.showMessageDialog(new JDialog(),"You have not yet registered.",
"User Not Found",JOptionPane.WARNING_MESSAGE);
System.exit(0);
} catch(DelegateException de) {
JOptionPane.showMessageDialog(new JDialog(),"You have not yet registered",JOptionPane.ERROR_MESSAGE);
System.exit(0);
currentUser = user;
txtFname.setText(currentUser.getFirstName());
txtLname.setText(currentUser.getLastName());
//This information will be display in my textfields Fname and Lname
//I can change my first and last name and hit button SAVE to save
public void actionPerformed(ActionEvent e)
if(e.getActionCommand().equals("SAVE")) submitChanges();
if(e.getActionCommand().equals("CHANGE_PASSWORD")) {
changepassword=new ChangePassword(new Frame(),name,badgeid);
public void submitChanges(){
String currentNTUsername = System.getProperty("user.name");
SimpleUser user =null;
try {
user = theABC.getID(ID);
user.setFirstName(txtFname.getText().trim());
user.setLastName(txtLname.getText().trim());
currentUser = user;
theABC.updateUserInfo(currentUser);
//IS this correctly if I want to use this method in singleton class ABC??
} catch(UserNotFoundException nfe)
JOptionPane.showMessageDialog(new JDialog(),"You have not yet registered",
"User Not Found",JOptionPane.WARNING_MESSAGE);
} catch(DelegateException de) {
JOptionPane.showMessageDialog(new JDialog(),"You have not yet registered",JOptionPane.ERROR_MESSAGE);
System.exit(0);
this.setVisible(false);
3) click on ChangePassword in my above GUI class A..will call this class B..and in this class B
I need to access method in a Singleton class- ABC class,,DO i need to inititates it agian, if not what should I do? thanks
package com.lockheed.vista.userinfo;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.border.*;
import javax.swing.table.*;
import javax.swing.tree.*;
import java.util.StringTokenizer;
import java.util.Vector;
import java.io.*;
import javax.swing.text.*;
import javax.swing.text.html.*;
import javax.swing.colorchooser.*;
import javax.swing.filechooser.*;
import javax.accessibility.*;
import java.beans.*;
import java.applet.*;
import java.net.*;
import org.apache.log4j.*;
import com.lockheed.common.gui.GraphicUtilities;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.border.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import vista.user.UserServicesDelegate;
import vista.user.SimpleUser;
import vista.user.UserNotFoundException;
import vista.user.*;
import com.lockheed.common.ejb.*;
import com.lockheed.common.gui.*;
import com.lockheed.vista.publish.*;
* This program allow users to change their Vista Web Center's password
public class ChangePassword extends JDialog
implements ActionListener{
protected final Logger log = Logger.getLogger(getClass().getName());
private UserServicesDelegate userServicesDelegate;
private User currentUser = null;
private JPasswordField txtPasswd, txtVerifyPW;
private JButton btnSubmit,btnCancel;
private JLabel lblName,lblBadgeID;
private String strBadgeID="";
* This is the constructor. It creates an instance of the ChangePassword
* and calls the method to create and build the GUI.
public ChangePassword(Frame owner,String name,String badgeid)
super(owner, "Change Password",true);
initScreen(name,badgeid);//build the GUI
* This method instantiates all the GUI widgets and places them into panels and
