When should I use static variable and when should not? Java essential
When should I use static variable and when should not? Java essential
Static => same value for all instances of the class.
Non-static => each instance can have its own value.
Which you need in which circumstances is completely up to you.
Similar Messages
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"requires unreachable" warning emitted when using static variables and the singleton pattern
I'm implementing code contracts on an existing code base and I've come across this situation in a few places. Wherever we have additional logic with code contracts where a singleton instance is instantiated, all code contracts emit a "reference use
unreached", or "requires unreachable" warning.
The code below demonstrates this.
static string instance = null;
static string InstanceGetter()
if (instance == null)
instance = "initialized";
AdditionalLogic(instance); // requires unreachable warning on this line
return instance;
static void AdditionalLogic(string str)
Contract.Requires(str != null);
Console.WriteLine(str);Would the class get unloaded even if the Singleton object has some valid/reachable references pointing to it?
On a different note, will all the objects/instances of a class get destroyed if the class gets unloaded.That's the other way round, really: instances cannot be garbage-collected as long as they are strongly reachable (at least one reference pointing to them), and classes cannot be unloaded as long as such an instance exists.
To support tschodt's point, see the JVM specifications: http://java.sun.com/docs/books/jvms/second_edition/html/Concepts.doc.html#32202
*2.17.8 Unloading of Classes and Interfaces*
A class or interface may be unloaded if and only if its class loader is unreachable. The bootstrap class loader is always reachable; as a result, system classes may never be unloaded.
Will the same case (Custom ClassLoader getting unloaded) occur if I write the singleton using the code wherein we lazily initialize the Singleton (and actually create a Singleton instance rather than it being a Class/static variable).You should check the meaning of this vocabulary ("object", "instance", "reference"): http://download.oracle.com/javase/tutorial/java/javaOO/summaryclasses.html
The difference between the lazy vs eager initializations is the time when an instance is created. Once it is created, it will not be garbage collected as long as it is reachable.
Now I'm wondering, whether being referenced by a static attribute of a class guarantees being reachabe. I'm afraid not (see the JLS definition of reachable: http://java.sun.com/docs/books/jls/third_edition/html/execution.html#44762). That is, again, unless the class has been loaded by the system class loader. -
Is is better to use static variables?
Hi,
Does anyone know if it's better to use static variables or to use normal variables?
Concerning the size of the code, it seems that declaring a variable as static is more consuming (for example plus 6 bytes for an object reference).
So this could mean that declaring variables as static should be avoided, but what about the execution time?
Some years ago, some javacard gurus were claiming that it's was better to use static variables (less processing required by the JVM to resolve adresses of static variables), but is it still the case?Hi Lexdabear,
Thanks for the answer.
I did the test (I converted my all code to use static variables and methods as much as possible), and did a bench before and after, on a JCOP31 card.
The conclusion is that today JVMs and processors are much powerful than 5 years ago, and that the difference is really difficult to measure, which anyway is a good thing for us ;-) -
Using Static Variable against Context Attribute for Holding IWDView
Dear Friends,
I have a method which is in another DC which has a parameter of the type IWDView. In my view, I will have an action which will call the method in another component by passing the value for the view parameter. Here, I can achieve this in 2 types. One is - I declare a static variable and assign the wdDoModifyView's view as parameter value and I can pass this variable as parameter whenever calling that method or the second way - create an attribute and assign the same wdDoModifyView's view parameter as its value. Whenever I call this method, I can pass this attribute as parameter. What is the difference between these two types of holding the value since I am storing the same value i.e., wdDoModifyView's view parameter. But when I trigger the action from different user sessions, the first type of code (using static variable) prints the same value in both the sessions for view.hashCode() and View.toString(), but the same is printing the different values when I pass the attribute which holds the view parameter.
