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Java Forum / General / November 2007Why can't i declare a static variable in a static method?
for example, class Test { public static void main(String[] args) { double radius = 5; static double PI = 3.15169; double area = radius * radius * PI; System.out.println("Area is " + area); } } I can't figure out why it is wrong thanks > Why can't i declare a static variable in a static method? A local variable exists (or rather before eligible for GC) only for the scope of the method, so the concept of static is not relevant. -- mike w >> Why can't i declare a static variable in a static method? > > A local variable exists (or rather before eligible for GC) only for > the scope of the method, so the concept of static is not relevant. There's nothing wrong with the idea of a static local variable, which would be a static variable whose scope was restricted to the block (hence method) in which it is declared. (Naturally an initialiser for such a variable would not be allowed to refer to non-static locals.) It's just that Java doesn't have them: I have no idea why.  SignatureChris "perhaps method calls generate friction => sparks?" Dollin Hewlett-Packard Limited registered office: Cain Road, Bracknell, registered no: 690597 England Berks RG12 1HN ZelluX schrieb: > for example, > class Test { [quoted text clipped - 8 lines] > I can't figure out why it is wrong > thanks java does not have this concept of a variable that doesn't change during calls.. what you can do is declare the variable outside the method static.. (static final is java's version of a constant) like class Test { private static final double PI = Math.PI ; public static void main(String[] args) { double radius = 5; double area = radius * radius * PI; System.out.println("Area is " + area); } } > ZelluX schrieb: >> for example, [quoted text clipped - 12 lines] > java does not have this concept of a variable that doesn't change during > calls.. You can declare locals `final` -- doesn't that count? SignatureChris "then ZZ1, ZZ2 ZZ3" Dollin Hewlett-Packard Limited registered office: Cain Road, Bracknell, registered no: 690597 England Berks RG12 1HN Chris Dollin schrieb: >> ZelluX schrieb: >>> for example, [quoted text clipped - 13 lines] > > You can declare locals `final` -- doesn't that count? nope ... doesn`t count ... the next call to the method the local will have the value to which it is initialized to in the current call.. not the value to which it was initialized to with the first call. Signaturejava only has global variables for this.. there are no global variable with scope to one method.. > Chris Dollin schrieb: >> [quoted text clipped - 15 lines] >> >> You can declare locals `final` -- doesn't that count? > nope ... doesn`t count ... the next call to the method the local will > have the value to which it is initialized to in the current call.. > not the value to which it was initialized to with the first call. Indeed, but it fits your "variable that doesn't change during calls" description, I thought. I don't see that it's anything to do with /changes/ to the variable. Java just doesn't allow statics that are scoped within a method. It has a notion of variables that don't change during calls -- final variables (local or not), and a notion of variables that do change (non-final, local or not.) SignatureChris "disentangling" Dollin Hewlett-Packard Limited registered office: Cain Road, Bracknell, registered no: 690597 England Berks RG12 1HN Chris Dollin schrieb: >>> You can declare locals `final` -- doesn't that count? Christian wrote: >> nope ... doesn`t count ... the next call to the method the local will >> have the value to which it is initialized to in the current call.. >> not the value to which it was initialized to with the first call. > Indeed, but it fits your "variable that doesn't change during calls" > description, I thought. Except that it does change between calls. > I don't see that it's anything to do with /changes/ to the variable. > Java just doesn't allow statics that are scoped within a method. > It has a notion of variables that don't change during calls -- final > variables (local or not), Except that a final local variable does change between calls. > and a notion of variables that do change (non-final, local or not.) Except that it's a different final local variable with each call. SignatureLew > Chris Dollin schrieb: >>>> You can declare locals `final` -- doesn't that count? [quoted text clipped - 8 lines] > > Except that it does change between calls. I don't think so ... > Except that it's a different final local variable with each call. ... and that's why; each call has it's own locals; but those locals don't exist once the call terminates. SignatureChris "can non-existant things change?" Dollin Hewlett-Packard Limited registered office: Cain Road, Bracknell, registered no: 690597 England Berks RG12 1HN >> Chris Dollin schrieb: >>>>> You can declare locals `final` -- doesn't that count? [quoted text clipped - 12 lines] > .... and that's why; each call has it's own locals; but those locals > don't exist once the call terminates. And you don't regard a brand-new instance as a change? SignatureLew (fx:snipping) >> .... and that's why; each call has it's own locals; but those locals >> don't exist once the call terminates. > > And you don't regard a brand-new instance as a change? It's /a/ change, but it's not a change to "the" variable. SignatureChris "existence is not a predicate, or something like that" Dollin Hewlett-Packard Limited Cain Road, Bracknell, registered no: registered office: Berks RG12 1HN 690597 England Chris Dollin schrieb: > (fx:snipping) > [quoted text clipped - 3 lines] > > It's /a/ change, but it's not a change to "the" variable. huh? void foo() { static Bar bar = new Bar(getCurrentTime()); bar.printTime(); //will print allways the same } java void foo() { final Bar bar = new Bar(getCurrentTime()); bar.printTime(); //will print allways the currentTime } so yes the variable is still called bar... but what does not change beside this? > Chris Dollin schrieb: >> [quoted text clipped - 14 lines] > so yes the variable is still called bar... > but what does not change beside this? Any given variable `bar` of `foo` has a single value that does not change. It doesn't matter that there are arbitrarily many different variables all called `bar` with different values at different times. [Saying "/the/ variable", as above, is misleading, IMAO.] SignatureChris "singular and mutable" Dollin Hewlett-Packard Limited registered office: Cain Road, Bracknell, registered no: 690597 England Berks RG12 1HN >> Chris Dollin schrieb: >>> [quoted text clipped - 19 lines] > > [Saying "/the/ variable", as above, is misleading, IMAO.] I hear static locals are coming. I'm not sure if they will be in 1.7 or not but they have been discussed at some length here recently. There is a name for them too, anybody remember what it is? CRS you know :-). SignatureKnute Johnson email s/nospam/knute/ >for example, >class Test { [quoted text clipped - 8 lines] >I can't figure out why it is wrong >thanks I have asked Sun to make that legal. It would give you a static variable accessible only by one method. It would not even need a change to the JVM, just the compiler. Perhaps you could submit an RFC. See http://mindprod.com/jgloss/rfc.html For now you must define a private static variable accessible to all methods in the class. