Static class reference

Below we have a class containing a static class variable and a static accessor method.
The method is responsible for assigning the variable, which it does on demand.
public class MyClass
 private static int INFO = -1;
 public static int getInfo()
 if(INFO < 0)
 try
 INFO = ...;
 catch(Exception e)
 return INFO;
 }When using this class is it not safe to assume that the assignment of the variable will ever only occur once for the life of the system that uses this class? (afterall is it static within the class)
I am experiencing the situation whereby EVERY time the method is called, the assignment of the variable is necessary. I can only assume that the static instance of this class in memory has been lost between each call and therefore the state/value of the variable has also been lost.
It is worth noting that no object instances of this class are ever created. The class is accessed only through its static interface. It seems that since no local reference to objects of this class are held in memory then the entire class, along with its internal static state, is cleared from memory (garbage collected?).
How is it possible to ensure that the state of static fields within such a class is preserved over time, without create instances of the class.
John

When using this class is it not safe to assume that
the assignment of the variable will ever only occur
once for the life of the system that uses this class?First of all, you haven't said what you assign it to, so we can't say. If you assign a value less than zero then it will be reassigned next time the method is called.
Secondly, it depends on whether or not you're expecting it to be thread-safe? It isn't.
If your code is not multi-threaded, then the section of code in your getInfo method that assigns the field should only execute once during a single run of an application.
Thirdly, you explicitly assign it (to minus one) at class initialisation time, so stricty speaking, the variale will always be assigned at least once, and will be assigned at least once more if the getInfo method is ever called.
I am experiencing the situation whereby EVERY time the
method is called, the assignment of the variable is
necessary. Can you offer any evidence to this? A small, complete, compilable and executable example that demonstrates the problem will help, and will probably get you an answer pretty quickly around here.
I can only assume that the static instance
of this class in memory has been lost between each
call and therefore the state/value of the variable has
also been lost.Then either there is a bug in your JVM, or at least two versions of the class have been loaded, and at least one of those was not loaded by the system classloader. Without further info, we aren't going to be able to help much more here.
The class is accessed only
through its static interface. It seems that since no
local reference to objects of this class are held in
memory then the entire class, along with its internal
static state, is cleared from memory (garbage
collected?).This is not allowed under the JLS if the class was loaded by the system classloader. If it was loaded by another classloader, then it is only allowed if it can be determined that the class will never again be used throughout the entire run of the application. Since you are referencing the class again, this should not be happening.
How is it possible to ensure that the state of static
fields within such a class is preserved over time,
without create instances of the class.From what you have told us so far, it should be. You will probably have to provide some code for us to help you further.

Similar Messages

Static class garbage collection

Can garbage collector can garbage collect static classes ?.
My doubt is that when you access a static class , that class is loaded through its class loader ( when first time that class is referenced ).
So when did the garbage collector collects this static class ( assume that static class no longer referred ).
Please do more information,
What are the ways to prevent garbage collector for a particular class ( Assume that i m implementing a singleton for my java runtime)
thanks and regards
Renjith.

Can garbage collector can garbage collect static classes ?. Static classes are nothing special in terms of class loading - they are only different in visibility for linking.
Perhaps you mean static members of classes?
My doubt is that when you access a static class ,
that class is loaded through its class loader ( when
first time that class is referenced ). Classes are always loaded through classloaders. Objects of those classes are allocated from the heap, and the object instances refer to the class object.
Objects (either instances, or classes themselves) are garbage collected when they are no longer live (i.e. no live object refers to them). (This is a somewhat recursive definition, and sometimes, you can have cyclic dependencies that make garbage collection tricky, but the GC, while it has to be conservative for correctness, usually gets it right).
So for a static member to be garbage-collected, the class has to be garbage-collected first. The class cannot be GC'ed until all references to it go away (this includes all dynamically allocated objects of that type, and the class loader that loaded that class).
And yes, class loaders can go away, but only if they are created by your program. The system class loader (which is the default classloader you get if you don't create any class loaders of your own) never goes away, so any class loaded from CLASSPATH will never be unloaded.
(As an example, servlet containers - e.g. Tomcat, Weblogic, etc.) allocate one or more classloaders for each webapp. When the webapp is un-deployed, the classloaders are "orphaned", and they, and any classes loaded by them (from the WEB-INF/lib and WEB-INF/classes directories) are unloaded and garbage-collected. (After all the dynamic objects of those classes are GC'ed, of course).
>
So when did the garbage collector collects this
static class ( assume that static class no longer
referred ).
Please do more information,
What are the ways to prevent garbage collector for a
particular class ( Assume that i m implementing a
singleton for my java runtime)
thanks and regards
Renjith.