Clarification on this is highly appreciated
The problem I face is when I use static variable to get the view instance and export the data using the UI element's id, the data belonging to different user sessions is mixed up where as when I use Context Attribute, the same problem doesn't arise. I want to know the reason why it is so. Is there any other place or way where I can get the current view instance of each session instead of wdDoModifyView?Hi Sujai ,
As you have specified the problem that we face when we use static attributes, when end users are using the application .
Static means i have n number of objects but the static variable value will remain same every where.
when it is context attribute for every object i.e nth object you have a nth context attribute i mean nth copy of the context attribute.
so every user has a unique Iview parameter , when context is used and
when static is used , assume you have userA , his iview is set this intially and u have another user B , when he is using , since the variable is static and when you access this variable you will get the value of userA.
Regards
Govardan Raj -
Can I use static variable in EJB?
Many books suggest developer don't use static variable in EJB,I want to know why?
I know there isn't any problem if the static varibale is read only
For writable static varible ,what will happen if I use a static Hashtable for share data
Help me!Thank you very much!!Greetings,
I know that "EJB business methods are not allowed to
block on synchronized resources" Just where do you "know" that from?? The EJB 2.0 Specification, at least, is nowhere in agrement with this statement. If it comes from a book I would question the author's reasoning. Contractually, there's no sound basis for this. In the case of Session Beans, they have an expressedly direct and single-threaded association with a client. From a design viewpoint, it certainly seems unnecessary for a bean to "block" its one-and-only client, but to say that it "is not allowed to" do so is without merit. Entity Beans, on the other hand, are concurrently accessible. Yet, with regard to a transactional context being in effect, the container does indeed block on a bean's business methods. For the bean to do so itself is, therefore, unnecessary. Furthermore, the specification explicitly concedes that a "Bean Provider is typically an application domain expert" and "is not required to be an expert at system-level programming." (EJB 2.0 Spec. 3.1.1) From these statements alone it is reasonable to assume the above statement is meritless since the Bean Provider is not expected to even consider synchronization issues when developing a bean.
But I'm mixed up why we could use "Hashtable" or otherApparently, because your sources are as well...
collection classes in the EJB ,in these method many
methods are synchronized In fact, not only "can we use" them but, with respect to multiple-row finders in Entity Beans we are [i]required to use them (or an iteration of them)! Not all Collection classes are synchronized (so called "heavy-weight collections"). As shown above, that the choice of a particular Collection class might be synchronized is of little consequence since a bean designed under strict adherence to the specification ensures that it is never concurrently writeable.
Could someone provide a good way for this problem?
Please Help Me!!!Regards,
Tony "Vee Schade" Cook -
Using Shared Variables and Initialize Front Panel Binding (to PSP)
Hi,
I use LV DSC RT 8.2.1
I have a Vacuum System That includes signals from - Pumps failure, Valves status, Vacuum gauge, start Pump ...
Each signal is read by a FieldPoint.
All the relevant FieldPoint Channels are read by the Server (a computer in the Ethernet Network) and published to the network in the form of Shared Variables.
I have a client VI that is reading the Shared Variables published by the Server using Front Panel Binding.
Problem :
Some of the Bindings are in the mode 'Write &Read' and that causes some initialization problems.
For example - Valve #1 is Open, and then a User start running the Client VI, (the Valve #1 Status mode is 'Write & Read")
if in the VI the status of Valve #1 is closed (before running it) then the Valve status is changing to Closed.
I want the Client VI to first read the Physical status of the instrument and then to change the Value if the User changes it.
But that's seems to be a problem when using Front Panel Binding... (is it?)
I know I can Deploy a lvlib in the Client Side and Item Bind to the Shared Variables or Use DataSocket.
(Is DataSocket is a Reliable method when connecting to Shared Variables? What are the disadvantages when using DataSocket?)
What is recommended by those of you that are experienced or by NI ?
Sincerely Yours,
Amitai Abramson.Amitai Abramson,
Hello and thanks for using the NI Forums.