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com >>for example, >>class Test { [quoted text clipped - 17 lines] >For now you must define a private static variable accessible to all >methods in the class. I'm not sure I'm seeing how a method scoped static would be used. Is the idea that the value would maintain its value between calls to the method, but keep the scope local to the method? If so how would you keep the declaration from overwriting the previously set value? Jim >I'm not sure I'm seeing how a method scoped static would be used. > >Is the idea that the value would maintain its value between calls to >the method, but keep the scope local to the method? If so how >would you keep the declaration from overwriting the previously set >value? It would behave exactly like a private static. It would be initialised to 0/null. However, it would be declared in a method, and any references to it outside that method would be illegal, detected an compile time. You would use it for example in void myMethod () { static Random wheel; // first use init. if( wheel == null ) wheel = new Random(); for (int i=0;i<100;i++) { System.out.println( wheel.nextInt( 6 ) ); } } You are declaring that wheel is purely for the use of myMethod, but other than that in is like a private static. To allow one-time initialisation would get fairly hairy. That is why I side-stepped the issue. Perhaps inability to init is a reasonably easily defined way is why Sun did not allow this. It would be a wart no matter how it was handled. Another syntax would be something like this where you declare outside the method. private to myMethod static Random wheel = new Random(); you could also have private instance methods too: private to myMethod Random wheel = new Random(); SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com >>I'm not sure I'm seeing how a method scoped static would be used. >> [quoted text clipped - 38 lines] > >private to myMethod Random wheel = new Random(); Thanks for the discussion. The first time initialization was one of the issues that had me questioning. Food for thought. Jim o allow one-time initialisation would get fairly hairy. That is why I One could just write class C { static void m() { static int i = 10; // ... etc. use i here } } And have it understood that the assignment is only executed once, on class load. It's a little hard to read, but not that much. > o allow one-time initialisation would get fairly hairy. That is why I > [quoted text clipped - 9 lines] > And have it understood that the assignment is only executed once, on > class load. It's a little hard to read, but not that much. Maybe it would be better with a different syntax: class C { private { static int i = 10; void m() { // ... etc. use i here } } void n() { // No access to i here. } } That is, group a method and its method-local fields in a "private block". I think this additional keyword use should be unambiguous - the other uses of "private" are followed by a keyword or an identifier. The variable would be declared static or not depending on whether it should have a single value for all instances or not. The braces show the scope limitation. The placement outside the method shows that it is a field, not a local variable. If the field is static, its initializer is executed on class load. If it is non-static, its initializer is executed during object creation. More generally, it is a normal private field declaration, with the single exception that it can only be referenced in one method. Patricia > More generally, it is a normal private field declaration, with the > single exception that it can only be referenced in one method. This scope construction also has the advantage that it scales to fields only referencable by *more* than one method, but not all. private { static int i = 0; public int next() { return i+1; } public void reset() { i = 0; } } Then again, is it really necessary to hide fields from their *own* class? /L SignatureLasse Reichstein Nielsen - lrn@hotpop.com DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html> 'Faith without judgement merely degrades the spirit divine.' >> More generally, it is a normal private field declaration, with the >> single exception that it can only be referenced in one method. [quoted text clipped - 6 lines] > public int next() { > return i+1; "return i++;" would be more interesting. > } > public void reset() { [quoted text clipped - 3 lines] > > Then again, is it really necessary to hide fields from their *own* class? That I am very unsure about. My general feeling is that it is possible the class is too large, and a candidate for refactoring, if this is really an issue. For example, maybe those methods in your example should be replaced by a nested class called something like "Counter". Patricia >>> More generally, it is a normal private field declaration, with the >>> single exception that it can only be referenced in one method. [quoted text clipped - 22 lines] > For example, maybe those methods in your example should be replaced by a > nested class called something like "Counter". The point is that no language will or can have every possible feature every possible programmer could possibly ever want. What it can have, as Java does, is a way to get every possible task done. If the idiom you crave doesn't exist, switch languages or use the equivalent feature that does exist. SignatureLew Roedy Green schreef: >> I'm not sure I'm seeing how a method scoped static would be used. >> [quoted text clipped - 38 lines] > > private to myMethod Random wheel = new Random(); Eiffel has ‘once’ methods, which seem to do exactly what you do here. Might be something worth looking at, or referencing in the bug you made at Sun’s. H. SignatureHendrik Maryns http://tcl.sfs.uni-tuebingen.de/~hendrik/ ================== http://aouw.org Ask smart questions, get good answers: http://www.catb.org/~esr/faqs/smart-questions.html >public static void main(String[] args) { > double radius = 5; > static double PI = 3.15169; > double area = radius * radius * PI; > System.out.println("Area is " + area); public class Main { public static void main( final java.lang.String[] args ) { double radius = 5; class Const { final static double PI = 3.15169; }; double area = radius * radius * Const.PI; java.lang.System.out.println( "Area is " + area ); }} Area is 78.79225 NB: Without the second »final« this instead yields: Main.java:4: inner classes cannot have static declarations class Const { static double PI = 3.15169; }; ^ Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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