Static variable and non-static

I have a quick question. The following syntax compiles.
private String t = key;
private static String key = "key";
But this doesn't compile.
private String t = key;
private String key = "key";
Can anybody explain how it is treated in java compiler?
Thanks.

jverd wrote:
doremifasollatido wrote:
I understand that completely. I didn't say that the OP's version with static didn't work. I was just giving an alternative to show that you don't need static, if you change the order that the instance variables are declared.My problem with the underlined is that, while technically true, I can see where a newbie would take it as "oh, so that's how I can get rid of static," and focus only on how to satisfy the compiler, rather than on learning a) when it's appropriate to make something static or not from a design perspective, and b) what the implications of that decision are, for both static and non.
I have just a wee bit of a prejudice against the "what do I type to make the error messages go away" approach. :-)That sounds good to me. We're currently trying to fix an issue caused by the fact that one class has most of its variables as static, when they should really be instance variables. There should only be one active instance of this class at a time, but the values of the static variables stick around when you call "new ThisClass()". So, old data interferes with new data, and various errors happen. The class has many static methods to access the static variables, but it should have been more of a Singleton--an actual object with non-static variables. The active instance of the Singleton would need to be replaced at logout/login (as opposed to shutdown completely and restart the JVM), but then at least when the new instance were created, then all of the variables would be initialized to empty--the old instance would disappear. The solution at the moment (solution by others--I don't agree) is to clear each static Vector and then set the reference to null (setting to null would be enough to drop the old data, but certain people don't get that...). This is fragile because many variables were missed in this "cleanup" process, and it will be easy to forget to "cleanup" new variables in the future. This class has been misdesigned for years (much longer than I've been here). The calls to static methods permeate the application, so it isn't easy to fix it quickly. There are several of these misdesigned classes in our application--a mix of static and non-static, when nothing or almost nothing should be static.

Similar Messages

Static and non-static variables and methods

Hi all,
There's an excellent thread that outlines very clearly the differences between static and non-static:
http://forum.java.sun.com/thread.jsp?forum=54&thread=374018
But I have to admit, that it still hasn't helped me solve my problem. There's obviously something I haven't yet grasped and if anyone could make it clear to me I would be most grateful.
Bascially, I've got a servlet that instatiates a message system (ie starts it running), or, according to the action passed to it from the form, stops the message system, queries its status (ie finds out if its actually running or not) and, from time to time, writes the message system's progress to the browser.
My skeleton code then looks like this:
public class IMS extends HttpServlet
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
        doPost(request, response);
   public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
      //get the various parameters...
         if (user.equalsIgnoreCase(username) && pass.equalsIgnoreCase(password))
            if(action.equalsIgnoreCase("start"))
                try
                    IMSRequest imsRequest = new IMSRequest();
                    imsRequest.startIMS(response);
                catch(IOException ex)
                catch(ClassNotFoundException ex)
            else if(action.equalsIgnoreCase("stop"))
                try
                    StopIMS stopIMS = new StopIMS();
                    stopIMS.stop(response);
                catch(IOException ex)
             else if(action.equalsIgnoreCase("status"))
                try
                    ViewStatus status = new ViewStatus();
                    status.view(response);
                catch(IOException ex)
         else
            response.sendRedirect ("/IMS/wrongPassword.html");
public class IMSRequest
//a whole load of other variables
public PrintWriter    out;
    public int                 messageNumber;
    public int                 n;
    public boolean         status = false; //surely this is a static variable?
    public String            messageData = ""; // and perhaps this too?
    public IMSRequest()
    public void startIMS(HttpServletResponse response) throws IOException, ClassNotFoundException
        try
            response.setContentType("text/html");
            out = response.getWriter();
            for(n = 1 ; ; n++ )
                getMessageInstance();
                File file = new File("/Users/damian/Desktop/Test/stop_IMS");
                if (n == 1 && file.exists())
                    file.delete();
                else if (file.exists())
                    throw new ServletException();
                try
                    databaseConnect();
               catch (ClassNotFoundException e)
//here I start to get compile problems, saying I can't access non-static methods from inside a static method
               out.println(FrontPage.displayHeader());
                out.println("</BODY>\n</HTML>");
                out.close();
                Thread.sleep(1000);
        catch (Exception e)
    }OK, so, specifially, my problem is this:
Do I assume that when I instantiate the object imsRequest thus;
IMSRequest imsRequest = new IMSRequest();
imsRequest.startIMS(response); I am no longer in a static method? That's what I thought. But the problem is that, in the class, IMSRequest I start to get compile problems saying that I can't access non-static variables from a static method, and so on and so on.
I know I can cheat by changing these to static variables, but there are some specific variables that just shouldn't be static. It seems that something has escaped me. Can anyone point out what it is?
Many thanks for your time and I will gladly post more code/explain my problem in more detail, if it helps you to explain it to me.
Damian