I'm glad that you've read the Using the LabVIEW Shared Variable Tutorial on our website. Check out these other resources:
Network Variable Technical Overview
Troubleshooting Network-Published Shared Variables
Why Do I See Unexpected Value Change Events for Shared Variables Using LabVIEW DSC?
All You Need to Know About Shared Variables
Creating a Value Change Event for Shared Variables
Alternative Method for Using Shared Variables Between Systems in LabVIEW 8.x
What Is The Difference Between Using Shared Variables And DataSocket VIs To Access OPC Tags?
The issue that you are seeing by having "Write & Read" bound items on both the server and client side is essentially a race condition, you don't know which one is being read/written at what time. To resolve this issue I would take a look at some of the documents below.
Using a Local, Global, or Shared Variable in Parallel Loops Can Cause Race Conditions
Using Local and Global Variables Carefully
Tutorial: Local Variable, Global Variable, and Race Conditions
Locking a Shared Resource or Variable in LabVIEW Using Semaphores
You mentioned not wanting to have two sets of shared variables (one on each side), but this is a great method to resovle this issue, that, or you can develope some sort of hand shaking to prevent these race conditions.
I would suggest that in the future when using these forums you try to ask only one question per thread and make it more concise. It's hard to tackle multiple questions and such broad questions as "I want to know all the ways that I can connect to Shared Variables, and I want to know the advantages and disadvantages." I suggest this because we want you to get your questions answered and more concise questions will result in quicker and better answers.
Message Edited by Ben S on 10-01-2009 06:05 PM
Ben Sisney
FlexRIO V&V Engineer
National Instruments -
A basic question about static variables and methods
What would be the effects if I declare lots of static variables and static methods in my class?
Would my program run faster?
Would the class take more memory spaces? (If so, how do I estimate an appropriate number of variabls and methods to be declared static?)
Thank you @_@when you declare a static var the var isn't created for every instance of the class, it just "live" in the class
when you declare a static method is the same, so if you have:
class MyClass
static int myMethod()
//Method work
you dont need to have a instance of the class to call myMethod, you can do such things like this.
int value = Myclass.myMethod();
if myMethod isn't static you can't do this.. so, if
class MyClass
int myMethod()
//Method work
you CAN'T call
int value = MyClass.myMethod();
instead, you have to write
MyClass m;
m = new MyClass();
value = m.myMethod(); -
The recipient cant see "middle of reply or typing" bubble in imessage. Could it be that I am using iOS 8 and they are not? Has any of your friends pose this as a minor problem? Is there a setting that I didn't enable?
''rojere [[#question-1055991|said]]''
<blockquote>
i try to keep my thumb palms off the mouse pad when i am typing but every now and then my screen resizes larger or smaller until i take my left palm off the keyboard base its only when my left palm is resting on the base next to the touch pad. i type pretty fast so it gets annoying and yes it also jumps screens or reverts back to previous message and i have to choose ctrl z to undo and get back to my message i was typing... it also switches tabs on me as well... not sure why my palm being on the left of the keypad on the base should effect anything but it does after a while. and i have to stop what i am doing...and go to the firefox menu and change the screen size back down to 100% sometimes it goes up to 200-300 other times it goes down to 70-80% i find it annoying i i wish there was a way to turn off the screen resize in the setup panel. if its just left to be 100% i am happy with that... i don't need it to be bigger or smaller... there should be a way to LOCK it down... i understand that there are people out there with eye problems and needs to have the screen size increase using the + or - but there should be a lock so it just stays no matter what is happening on my keypad or touch pad or base of the pad... that screen size will not increase or decrease. unless i turn off the lock... perhaps this is something they can implement in the next version or do a quick update of the browser
</blockquote>
sorry that is all greek to me thanks for attempting to explain what you were trying to explain sorry i just not tech person to be able to understand or where to begin ... have good day and thanks for again trying -
my document.numbers can not be opened how can i recover the version from yesterday? i was not using time machine and it was not in icloud. is there an app i can buy that might help
Thanks Andy,
I wish I could have told my sister about this as she could not open my homepage back before my new iWeb site. She now can open my site without the yet in case denial window does appear = I thank you.