* onto the frame.
private void initScreen(String strname,String strBadgeid)
txtPasswd = new JPasswordField(20);
txtVerifyPW=new JPasswordField(20);
txtPasswd.setEchoChar('*');
txtVerifyPW.setEchoChar('*');
JPanel pnlMain=new JPanel();
pnlMain.setLayout(new BoxLayout(pnlMain, BoxLayout.Y_AXIS));
pnlMain.setBorder(BorderFactory.createEmptyBorder(20,0,20,0));
JPanel pnlPW=new JPanel();
pnlPW.setLayout(new BoxLayout(pnlPW, BoxLayout.X_AXIS));
pnlPW.setBorder(BorderFactory.createEmptyBorder(0,96,0,30));
pnlPW.add(new JLabel("Password:"));
pnlPW.add(Box.createRigidArea(new Dimension(5,0)));
pnlPW.add(txtPasswd);
JPanel pnlVerifyPW=new JPanel();
pnlVerifyPW.setLayout(new BoxLayout(pnlVerifyPW, BoxLayout.X_AXIS));
pnlVerifyPW.setBorder(BorderFactory.createEmptyBorder(0,63,0,30));
pnlVerifyPW.add(new JLabel("Verify Password:"));
pnlVerifyPW.add(Box.createRigidArea(new Dimension(5,0)));
pnlVerifyPW.add(txtVerifyPW);
JPanel pnlTop= new JPanel();
pnlTop.add(pnlPW);
pnlTop.add(Box.createRigidArea(new Dimension(0,10)));
pnlTop.add(pnlVerifyPW);
pnlMain.add(pnlTop);
btnSubmit = new JButton("Submit");
btnSubmit.setActionCommand("SUBMIT");
btnSubmit.addActionListener(this);
btnCancel = new JButton("Cancel");
btnCancel.setActionCommand("CANCEL");
btnCancel.addActionListener(this);
JPanel pnlBottom = new JPanel();
pnlBottom.setLayout(new BoxLayout(pnlBottom, BoxLayout.X_AXIS));
pnlBottom.setBorder(BorderFactory.createEmptyBorder(25,55,20,30));
pnlBottom.add(btnSubmit);
pnlBottom.add(Box.createRigidArea(new Dimension(25,0)));
pnlBottom.add(btnCancel);
pnlMain.add(pnlBottom);
this.setContentPane( pnlMain);
setSize(350,230);
setVisible(true);
public void actionPerformed(ActionEvent e)
if(e.getActionCommand().equals("CANCEL")) this.setVisible(false);
if(e.getActionCommand().equals("SUBMIT")) submitPW();
* This method is called when the submit button is clicked. It allows user to change
* their password.
public void submitPW(){
myABC= ABC.getABC();//Is this correct?
char[] pw =txtPasswd.getPassword();
String strPasswd="";
for(int i=0;i<pw.length;i++){
strPasswd=strPasswd+pw;
char[] vpw =txtVerifyPW.getPassword();
String strVerifyPW="";
for(int i=0;i<vpw.length;i++){
strVerifyPW=strVerifyPW+pw;
if((strPasswd==null)||(strPasswd.length()==0)) {
JOptionPane.showMessageDialog(new JDialog(),"You have not enter a password. Please try again.",
"Invalid Password",JOptionPane.ERROR_MESSAGE);
if((!strPasswd.equals(strVerifyPW)))
//password and verify password do not match.
JOptionPane.showMessageDialog(new JDialog(),"Your passwords do not match. Reenter and try again.",
"Invalid Password",JOptionPane.ERROR_MESSAGE);
try
myABC.setUserPassword(strPasswd);//try to use a method in Singleton class
txtPasswd.setText("");
txtVerifyPW.setText("");
this.setVisible(false);
} catch(DelegateException e) {
JOptionPane.showMessageDialog(new Frame(),
"Error.",
"Unable to change password information.",JOptionPane.WARNING_MESSAGE);
} catch(UserNotFoundException e) {
JOptionPane.showMessageDialog(new Frame(),
"Error.",
"Unable to change password information.",JOptionPane.WARNING_MESSAGE);
And ofcourse I have other EJB classes to work with these classes.
***It compiles okey but when I ran, it say "NullPointerException"
I think I call my Singleton wrong.
Please help me.thanks

