Instance method writes to a static field. This is tricky to get correct
hi,
i am assinig a static refence to an obect, and acess this refrence outside the class.
public static loadingDialog StatusWndw = null;
StatusWndw = new loadingDialog(this);
i want toacess StatusWndw this out side the class but find bug show me generally bad practice.
pls reply.
regards,
lkkj
There are lot of false positives with find bugs so it is up to you to decide whether you want to keep the code.
If there is a good reason to keep the variable StatusWndw as static leave it as it is. In fact most singleton classes need to have static variable that is initialized at some point.
However you might want to make the StatusWndw as private and provide a gettor for it?
Similar Messages
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Instance methods faster than sync. static methods in threaded env?
consider the following please:
(-) i have "lots" of instances of a single Runnable class running concurrently..
(-) each instance uses a common method: "exponential smoothing" and they do a lot of smoothing.
so:
(option #1): include a "smooth()" instance method in the Runnable class.
(option #2): include a "smooth()" synchronized static method in the Runnable class.
(option #3): create a MathUtility class, and have "smooth()" as an instance method in this class.
(option #4): make "smooth()" a synchronized static method in the MathUtility class.
from OOP point of view, i think i should externalize "smooth()" to a MathUtility class, and then make
is "synchronized static".
but then from a performance point of view....
would not it be optimal to make "smooth()" an instance method in MathUtility and then have each
instance of the Runnable create its own MathUtility instance so that each thread has its own copy
of the "smooth()" method??
well, i can't image there would be a measurable difference so maybe i should not post.
but, if there is a flaw in my thinking, please let me know.
thanks.kogose wrote:
from OOP point of view, i think i should externalize "smooth()" to a MathUtility class, and then make
is "synchronized static".From an OOP point of view you should probably have a class that represents the data that provides a (non-static) smooth() method that either modifies the data or returns a new smoothed data object (depending on whether you want your data objects to be immutable or not).
but then from a performance point of view....
would not it be optimal to make "smooth()" an instance method in MathUtility and then have each
instance of the Runnable create its own MathUtility instance so that each thread has its own copy
of the "smooth()" method??No, methods are not "copied" for each instance. That just doesn't happen.
well, i can't image there would be a measurable difference so maybe i should not post.If you don't know, then you should probably try it.
but, if there is a flaw in my thinking, please let me know.The flaw in your thinking is that you can think that you can intuitively grasp the difference in performance of a change at that level.
I have yet to meet anyone who can reliably do that.
Performance optimization is not an intuitive task at that level and you should never do performance optimizations without proving that they are improvements for your particular use case.
First part: Is the smooth() method really thread-unsafe? Does it use some shared state? My guess would be that it uses only local state and therefore doesn't need any synchronization at all. That would also be the fastest alternative, most likely. -
Write to static field from instance method
instance method writes to a static field. This is tricky to get correct if multiple instances are being manipulated, and generally bad practice.
public static loadingDialog StatusWndw = null;
StatusWndw = new loadingDialog(this);
iwant to acces this satic field out side the class.
how to slove this problemMake the LoadingDialog a singleton class. Lookup the singleton pattern.
-
Static field changes for both instances
Whenever I construct a new object with a static field or use a set method on any of the instances of that object, the static field will change to the last modification made. In the case of my program, whenever the static String color field is changed to the last modification made. This only happens with the static fields, the other fields work just fine.
Why does this problem occur?Sorry, I can't understand the question. If you have lots of different objects, all of the same class, they all share one copy of the static field, so if you invoke a method that changes the static field on one instance, all the other instances will see that change.
if you don't want this behaviour, don't make the field static; then each instance will have its own copy of the field. -
Name space conflict between static and instance method
Hello,
there seems to be a very unfortunate name space conflict between static and instance method name. Consider:
static String description() that should return a description of a class.
String description() that should return a description of an instance.
That seems to confuse the compiler. How come? Static and instance methods don't have anything to get confused about.
Thanks for any insights :-)Umm...jeez.
It's not a bug, it's the way it's supposed to be.
Since a static method can be called the same way an instance method
A instance = new A();
A.staticMethod();
instance.staticMethod();it's not allowed.