I think my iWeb site works as Noone has said otherwise from any platform etc...
Alex L
I do agree that should not have to be a consideration for any users so i agree that it would be great if Apple could allow simplicity to run seemlessly.
I have had this problem since Homepage, and here is
what has worked for me. I send out the link, and
then for PC users I send the link with "<" after the address (without quotation
marks. e.g., <http://site.com>).
I also put a note that if that doesn't work, to just
copy and paste the link into a browser. With that I
have only had 1 person say they still can't open.
Everyone else gets it to work.
It's a lot to do and seems like a hassle, but you get
used to it.
Hope this helps!
Andy Martini
PS - APPLE! When are you going to fix this??! It
seems like such a small thing yet you guys have been
stumped for YEARS on this! -
My question has not be answers yet,I am using iPad 3 and I can not get to connect to iTunes Store to download anything.
The iPad Screen is almost square, HD TVs are rectangular so will never be full screen. When watching movies, the movies native aspect ratio will be used rather than the iPads screen aspect ratio so will look full screen if the movie is full screen, but most likely will be seen in wide screen letter box format. with black bars across the top and bottom as that what most movies from iTunes are in, format wise
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Hello, I'm using Mountain Lion and mail does not leave the mails on the server, either on iCloud nor on other suppliers (orange). They are both using iMap protocol... Any place where I could change this ? Thanks for any hall; Christian
Under what circumstances does mail "not leave the mails on the server"?
If you mean that you delete them (or move them out of the inbox) on one device and then find them not in the inbox on the server when you look from another device, that's how IMAP is supposed to work. (Note that the deletion or moving might be the result of rules you have set up in Mail.)
If you mean something else, please explain your issue in more detail. -
I am using laptop hitting and slow system not working
i am using laptop hitting and slow system not working and very late opan file browser every think . plez help me
Hi,
Shut down the notebook. Tap away at the esc key as you start the notebook to enter the Start-up Menu. Select the Bios option ( usually f10 ) and under the Advanced or Diagnostic tab you should find the facility to run tests on both the Hard Drive and Memory. Post back with the details of any error messages.
Note: If the option to run these tests is not available in the Bios Menu, use the f2 diagnostic menu instead.
Can you also post back with the following details.
1. The full Model No. and Product No. of your notebook - see Here for a guide on locating this information.
2. The full version of the operating system you are using ( ie Windows 7 64bit ).
Regards,
DP-K
****Click the White thumb to say thanks****
****Please mark Accept As Solution if it solves your problem****
****I don't work for HP****
Microsoft MVP - Windows Experience -
How to use line wrapping and line spacing in java?how to use line wrapping
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Hello,
You should post your question at:
http://java.forum.sun.com
Thanks
- Rose -
I recently purchased a used ibook g4 and i want to update to itunes 10.2. It tells me that I need to update to mac os 10.5. Today is the fist time that I have ever used a mac and I am not very well versed on computers to begin with. From what I gather I need to purchase the mac os 10.5 but there are a lot of different options and I dont know what I need to do. Can somebody point me in the right direction? I dont want to spend too much money because i just want my ipod touch to be compatible with my ibook. thank you
Hi
Your question is more suitable for the 10.4's General Forum rather than 10.4 OSX Server. Post here instead:
https://discussions.apple.com/community/mac_os/mac_os_x_v10.4_tiger?categoryID=1 77
You'll have more of a chance of someone responding.
HTH?
Tony -
I just downloaded a few ITunes Apps to my PC using windows 8 and they will not work. Suggestions? Thank you.
The apps can only be run on an iOS device. They will not run on your PC.
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