1. When replying, use <reply>, don't post a new topic.
2. Implementing a singleton is a frequently asked question. Search before you post.
3. This is not a question about Swing. A more appropriate forum would be "New To Java Technology" or perhaps "Java Programming", but see point 1.
4. When posting code, keep it short. It increases the chance of readers looking at it. And in composing your shorter version for the forum, you just may solve your problem.

How to access the parent class variable or object in java

Hi Gurus,
I encounter an issue when try to refer to parent class variable or object instance in java.
The issue is when the child class reside in different location from the parent class as shown below.
- ClassA and ClassB are reside in xxx.oracle.apps.inv.mo.server;
- Derived is reside in xxx.oracle.apps.inv.mo.server.test;
Let say ClassA and ClassB are the base / seeded class and can not be modified. How can i refer to the variable or object instance of ClassA and ClassB inside Derived class.
package xxx.oracle.apps.inv.mo.server;
public class ClassA {
    public int i=10;
package xxx.oracle.apps.inv.mo.server;
public class ClassB extends ClassA{
    int i=20;
package xxx.oracle.apps.inv.mo.server.test;
import xxx.oracle.apps.inv.mo.server.ClassA;
import xxx.oracle.apps.inv.mo.server.ClassB;
public class Derived extends ClassB {
    int i=30;
    public Derived() {
       System.out.println(this.i);                  // this will print 30
       System.out.println(((ClassB)this).i); // error, but this will print 20 if Derived class located in the same location as ClassB
       System.out.println(((ClassA)this).i); // error, but this will print 20 if Derived class located in the same location as ClassA
    public static void main(String[] args) {
        Derived d = new Derived();
Many thanks in advance,
Fendy

Hi ,
You cannot access the controller attribute instead create an instance of the controller class and access the attribute in the set method
OR create a static method X in the controller class and store the value in that method. and you can access the attribute by
Call method class=>X
OR if the attribute is static you can access by classname=>attribute.
Regards,
Gangadhar.S
Edited by: gangadhar rao on Mar 10, 2011 6:56 AM

How to access class variables in anonymous class??.

I have a boolean class level variable. Fom a button action , this boolean will set to true and then it used in anonymous class. In anonymous class, I am getting default value instead of true. Could u anyone help in this plzzz.

first of all, you don't want parent because that is something that Containers use to remember their containment hierarchy. you are thinking of super which is also incorrect, because that has to do with inheritance.
the problem here is a scoping problem. you generally would use final if you were accessing variables in an anonymous class that are in the local scope. in this case, you just need to create some test code and play with it. snip the code below and play with it. it shows both the given examples and some additional ways to change/display class variables.
good luck, hackerx
import javax.swing.*;
import java.awt.event.*;
public class Foo extends JPanel
     private boolean the_b = true;
     public static void main(String[] args)
          Foo f = new Foo();
          JFrame frame = new JFrame();
          frame.getContentPane().add(f);
          frame.pack();
          frame.show();
     public Foo()
          // get your button
          JButton b = new JButton("Not!");
          b.addActionListener(new ActionListener()
               public void actionPerformed(ActionEvent ae)
                    // *** uncomment these to play around ***
                    // Foo.this.the_b = false; // this will work, but unnecessary
                    // the_b = false; // this works fine too
                    notFoo();
          this.add(b);
          // something to show the value that accesses a class variable
          // using an inner class instead of final nonsense
          DisplayThread t = new DisplayThread();
          t.start();
     private void notFoo()
          the_b = !the_b;
     class DisplayThread extends Thread
          public void run()
               while(true)
                    System.err.println("boolean value: " + the_b);
                    try {
                    sleep(1000);
                    } catch(InterruptedException ie) {}
}

Singleton class + JTable mouseClicked

Hi,
I have a problem with this topic:
singleton class OR create new class and use one function (not a static func) of this class from else class.
I have singleton class that return JScrollPane with table, in this table I have tasks..
in this class I have: public void mouseClicked(MouseEvent e) function.
when the user clicked --> I create another JTable (table with sub tasks)
and if the user clicked in this table, I create another JTable (table with sub sub tasks).............................
I need your help because: if the class that create a JTable singleton type, then I can not get another mouseClicked of jtable that below the new jtable (I can't get another mouseClicked of sub tasks table if I created the sub sub tasks table...).
But, if I create everytime NEW class that return the scroll JTable --> I get different problem:
I need to use with some functions from scroll JTable class and I don't know how to do it..I can't do: TaskTable.getInstance().getMyFunc();
This function must to be in this class, and not static!
I'm new :) maybe I don't know some else way to get function from regular class.
Sorry about my english.
I'm really need your help :)
Thasks.