Can I just ask you one more question though?Okay, but I warn you: it's 1:00 a.m., I've been doing almost nothing but Java for about 18 hours, and I don't do servlets, so don't take any of this as gospel.
If, however, from another class (FrontPage for
example), I call ((new.IMSRequest().writeHTML) or
something like that, then I'm creating a new instance
of IMSRequest (right?)That's what new does, yes.
and therefore I am never going
to see the information I need from my original
IMSRequest instance. Am I right on this?I don't know. That's up to you. What do you do with the existing IMS request when you create the new FrontPage? Is there another reference to it somewhere? I don't know enough about your design or the goal of your software to really answer.
On the other hand, IMSRequest is designed to run
continuously (prehaps for hours), so I don't really
want to just print out a continuous stream of stuff to
the browser. How can I though, every so often, call
the status of this instance of this servlet?One possibility is to pass the existing IMSRequest to the FrontPage and have it use that one, rather than creating its own. Or is that not what you're asking? Again, I don't have enough details (or maybe just not enough functioning brain cells) to see how it all fits together.
One thing that puzzles me here: It seems to me that FP uses IMSReq, but IMSReq also uses FP. Is that the case? Those two way dependencies can make things ugly in a hurry, and are often a sign of bad design. It may be perfectly valid for what you're doing, but you may want to look at it closely and see if there's a better way.

Non-static variable from a static context

This is the error i get . If i understand the error correctly it says im using a static variable when i shouldnt be? Or is it the other way round? below the error is the actual code....
The error...
Googler.java:27: non-static variable this cannot be referenced from a static context
          submitButton.addActionListener(new ButtonHandler());The code...
          JButton submitButton = new JButton("Submit Query");
          submitButton.addActionListener(new ButtonHandler());

thanks for the response.
I have already tried what you said but I tried it again anyway and i get the same error more less...
Googler.java:28: non-static variable this cannot be referenced from a static context
          ButtonHandler buttonHandler = new ButtonHandler();here is part of my code
public class Googler
static JTextField input1, input2;
     public static void main(String[] args)
          JFrame myFrame = new JFrame("Googler v1.0");
          Container c = myFrame.getContentPane();
          JLabel lab1 = new JLabel("Enter Google Query:");
          JLabel lab2 = new JLabel("Enter Unique API Key:");
          input1 = new JTextField(15);
          input2 = new JTextField(15);
          JRadioButton radSearch = new JRadioButton("Search Query");
          JRadioButton radCached = new JRadioButton("Cached Query");
          JButton submitButton = new JButton("Submit Query");
          ButtonHandler buttonHandler = new ButtonHandler();
          submitButton.addActionListener(buttonHandler);
          ButtonGroup group = new ButtonGroup();
          group.add(radSearch);
          group.add(radCached);Ive tried declaring buttonHandler as a static variable and this dosn't work either. I've never had this problem before it must be something silly im missing...?
Thanks
Lee

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.

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();

Static variables and JAR files problem

Hello All,
I am trying to get two applets to communicate.
Both are held in the same JAR file loaded from the same location. The JAR file also contains a class with some static variables which are used to comunicate between the two applets.
The applets are on separate frames in the browser and the system works fine when I don't use a JAR file but when I put the class files into a JAR file, each applet has it's own static variables (not very static).
Can anyone tell me why static variables stop working when the code is loaded from a JAR file.
Thanks in advance,
Alastair.