Static Class Function vs. Instance Variables

I'm making a Wheel class to spin the wheels on some toy
trains as they move back and forth.
Each wheel on the train is an instance of the Wheel class and
there are several of them.
I thought it would be great to just have a static class
function to tell all the Wheels to start turning:
Wheel.go();
The Wheel class keeps a static array of all of its instances
so I thought I would just loop through all of those instances and
issue the wheelInstance.roll() method.
So far it all works. But I was planning to use a setInterval
to call the roll() method and each instance has its own rollID
property that I would like to assign the setInterval ID to. Here is
the problem.
Since the rollID is an instance property I can't access them
from a static class function. Is there any way to do this?
Currently I"m just using an onEnterFrame which doesn't require me
to use the instance properties.

Technically yes, realistically for this class no.A
class will probably take several hundred bytes at
least to load, with no data of your own. So adding4
bytes for a int is less than 1% of the total size.
And if you are loading millions of differentclasses
then you should rethink your design.If you don't instantiate the class when you reference
a static variable why would you consume memory for the
class other than the variable itself? I don't
understand what you are talking about with the
"millions of different classes", it's not germane to
the question. Bottom line, referencing a static
variable more than once will save memory.Using a class, static or by instance, requires that the class be loaded. A loaded class creates, at the very least, an instance of java.lang.Class. Any static members of the class are in addition to the storage space needed for the instance of java.lang.Class and for any internal storage needed by the JVM in addition to that.
Thus if one has a static data member when the class is used in any way, the static data member takes storage space. However a member (non-static) does not take storage space.
Of course the meta data for the member could take as much space as the static member so the point could be moot. Is that what you were referring to?

Static class to change TextField

Hi,
I'm trying to make a static class that will change the text written in a TextField inside a SWF file.
In this case I have a MovieClip with a TextField inside with a message. I want to be able to make a class that giving this TextField and a message, that will change in the button.
Certainly that I'm forgetting something obvious, I get the correct trace messages, but the textfield disappears. The initial text is "Start" and I want to change that to "OK". For now I just want to change the text, but after I want to be able to change the text format.
So far I have this:
package
          import flash.display.MovieClip;
          public class ChangeTextField extends MovieClip
                    public function ChangeTextField(_textField:Object, _text:String):void {
                    public static function change(_textField:TextField, _text:String):void {
                         trace("CHANGETEXTFIELD " + _textField); //Return "Start"
                         _textField.text = _text;
In another class I've loaded the swf and it's content is inside a MovieClip so that I can reference it. To change the TextField text I would do something like:
ChangeTextField.change(okButton.normal.label_tx, "OK");
trace(okButton.normal.label_tx.text); //Returns "OK"
By the trace messages everything is doing what it's supposed to do, but the TextField disappears.
Any idea?
Cheers.

That was simple
Thanks!

Are private fields public in a static class?

If a variable in a static class is declared as private, it still seems to be visible. Are access specifiers without any importance in a static class?

Roxxor wrote:
tschodt wrote:
The outer class can access all the fields of the nested class, yes.Yes, but only if the nested class is static, otherwise not.
This is what I have noticed:
If the nested class is not static, then the outer class has to instantiate the nested class to access its variables. Well, not really. If the nested class is not static (i.e., it's a nested class), then its variables don't even exist until an instance is created.
If the nested class is not static, then the nested class can access the outer class´ variables without instantiate it. The inner class can only exist if there exists an instance of the outer class. An inner class (a non-static nested class) contains a reference to the wrapping class. So, it doesn't have to instantiate the outer class, because an instance already exists.
If the nested class is static, then the nested class has to instantiate the outer class to access its variables. I guess. It seems like a really strange design though.