..if I create everytime NEW class that return the scroll JTable --> I get different problem:
I need to use with some functions from scroll JTable class and I don't know how to do it..
TaskTable t1 = new TaskTable();
t1.getMyFunc();

(retain) properties in the singleton class

Hi,
What happens with the properties declared with (retain) in the singleton class:
MyClass *foo;
@property (nonatomic, retain) MyClass *foo;
I am using a singleton class as explained http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/CocoaFunda mentals/CocoaObjects/CocoaObjects.html#//apple_ref/doc/uid/TP40002974-CH4-SW32 which should NOT be released.
I see no way to call
[foo release].
My guess is that these properties will be allocated once and persist though entire life of the singelton - which is not a leak.
correct?

leonbrag wrote:
My guess is that these properties will be allocated once and persist though entire life of the singelton - which is not a leak.
correct?
Yes. Of course, there are still ways to get rid of singletons if you really want/had to. In some cases, you could have a singleton that allocated system resources that aren't automatically released when a program ends. There are ways to clean up such things if you run across them. Look at the "applicationWillTerminate:" notification, for example.

How to make Singleton Class secure

I have a Singleton class which I instantiate using reflection by a call to a public getInstance method that return the object from a private constructor. I have to use reflection as I dont have any other options since my class instantiation is decided on runtime. How will I secure other form instantiating the same class again by using reflection on my private constructor? Any ideas?

How much do these different implementations of your singleton have in common? What I'm getting at is, is there some common code you could extract out of them all, and make that a singleton in it's own right? Needing to do this sort of hacking is often a sign you're fudging something together, that could be solved more elegantly. We use the Singleton pattern generally when the state of the object needs to exist exactly once. Can you extract that from the disparate objects?
The singleton classes that I load do not have anything in common. They are all similar but unique, and independant in their own respect. I will have to decide only on what singleton object I have to load in runtime, based on certain criterion. This is done by reflection, by a call to the public getInstance() method of the respective singleton classes, since I am deciding on runtime. My problem is that can I prevent others from accessing my private constructor from outside the class using reflection, which enables them to create another duplicate object. Can I restrict them to use only my getInstance() method instead?? I know this is hacking to access the private members, but I want to know whether there is any way where I can restrict this hacking???In the above code I dont want these statements to work at any case
java.lang.reflect.Constructor[] c = cl.getDeclaredConstructors();
c[0].setAccessible(true);
A anotherA = (A) c[0].newInstance(new Object[]{"Duplicate"});

TestStand create different object for singleton class

Hi all,
we have a singleton class which has some functions used to do testing a harware.
Our main Exe will create an object for that singleton class(which opens Com1 port and communicate with hardware). so the Exe will do basic communication test with hardware it is working well. We are using Teststand operator interface to do various testing by using sequence files. Main exe will use teststand usercontrols to execute tests when the user clicks Testbutton. after that, teststand try to create an object for that singleton but it returns new object not the existing one which is created by EXE. So it throwing me exeception "Com1 port access denied." (since we created object for signleton class @ very first in EXE)
My question is Since that teststand runs in a separate Appdoamin will the singltonclass create separate object for different appdomain? if so is there any solution to reslove this?
Hope i clearly explained my probs.
Thanks in advance
Srini

Hi Srini,
How are you calling the executable? From a Call Executable step? Or are you using another means of calling it? Also, why is TestStand trying to recreate the object? As long as you have the correct handle to the object I don't think it matters what app domain you are in.
Regards,
jigg
CTA, CLA
teststandhelp.com
~Will work for kudos and/or BBQ~

Singleton Class Variable

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