Also in the class, you can call
public void myMethod() {
instanceMethodInClass(); // You don't need this, it's superfluous
staticMethodInClass(); // You don't need the class name.
}If you didn't understand, then just accept the fact that it is so. Some day you'll understand it. -
Class methods vs. Instance methods
When shall we use class methods instead of instance methods , and vice versa?
By going through the Java Tutorial, it only says the following.
A common use for static methods is to access static fields....
I am definitely sure that class methods aren't just exist for accessing static fields of a class.
PS. I understand that instance methods belong to the object that is instantiated and class methods belong to the class itself.Jay-K wrote:
JoachimSauer wrote:
P.S.: I wouldn't call them "class methods". They are called static methods in Java. It helps to stay with the common, widely-used terminology.Thanks, its good to know what the norms are.
but i wonder why the Java Tutorial states it clearly "Class Methods" over here [http://java.sun.com/docs/books/tutorial/java/javaOO/classvars.html|http://java.sun.com/docs/books/tutorial/java/javaOO/classvars.html]
Because strictly speaking that's what they are. They belong to classes, rather than instances of classes. The world-at-large, though, has found that term to be a bit ambiguous, especially when discussing fields - it's not uncommon to see instance variables referred to as "class variables" by newbies - so it's kind-of self-corrected. -
This static method cannot hide the instance method from...
What means the error message "This static method cannot hide the instance method from Referee"?
Referee.java is a interface. I implemented it in the class RefereeMyName.java and made a method in that class to be static. Thats why I received that error message.
But can't I use static methods when I have implemented a interface in that class? I want to have a Player.java class which I want to access RefereeMyName.getTarget(). I cannot use a instance instead because I wouldn't receive a valid return value then (RefereeMyName is the client class).
What is the solution?Hi,
Well i do not think that you can do that b'cos that way you are not giving the same signature for the method as that exists in the interface. I do not know how other way it can be done but if something urgent for you then you can remove that method from that interface and only define in the class.
Regards,
Shishank -
Can one combine static and instance methods?
Hi,
Can one define a method so that is can be used as both a static or an instance method?
Basically I'm trying to simplify my class to so that I can call a method either statically with parameters or instantiated using it's own attributes.
In other words, I'm trying to accomplish both of these with one method:
zcl_myclass=>do_something_static( im_key = '1234' ).
lv_myclass_instance->do_something( ). " key in private attributes
Why would I want to do that?
I would like to avoid instantiation in some cases for performance reasons and would like to keep it simple by having only one method to do basically the same thing.
Any input or alternative suggestions welcome.
Cheers,
MikeOk, I may be reaching here a bit, I know, but maybe this may give you some ideas. After creating the object, pass it back to the method call.
report zrich_0001.
* CLASS lcl_app DEFINITION
class lcl_app definition.
public section.
data: a_attribute type string.
class-methods: do_something importing im_str type string
im_ref type ref to lcl_app optional.
endclass.
* CLASS lcl_app IMPLEMENTATION
class lcl_app implementation.
method do_something.
if not im_ref is initial.
im_ref->a_attribute = im_str.
write:/ 'Do_Something - ', im_ref->a_attribute.
else.
write:/ 'Do_Something - ', im_str.
endif.
endmethod.
endclass.
data: o_app type ref to lcl_app.
start-of-selection.
create object o_app.
call method o_app->do_something(
exporting
im_str = 'Instansiated'
im_ref = o_app ).
call method lcl_app=>do_something(
exporting
im_str = 'Static' ).
Regards,
Rich Heilman -
Purpose of declaring the method or class or static and as instance
what is the purpose of declaring a method in a class or the class itself as static.
Does it mean we cannot create a copy of that class or method if we declare it as static.
if so then why do they dont want that class to be created as a copy ?
Why do they want to declare a class as static
please provide some conceptual undersatnding regarding the static and instance class with one exampleStatic methods are often used for the implementation of utility methods. Please have a look at the class CL_ABAP_CHAR_UTILITIES for example.
You use the methods of this class in the same way as you would use a function in ABAP (like
LINES( itab )
). You use it in a static way because the functionality is always the same no matter in what context you are calling the function.