Wire: Function Conflict, Static VI Reference

Hello Community,
Sorry if this has already been answered before, but I have spent an hour searching. I am trying to learn a little about using subVIs to clean up my code.
I'm trying to connect a static VI reference of a Gaussian function that I made to the Nonlinear Curve Fit.vi, but I get a broken wire which says "function conflict".
When I create constants for the output of my Gaussian VI and the input of the Nonlinear Curve Fit.vi, they are clearly different classes.
Could someone show me how to edit my Gaussian VI to match the class properly? I'd greatly appreciate it.
Thanks,
-Patrick
Solved!
Go to Solution.
Attachments:
pat_gaussian_fit.vi ‏24 KB
pat_gaussian_fit.vi ‏24 KB

Your model subvi needs a connector pattern that is identical to the template. Start with the template mentioned in the help and only modify the code, not the existing controls and indicators. Simply leave unused terminals disconnect, don't delete them.
LabVIEW Champion . Do more with less code and in less time .

What is static class ?

I know about static function.
What is static class?
What is the use of static class?
how can i use static class.?
Give me a static class example?

Static class and singleton are similar but they are not the same. A singleton is instantiated, and you have a reference to it - just one single reference no matter how many times you instantiate it. Singletons are good for data storage, config settings, loggers things that you may need all around your application, but you always want the same one.
A static class is not instantiated - an example of this is Flash's Math class. You can just do Math.sin() and get a result, you don't instantiate it in order to use it. Static classes are good for utility classes... essentially they are simply a namespace for a group of methods.
Oh, and you said you know about static function - what is static class - a static class then is just one that contains only static functions/methods.

About static class

Dose Java has Static class?
What is diff between static class and a final class which all method are static in java?

Dose Java has Static class?yes, but only if the class is an inner class
a static inner class does not have a reference to its enclosing class - because of this, a static inner class cannot use this to access any instance variable of the enclosing class
however, a static inner may access all static variables of the enclosing class
What is diff between static class and a final class
which all method are static in java?a static inner class is linke any other class in java
a final class with static methods is like the java.lang.Math class - you cnnot subclass it and do normally not instantiate it
hope that helps
Harry

Benefit of static classes

Hello
What is the benefit of static classes? As I know, static classes can be only used as an inner class. But why should I use them?
Thanks in advance..

What is the benefit of static classes? As I know,
static classes can be only used as an inner class.
But why should I use them?A static class won't keep a reference to the outer class.
Kaj

How to use application class reference in the controller methods of BSP

Hi,
I have created a bsp application and also created an application class and assigned it to the application class. In the application class, I have created attribute TEXT type string(public and instance parameter).
In the controller let's say main.do, I am trying to give a value to to the text by adding the following code.
application->text = 'test'.
I am getting syntax error saying field 'text' is unknown. It is not contained in one of the specified tables nor defined by DATA statement.
Please can someone let me know how to use the application class in the coding with an example. I couldn't find how exactly this has to be reference. Please help.
Best regards
Siva

Hi,
if you are having main controller and sub-controller then you may need to use below coding to use application class reference.
*Data declaration
DATA: obj_cntrl        TYPE REF TO cl_bsp_controller2,
         obj_sub_cntrl   TYPE REF TO z_cl_sub_cntl,
         application TYPE REF TO z_cl_application.
*Get the controller
CALL METHOD obj_main_cntrl->get_controller   "obj_main_cntrl is the object of main controller
    EXPORTING
      controller_id       = 'SUB'   "Controller ID
    RECEIVING
      controller_instance = obj_cntrl .
obj_sub_cntrl ?= obj_cntrl .
application ?= obj_sub_cntrl ->application.
or simply use below code in your controller method.
application ?= me->application.
Thnaks,
Chandra