Ok,
I've just tried the below setup and I get the following security exception:
java.security.AccessControlException: access denied (java.util.PropertyPermission java.home read)
at java.security.AccessControlContext.checkPermission(Unknown Source)
at java.security.AccessController.checkPermission(Unknown Source)
at java.lang.SecurityManager.checkPermission(Unknown Source)
at java.lang.SecurityManager.checkPropertyAccess(Unknown Source)
at java.lang.System.getProperty(Unknown Source)
at sun.plugin.security.PluginClassLoader.getPermissions(Unknown Source)
at java.security.SecureClassLoader.getProtectionDomain(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at sun.applet.AppletClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.applet.AppletClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
at appSend.init(appSend.java:62)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
I now have one JAR file which contains the two classes.
appSend.class & appReceive.class
and a separate details.class file
The two HTML files are in separate folders but the JAR file is still the same for both.
Classes\ -> contains the JAR file (myCode.JAR) and details.class
Sender\ -> contains the HTML file appSender.html (see below for
Applet tag)
Receiver\ -> contains the HTML file appReceiver.html (see below for
Applet tag)
appSender.html
<HTML>
<HEAD>
</HEAD>
<BODY BGCOLOR="000000">
<CENTER>
<APPLET
name = "appSender"
code = "appSend.class"
archive = "myCode.JAR"
codebase = "../Classes"
width = "500"
height = "300"
>
</APPLET>
</CENTER>
</BODY>
</HTML>
appReceiver.html
<HTML>
<HEAD>
</HEAD>
<BODY BGCOLOR="000000">
<CENTER>
<APPLET
name = "appReceiver"
code = "appReceive.class"
archive = "myCode.JAR"
codebase = "../Classes"
width = "500"
height = "300"
>
</APPLET>
</CENTER>
</BODY>
</HTML>
Is this how it should be? or have I made a mistake with the tags?
Can the ClassLoaders in either applet see the details.class file through the codebase tag?
I'm guessing from the security exception that the ClassLoader for each applet can only load classes from within the JAR file? and can't just use the codebase tag, which is not very helpful!
Thanks for your help so far.
Regards,
Alastair.

How to reference a static variable before the static initializer runs

I'm anything but new to Java. Nevertheless, one discovers something new ever' once n a while. (At least I think so; correct me if I'm wrong in this.)
I've long thought it impossible to reference a static variable on a class without the class' static initializer running first. But I seem to have discovered a way:
public class Foo {
public static final SumClass fooVar; // by default initialized to null
static {
fooVar = new SumClass();
public class Bar {
public static final SumClass barVar;
static {
barVar = Foo.fooVar; // <<<--- set to null !
}Warning: Speculation ahead.
Normally the initial reference to Foo would cause Foo's class object to instantiate, initializing Foo's static variables, then running static{}. But apparently a static initializer cannot be triggered from within another static initializer. Can anyone confirm?
How to fix/avoid: Obviously, one could avoid use of the static initializer. The illustration doesn't call for it.
public class Foo {
public static final SumClass fooVar = new SumClass(); // either this ..
public class Bar {
public static final SumClass barVar = Foo.fooVar; // .. or this would prevent the problem
}But there are times when you need to use it.
So what's an elegant way to avoid the problem?

DMF. wrote:
jschell wrote:
But there are times when you need to use it. I seriously doubt that.
I would suppose that if one did "need" to use it it would only be once in ones entire professional career.Try an initializer that requires several statements. Josh Bloch illustrates one in an early chapter of Effective Java, IIRC.
Another classic usage is for Singletons. You can make one look like a Monostate and avoid the annoying instance() invocation. Sure, it's not the only way, but it's a good one.
What? You only encounter those once in a career? We must have very different careers. ;)
So what's an elegant way to avoid the problem? Redesign. Not because it is elegant but rather to correct the error in the design.<pff> You have no idea what my design looks like; I just drew you a couple of stick figures.If it's dependent on such things as when a static initializer runs, it's poor. That's avoidable. Mentioning a case where such a dependency is used, that's irrelevant. It can be avoided. I know this is the point where you come up with a series of unfortunate coincidences that somehow dictate that you must use such a thing, but the very fact that you're pondering the problem with the design is a design problem. By definition.
Besides, since what I was supposing to be a problem wasn't a problem, your "solution" isn't a solution. Is it?Well, you did ask the exact question "So what's an elegant way to avoid the problem?". If you didn't want it answered, you should have said so. I'm wondering if there could be any answer to that question that wouldn't cause you to respond in such a snippy manner. Your design is supposedly problematic, as evidenced by your question. I fail to see why the answer "re-design" is unacceptable. Maybe "change the way the Java runtime initializes classes" would have been better?
This thread is bizarre. Why ask a question to which the only sane answer, you have already ruled out?