The purpose of instance methods is that their logic is in some way related to an attribute of the object instance that you use to call it.
For example, you create an instance of object PO (a purchase order) called MY_PO. Then the method
MY_PO->ADD_POSITION
would add a position to a concrete PO that has a unique number etc. But if the object has a static method DELETE_POSITION then it just deletes the current position of a PO, regardless on which concrete PO you are acting at the moment.
I hope this clarifies it for you.
Regards,
Mark -
Should I use a static method or an instance method?
Just a simple function to look at a HashMap and tell how many non-null entries.
This will be common code that will run on a multi-threaded weblogic app server and potentially serve many apps running at once.
Does this have to be an instance method? Or is it perfectly fine to use a static method?
public static int countNonNullEntries(HashMap hm){
if(hm==null)return 0;
int count=0;
for(int i=0; i<hm.size(); i++) {
if(hm.get(i)!=null)
{ count++;}
return count;
OR
public int countNonNullEntries(HashMap hm){
if(hm==null)return 0;
int count=0;
for(int i=0; i<hm.size(); i++) {
if(hm.get(i)!=null)
{ count++;}
return count;
}TPD Opitz-Consulting com wrote:
The question is the other way around: Is there a good reason to make the method static?
Ususally the answer is: no.The question is: does this method need state? Yes -> method of a class with that state. No -> static.
The good thing of having this method statig is that it meight decrese memory foot pring but unless you're facing memory related problem you should not think about that.I doubt there is any difference between the memory foot print of a static or not method.
I'm not shure if this method beeing static maks problems in multithreaded environments like the one you're aiming at. But I do know that an immutable object is always thread save.Does the method use shared state (data)? No -> no multi threaded problems.
Can the parameters be modified by different threads? Yes, if multiple threads modified the parameter map, but nothing you can do about it here (no way to force the calling thread to lock on whatever you lock on).
So my answer to your question is: yes, it should be non static.The method should be static since it uses no state.
It is thread-safe when only the calling thread can modify the passed map (using a synchronized or ConcurrentHashMap is not enough, since you don't call a single atomic method on the map).
// Better use Map instead of HashMap
// We don't care about the generic type, but that does not mean we should use a raw type
public static int countNonNullEntries(Map<?, ?> map) {
// whether to accept null map or not, no need for explicit exception
// since next statement throw NPE anyway
Collection<?> values = map.values();
// note your method is called countNonNull, not countNull!
// (original code it would need to by if(null != mapValue) notNullsCounter++;
return values.size() - java.util.Collections.frequency(values, null);
} -
Hi,
I am a self-taught Java programmer and have been writing Java code over the past 18 months. I am comfortable with general coding, but I am having great difficulty deciding whether a method should be static or instance.
In many cases, I decide to create a class which has maybe 1 or 2 methods. A good example of this is the class I wrote which allows me to pass it a JFrame and it then centres the JFrame on screen. Seems to me I can do 1 of 2 things.
1. Put the code in the class constructor and then call it with:
CentreMe cm = new CentreMe(this);
where CentreMe is the name of the class and this refers to the JFrame that is being passed
2. Put the code in a static method within the class and then call it with:
CentreMe.CenterFrame(this);
Assuming that all the CentreMe class does is to centre a frame in this way, which is the correct (or at least best) approach. Seems to me there is no point in creating objects as per option 1 when they are never used therafter?
If anyone has advice or a URL that deals with this particular "design" issue I would be very grateful.
Thanks
MarkGiven that design, then your intuition is correct, that method should be static. No need for objects there. But on a higher level, the design is backwards. Instead of having a separate class that knows how to centre a JFrame, it would be better to create a subclass of JFrame that knows how to centre itself. Have a look at Bruce Eckel's online book "Thinking in Java" ( http://www.bruceeckel.com ); it has a lot of useful information about the design of objects.
-
Interfaces: static variables but instance methods, why?
Why do variables need to be static in interfaces but the methods in them are not allowed to be static? Or, asked differently: Why methods must not be static while variables have to be static in interfaces?