Purpose of static classes

What is the purpose of making a class static?
public static class December {
}The only reason I can see is to access inner-classes without instantiating the outer-class? Are there any other?

see these
http://forum.java.sun.com/thread.jsp?forum=31&thread=277820
http://forum.java.sun.com/thread.jsp?forum=31&thread=305260

Creation of a static class with private methods

I'm new to java programming and am working on a project where I need to have a static class that does a postage calculation that must contain 2 private methods, one for first class and one for priority mail. I can't seem to figure out how to get the weight into the class to do the calculations or how to call the two private methods so that when one of my other classes calls on this class, it retrieves the correct postage. I've got all my other classes working correct and retrieving the information required. I need to use the weight from another class and return a "double". Help!!!
Here's my code:
* Title: Order Control 
* Description: Order Control Calculator using methods and classes
* Copyright: Copyright (c) 2002
* Company: Info 250, sec 001, T/TH 0930
* @author Peggy Blake
* @version 1.0, 10/29/02
import javax.swing.*;
public class ShippingCalculator
static double firstClass, priorityMail;
//how do I get my weight from another class into this method to use??? not sure I understand how it works.
public static double ShippingCalculator(double weight)
String responseFirstClass;
double quantity, shippingCost;
double totalFirstClass, firstClass, priorityMail, totalShipping;
double priorityMail1 = 3.50d;//prioritymail fee up to 1 pound
double priorityMail2 = 3.95d;//prioritymail fee up to 2 pounds
double priorityMail3 = 5.20d;//prioritymail fee up to 3 pounds
double priorityMail4 = 6.45d;//prioritymail fee up to 4 pounds
double priorityMail5 = 7.70d;//prioritymail fee up to 5 pounds
quantity = 0d;//ititialization of quantity
// weight = 0d;//initialization of weight
// shippingCost = 0d;
//calculation of the number of items ordered..each item weights .75 ounces
quantity = (weight/.75);
if (quantity <= 30d)
//add 1 ounce to quantities that weigh less than 30 ounces
weight = (weight + 1);
else
//add 2 ounces to quantities that weigh more than 30 ounces
weight = (weight + 2);
if (weight > 80d)
//message to orderclerk ..order over 5 lbs, cannot process
JOptionPane.showMessageDialog(null, "Order exceeded 5 lbs, cannot process");
//exit system, do not process anything else
System.exit (0);
else
if (weight < 14d)
//send message to customer: ship firstclass or priority, y or n
responseFirstClass = JOptionPane.showInputDialog(null, "Ship first class? y or n?");
if (responseFirstClass.equals("y"))
//compute FirstClass shipping cost
totalFirstClass = ((weight - 1) * .23d) + .34d;
firstClass = totalFirstClass;
else
//compute PriorityMail cost for orders less than 14 ounces
priorityMail = (priorityMail1);
else
if (weight <=16d)
//compute totalshipping for orders up to 16 ounces
priorityMail = (priorityMail1);
else
if (weight <=32d)
//compute totalshipping for orders up to 32 ounces
priorityMail = (priorityMail2);
else
if (weight <=48d)
//compute totalshipping for orders up to 48 ounces
priorityMail = (priorityMail3);
else
if (weight <= 64d)
//compute totalshipping for orders up to 64 ounces
priorityMail = (priorityMail4);
else
//compute totalshipping for orders up to 80 ounces
priorityMail = (priorityMail5);
priorityMail = 0d;
firstClass = 0d;
firstClassMail ();
priorityMailCost ();
//I think this is where I should be pulling the two methods below into my code, but can't figure out how to do it.
shippingCost = priorityMail + firstClass;
return (shippingCost);
}//end method calculate shipping
private static double firstClassMail()//method to get first class ship cost
return (firstClass);
}//end method firstclass shipping
private static double priorityMailCost()//method to get priority mail cost
return (priorityMail);
}//end method priorityMail
}//end class shipping calculator

public class A {
public String getXXX () {
public class B {
A a = new A();
public void init () {
a.getXXX();
}