Difference between calling static method and not static method?

Hi,
Suppose i want to write a util method, and many class might call this method. what is better? writing the method as static method or not static method. what is the difference. what is the advantace in what case?

writing the method as static method or not static
method. what is the difference.The difference is the one between static and non-static. Any tutorial or the JLs will clarify the difference, no need to repeat it here.
what is the advantace in what case?It's usually not like you have much of a choice. If you need access to an instance's attributes, you can't make it static. Otherwise, make it static.

Non Static Variable addressed to Static Variable

Hi,
I am new to java, I am getting (Non Static Variable sb,serverAddress addressed to Static Variable)
Here is the code. Thanks for reading, any help or explanation would be appreciated-
* To change this template, choose Tools | Templates
* and open the template in the editor.
package webcheck;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.ProtocolException;
import java.net.URL;
* @author
public class checkhttp {
      private URL serverAddress = null;
      private StringBuilder sb = null;
      public checkhttp(java.net.URL serverAddress,java.lang.StringBuilder StringBuilder)
          this.serverAddress=serverAddress;
          this.sb=sb;
public static void main(String[] args) {
      HttpURLConnection connection = null;
      OutputStreamWriter wr = null;
      BufferedReader rd = null;
      String line = null;
      int x;
      //checkhttp check= new checkhttp();
      try {
          serverAddress = new URL("http://www.yahoo.com");
          //set up out communications stuff
          connection = null;
          //Set up the initial connection
          connection = (HttpURLConnection)serverAddress.openConnection();
          connection.setRequestMethod("GET");
          connection.setDoOutput(true);
          connection.setReadTimeout(10000);
          connection.connect();
          //get the output stream writer and write the output to the server
          //wr = new OutputStreamWriter(connection.getOutputStream());
          //wr.write("");
          //wr.flush();
          //read the result from the server
          rd = new BufferedReader(new InputStreamReader(connection.getInputStream()));
          sb = new StringBuilder();
          while ((line = rd.readLine()) != null)
              sb.append(line + '\n');
          System.out.println(sb.toString());
          System.out.println("Server is up");
      } catch (MalformedURLException e) {
          e.printStackTrace();
      } catch (ProtocolException e) {
          e.printStackTrace();
      } catch (IOException e) {
          e.printStackTrace();
      finally
          //close the connection, set all objects to null
          connection.disconnect();
          //rd = null;
          //sb = null;
          //wr = null;
          connection = null;
}

Someone please correct me if I'm wrong, but since main is static, any fields it access must also be static.
The static keyword declares that something is accessable no matter the class state, thus you can call main() and run your program without having something make an instance of the object that your progam defines. For example:
class Foo {
      static public String strText = "Hello World";
//Later in some method, this is valid.
String MyString = Foo.strText;
//However, if strText was not static you need to
Foo fooExample = new Foo();
String myString = fooExample.strText;Static should not override private, so static private fields/members are not accessable. To be accessable you still need to be public.
Edited by: porpoisepower on Jan 21, 2010 2:18 PM

Best Practice: Using a static variables and methods vs singleton pattern

Just curious, since when anything, within a class, is denoted as static it typically will be stored in main memory as a class property(method, variables, etc). Is it a good practice to be doing this all the time or is the singleton pattern a better choice. Please explain why. Thanks.

I wouldnot make anything other than the constants and the instance itself static. And I cant think of a reason wjy one would.