I don't understand the reason for that difference.An interface defines functionality but can not be instantiated as an object. So for the static variables, it can define variables that can be accessed by all implementing classes, but there can only be one definition of them, hence static. Non-static variables fall in the realm of implementation, which is not what interfaces are for.
Static methods are used to define an implementation that is common among all instances of an object, and must be implemented when defined. Since interfaces can not provide any implementation, static methods are invalid. Remember that static methods are commonly used without instantiating an object (like java.lang.Math methods) so the implementation must be provided when defined. -
Does main method have to be static? Is there a way to go around it?
I am trying to write a class that will recursively walk through directory listing of a given ftp adress and store it in a text file in this format:
<path> , DIR // if directory
<path> , file // if file
Since I am not yet experienced to do my own socket programming(tried and did not work out) I used Secure iNet Factory's library. And since I will be running multiple instances of this class I concluded that my fields and methods should not be static. Here is the class I wrote:
import com.jscape.inet.ftp.Ftp;
import com.jscape.inet.ftp.FtpException;
import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.net.URL;
import java.util.Enumeration;
import java.util.logging.Level;
import java.util.logging.Logger;
public class test {
private String path = "/";
private String name;
private String adres;
// create new Ftp instance and connect.
public test(String Name, String Adres) throws IOException {
name = Name;
adres = Adres;
System.out.println("Test class: " + name);
try {
Ftp ftp = new Ftp(adres,"anonymous","anonymous");
ftp.connect();
Enumeration e = ftp.getDirListing();
iterate(path, ftp);
ftp.disconnect();
catch (FtpException ex) {
Logger.getLogger(test.class.getName()).log(Level.SEVERE, null, ex);
void iterate(String aStartingDir, Ftp ftp) throws FtpException, IOException {
//Set the path.
path = path + aStartingDir + "/";
// Change directory.
ftp.setDir(aStartingDir);
// Iterate through directory list.
Enumeration e = ftp.getDirListing();
while(e.hasMoreElements()){
Object f = e.nextElement();
// Recursive call if item is directory.
if(f.toString().startsWith("drwxrwxrwx") &&
// Ignore "." and ".."
!f.toString().substring(55).trim().equalsIgnoreCase(".") &&
!f.toString().substring(55).trim().equalsIgnoreCase("..")) {
// Write to index file.
URL dirUrl = FtpCheck.class.getResource("./index/"); // get the directory.
URL fileUrl = new URL(dirUrl, name + ".txt"); // construct the file path.
String filePath = fileUrl.getPath().replaceAll("%20", " "); // fix spaces.
BufferedWriter file = new BufferedWriter(new FileWriter(filePath,true));
file.write((path + f.toString().substring(55).trim()).substring(3)
+ ",DIR\n");
file.close();
iterate(f.toString().substring(55).trim(),ftp);
// Skip entries "." and ".."
else if (f.toString().substring(55).trim().equalsIgnoreCase(".") ||
f.toString().substring(55).trim().equalsIgnoreCase("..")) {
continue;
// Add files to the index.
else {
URL dirUrl = FtpCheck.class.getResource("./index/"); // get the directory.
URL fileUrl = new URL(dirUrl, "index.txt"); // construct the file path.
String filePath = fileUrl.getPath().replaceAll("%20", " "); // fix spaces.
BufferedWriter file = new BufferedWriter(new FileWriter(filePath,true));
file.write((path + f.toString().substring(55).trim()).substring(3)
+ ",file\n");
file.close();
// End of listing reached, go one directory up.
ftp.setDirUp();
// And remove the last directory name from path.
path = path.substring(0,path.lastIndexOf("/"));
path = path.substring(0,path.lastIndexOf("/")) + "/";
}which compiles fine.