Alternative to Static Class Inherit

Greetings,
I’ve been using some code for years in a different number of applications but that involves a lot of copy paste and is a nightmare to maintain. For that reason I’ve took some time to re-organize some of my codes and that mainly involves
splitting it into multiple projects for latter inclusion in a number of solutions.
So far so good and I’d say 95% is done but I’m now stuck with a Static Class I need to use. I basically need a few “Global” parameters and settings to be available across every solution. I know I can’t Inherit a Static Class nor override
a Field to return a different value unfortunately.
In a nutshell:
myFrameworkProject
namespace
myFramework.Classes{
 //these contains general stuff
used in every project
 public static class Globals{
 public static
XPTO veryImportantProperty = new XPTO()
 public static
Settings Settings = new Settings();
public void doSomethingImportant();
Public class Settings{
 Private
string Username;
 Private
string Password;
 Private
string Whatever;
myFancyProject
namespace
myFancyProject.Classes{
 //these add specific stuff used
in this project
 public static class Globals : myFramework.Classes.
Globals {
 public static
override mySettings Settings = new mySettings();
Public class mySettings: myFramework.Classes .Settings{
 //some
more specific settings I need
 Private
string SomethingElse;
What are my options? I’ve been trying to play with Singletons but for some reason I just can’t seem to make it work the way I want to…probably because it’s just not the right way to do it in the first place.

I think you are over thinking it a little bit. The example you have above would be great if you had multiple setting types you wanted to retrieve within the same application domain instance using keys or specific types. The way I understand your problem
is you have one settings type per application that you run with various applications sharing a code base. Here is an example I created for you which better illustrates the solution I had in mind when I read your problem.
There are 2 namespaces, the one for your custom project(s) named FancyProject and then the Core / framework. Hope this helps!
namespace JF.FancyProject
using JF.Framework.Classes;
// in initialization of your default application domain (ie. in your application startup)
// inject your MySingleton with your custom implementation of ISettingsBuilder
// this can be done in many ways from hard coding in each startup routine to IoC / Dependency Injection
// the end result is this
public static class Console
public static void Main(params string[] args)
MySingleton.Initialize(new ConcreteSettingsBuilder());
// now you can use it where ever
var temp = MySingleton.SettingsConcreteInstance<ConcreteSettings>().SomethingCustom;
// or base
var temp1 = MySingleton.SettingsInstance.SomeBaseThing;
public sealed class ConcreteSettings : JF.Framework.Classes.SettingsBase
public string SomethingCustom { get; set; }
public sealed class ConcreteSettingsBuilder : BaseSettingsBuilder, ISettingsBuilder
public override SettingsBase CreateSettings()
var settings = new ConcreteSettings();
// call the base if you need to get standard settings populated
base.populateBaseSettings(settings);
// populate all of your custom settings
return settings;
namespace JF.Framework.Classes
public abstract class SettingsBase
public string SomeBaseThing { get; set; }
// base settings and behavior that can be abstracted
public static class MySingleton
private static SettingsBase _settings;
private static Lazy<SettingsBase> _lazyCreationMethod;
public static void Initialize(ISettingsBuilder builder)
_lazyCreationMethod = new Lazy<SettingsBase>(builder.CreateSettings);
public static SettingsBase SettingsInstance
get { return _lazyCreationMethod.Value; }
public static T SettingsConcreteInstance<T>() where T : SettingsBase
return (T) _lazyCreationMethod.Value;
public abstract class BaseSettingsBuilder : ISettingsBuilder
public abstract SettingsBase CreateSettings();
protected virtual void populateBaseSettings(SettingsBase settings)
// if you find many of your settings are created the same way use a base class
public interface ISettingsBuilder
SettingsBase CreateSettings();
Edit:
One more thought. I do agree with some of the other posters that for bigger projects the use of a Singleton pattern is not a good idea. For little applications it does not matter too much because chances are there is not enough substance that it will
hinder your code and the development cycles are generally very short.
Should you want to reconsider the Singleton pattern then I recommend you look at something like
Autofac, you could use this to inject instances of your ISettingsBuilder, or other concrete instances, directly into your dependent classes and you could take it further in developing self containing services that you could
inject as well.
Again, for small projects its not a big deal but if you ever start on something a bit more complex its worth looking into.
-Igor