Help with static and non static references

Hi all,
I'm new to Java I'm having some difficulties with accessing non static methods.
I'm working with Tree data structures and all the interfaces and methods are non-static.
I have to write a tester main method to create and modify trees but how do I access the non static fields from main which is static?
Any help would be appreciated
Thanks.

Create an Instance for that particular class then call the method using that instance.

(JC) static variable and derived object

Hi there!
It is glad to know you from Java Card Forum. Can I ask for your help on the following question?
It is about static variable. The following is my sample code:
========================================
package com.Test01;
import javacard.framework.*;
import javacard.security.*;
import javacardx.crypto.Cipher;
public class Test01 extends Applet {
OwnerPIN Pin;
static DESKey[] keys;
protected Test01(byte[] buffer, short offset, byte length) {
keys = new DESKey[4];
length = 0;
while (length < 4) {
keys[length] = (DESKey)KeyBuilder.buildKey((byte)3, (short)0x80, false);
length = (byte)(length + 1);
public static void install(byte buffer[], short offset, byte length) {
new Test01(buffer, offset, length);
===========================================================
If there are two instances, A and B, created in the package.
My issues:
1. Are keys[0]~ keys [3] kept while B is deleted?
2. Does each instance have itsown object while "keys[length] = (DESKey)KeyBuilder.buildKey((byte)3, (short)0x80, false);"? or they share the same object?
3. follow item 2, if A and B share the same object, is the object kept while B is deleted? Where can I get the information in Sun's spec?
Thank you very much.
Best regards,
kantie

Thanks a lot. You mean that keys variable will be removed while instance B deleted, right? I think the idea of database applet is very good, but I can't force applet provider the way of their implementations : )
I still got question, does it get no use to set the keys variable to be "static"? So that I can keep it to other instances, if applied.
And I think that any object derived under the static variable shall be kept, until the package is deleted.
For example, if you declare a static pointer, and it points to an object newed by the instance at the first time. We say every instances (A and B) of this package (PckM) share this same static pointer and this same object, right? There are two situations:
1. if this referred object is removed when B is deleted, so the memory of this object will be released. Then user might create an instace C of another package (PckN), and instance C new its objects just overlapping on the released memory. In this case, it will cause instance A to be crashed, because its static pointer has been referred to illeagle address.
2. if this referred object is kept when B is deleted, then instance C will new its objects in other free memory. In this case, instance A will work well, because its static pointer still refers to correct object.
What do you think? Am I missing any concepts?
Thank you for your great opinions.
Best regards,
kantie

"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.

Problems with Static and non-static stuff

Right now I am trying to work on a RPG and trying to make it all using classes instead of having a Applet run everything and want to call Methods to make / remove the buttons here is the Method I'm dealing with
    public void placeButtons()
        attackBtn.setBounds(200,700,75,25);
        defendBtn.setBounds(200,725,75,25);
        magicBtn.setBounds(275,700,75,25);
        itemBtn.setBounds(275,725,75,25);
        game.add(attackBtn);   // Here is where I am having the problem (non-static method add(java.awt.Component) cannot be referenced from a static context)
        attackBtn.addActionListener(this);
    }Normally I would be making all my buttons in my main class but I want to know if I can make them in a separate class and load them in my main class so I can shorten my code by a ton.

Generally speaking: Yes: you are only limited by your ingenuity and knowledge.
//call from other class
MyButtonClass mbc = new MyButtonClass();
MyPanel p = new MyPanel();
p.add(mbc.getMyMainButton());
//what everAlthough I would think that it may be better to get your entire Frame back for display or a panel... It just seems like it's going to be awkward to work with unless you're making some kind of button generator or something, but then it's your app and from your description, the code given should illustrate a way of getting to the answer you seek.

Abstract method versus static and non-static methods

For my own curiosity, what is an abstract method as opposed to static or non-static method?
Thanks

>
Following this logic, is this why the "public static
void main" 0r "Main" method always has to be used
before can application can be run: because it belongs
to the class (class file)?
Yes! Obviously, when Java starts up, there are no instances around, so the initial method has to be a static (i.e. class) one. The name main comes from Java's close association with C.
RObin

Static variable and non-static

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