Here is the class that invokes it:
import java.io.FileNotFoundException;
import java.io.IOException;
import java.net.MalformedURLException;
import java.util.logging.Level;
import java.util.logging.Logger;
* @author Sentinel
public class NewClass {
public NewClass() {
public static void main(String args[]) throws MalformedURLException, FileNotFoundException, IOException {
index t = new index("El Naga","elnaga.sytes.net");
t.start();
index t1 = new index("Afacan","afacan.myftp.org");
t1.start();
public class index extends Thread {
String name;
String adres;
public index(String Name,String Adres) {
name = Name;
adres = Adres;
@Override
public void run() {
try {
test checker = new test(name,adres);
catch (IOException ex) {
Logger.getLogger(NewClass.class.getName()).log(Level.SEVERE, null, ex);
}when I hit run build fails:
init:
deps-jar:
Compiling 1 source file to C:\Documents and Settings\Sentinel\My Documents\NetBeansProjects\ftpCheck\build\classes
C:\Documents and Settings\Sentinel\My Documents\NetBeansProjects\ftpCheck\src\NewClass.java:19: non-static variable this cannot be referenced from a static context
index t = new index("El Naga","elnaga.sytes.net");
^
C:\Documents and Settings\Sentinel\My Documents\NetBeansProjects\ftpCheck\src\NewClass.java:20: non-static variable this cannot be referenced from a static context
index t1 = new index("Afacan","afacan.myftp.org");
^
2 errors
BUILD FAILED (total time: 0 seconds)I (now)know that main method has to be static. I'm stuck and in need of help to overcome this obstacle. Is there a way to do this?a stab in the dark, but what if you make index a non-internal class. make it a stand-alone class in it's own file. Either that or make it a static inner class (almost the same thing).
Your problem is that an inner class needs an instance of the outer class to start on, and you're not doing this. Another solution would be to do this kludge:
index t = new NewClass().new index("El Naga","elnaga.sytes.net"); // I think this is how to call itBut again better is to make index a stand-alone class.
Edited by: Encephalopathic on Oct 28, 2008 8:06 AM -
Compiler thinks getDeclaredFields method is in a static context
Hi folks,
I have the following lines of code at the beginning of a non-static method ;
public void streamOutput(Object objectToStream, boolean autoFlush)
Class objectClass = objectToStream.getClass();
Field[] fieldArray = Class.getDeclaredFields();
When using javac to compile this code I get the following error ;
Can't make static reference to method java.la
ng.reflect.Field getDeclaredFields()[] in class java.lang.Class.
Field[] fieldArray = Class.getDeclaredFields();
^
The thing is that I can't see how javac thinks that the method is being called within a static context. The method is non-static, the containing class in non-static, and I even have a reference to 'this' later on inside the method which the compiler accepts ( which it surely wouldn't if I was working within a static context ).
Any ideas what is wrong ?The line "Class.getDeclaredFields();" is written as if the method getDeclaredFields() is a static method. It isn't. You have to use an instance of some class. Maybe what you need is objectClass.getClass().getDeclaredFields();
-
Using Objects to Initialize Static Fields: Good or Bad
I have a Command interface and a bunch of Commands that implement it (i.e. CmdJoin,CmdQuit). These Commands' settings change at runtime (i.e. required access levels, String identifiers). I don't want to have to store an instance of each Command in a database to save their settings , so instead I'm using the obvious solution: making static fields in these commands for these settings. This way, I can use a Simple Factory to return a Command, change its settings, execute it, and forget it, and still have the settings for that Command apply to all Commands. Yet I want to be able to modify and access fields of different Commands polymorphically. How can I have these commands' settings-related fields be static while modifying and accessing these fields polymorphically?
Here's what I have though of. First of all, interfaces can't have static methods. Secondly, neither can abstract classes. I also can't extend a base class which implements these settings-related fields and their interface, because then the fields would belong to all child classes of this base class, whereas I just want it to belong to a certain child class of the base class (i.e. all instances of CmdJoin or CmdQuit).
I've thought of two solutions.
The first is implementing a concrete interface in an abstract base class (getting rid of the Command interface) and overriding it in child classes, so that I can use the interface of the base class and the fields of the child classes.
The second is having no base class, and just a bunch of Commands implementing the interface with their own static fields. I would initialize these fields by passing arguments to their constructors.
These solutions seem very sloppy though! Are there any better ways?To clarify, I want all objects of type A to be able to respond to a static method declared in type A yet still remember their implementation of this static method. I provided two solutions that I have thought of, and I find them sloppy, so I'm asking if there's a better way.
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