How to access a property in a static vi reference

Hi,
I'm completely new to Labview and I'm working on a project that includes controlling CANalyzer from LabView by an ActiveX control.
In CANalyzer I have a CAPL function that I need to call from LV. The problem is that the function must be created in the onInit event when starting a canalyzer measurement. The only way I have found to create the function is to use a "reg event call back" and create the function in a "static vi reference". This works and I can call the funtion from that vi, but I'm not able to call the function from anywhere else but from the "static vi reference" and since it must be strictly typed I can not add any output connector where I can add a reference.
I've been trying to solve this for thee days now and it's starting to feel hope less.
In VB the complete code would look something like:
    Dim WithEvents gCANApp As CANalyzer.Application
    Dim WithEvents gMeasurement As CANalyzer.Measurement
    Dim gSendFunction As CANalyzer.CAPLFunction
    Dim Y
    Set gCANApp = New CANalyzer.Application
    gCANApp.Open ("C:\...xxxx.cfg")
    Set gMeasurement = gCANApp.Measurement
    gMeasurement.Start
    gSendFunction.Call
   'Measurement OnInit event handler
   Private Sub gMeasurement_OnInit()
       Set gSendFunction = gCANApp.CAPL.GetFunction("MyFunction")
   End Sub
This would work fine since I have a reference to gSendFunction in my main code.
My LV code looks like this:
and my static vi ref looks like this:
What I think I need is a reference to the data thread coming from "GetFunction"
If any one can help me solve this I would be very happy!
Best Regards
Henrik

Hi,
you could add another input to your static vi ref where you input the reference to a reference display element which sits in your other vi (upper image).
Then you could make a signaling value change to the reference display element from within your static vi ref.
In your other vi set up an event structure to detect the value change. In that event close the vi - this will let you have the desired reference as output of that vi.
Regards Florian

Static Class JMS Listener

I am designing an EJB component (MessageDrivenBean) that requires
dynamic ("hot") configuration changes. As such, the bean must be
notified somehow of configuration changes in the DB (rather than have
to poll the DB for changes at every method call).
I'm thinking of implementing this functionality using a static class
instance (singleton) that is shared between the multiple instances of
the bean. This static instance would listen on a JMS topic for
configuration change events and update its internal cache accordingly
(by getting the configuration information from the DB). The beans
would then access this in-memory static instance everytime they need
configuration info instead of polling the database. By using the JMS
topic, I guarantee that clustered instances of Weblogic will all be
notified of the database changes (each instance would have its own
"cache" singleton).
I'm pretty sure all of this will work, but I'm equally sure that I'm
violating some EJB rules. Singletons and JMS listeners are generally
discouraged. Are there any alternatives to this design?

Hi Joost,
This all seems to be pretty complicated. Not to mention that you can not
guarantee that members of a cluster will be notified simultaneously.
Why don't you want using the database?
"Joost Ouwerkerk" <[email protected]> wrote in message
news:[email protected]..
I am designing an EJB component (MessageDrivenBean) that requires
dynamic ("hot") configuration changes. As such, the bean must be
notified somehow of configuration changes in the DB (rather than have
to poll the DB for changes at every method call).
I'm thinking of implementing this functionality using a static class
instance (singleton) that is shared between the multiple instances of
the bean. This static instance would listen on a JMS topic for
configuration change events and update its internal cache accordingly
(by getting the configuration information from the DB). The beans
would then access this in-memory static instance everytime they need
configuration info instead of polling the database. By using the JMS
topic, I guarantee that clustered instances of Weblogic will all be
notified of the database changes (each instance would have its own
"cache" singleton).
I'm pretty sure all of this will work, but I'm equally sure that I'm
violating some EJB rules. Singletons and JMS listeners are generally
discouraged. Are there any alternatives to this design?Regards,
Slava Imeshev

Static class reference

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