AppletTalk.com Java discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Test to see of a variable has been used/initialised Goto page 1, 2, 3  Next          AppletTalk.com Forum Index -> Java Language Programming View previous topic :: View next topic   Author Message Anon Guest Posted: Mon Apr 26, 2004 7:11 pm    Post subject: Test to see of a variable has been used/initialised Hi, How do I find out if a variable has been used or initialised yet? Thanks, Anon. Back to top Eric Sosman Guest Posted: Mon Apr 26, 2004 8:16 pm    Post subject: Re: Test to see of a variable has been used/initialised Anon wrote: Quote: Hi, How do I find out if a variable has been used or initialised yet? If it's a local variable in a method, the compiler will complain if it thinks the variable might be used before being initialized. The compiler will sometimes issue bogus complaints, but if it issues no complaint you can be sure the variable is initialized before use. If it's an argument of a method or constructor, it is initialized by the method's caller or by the `new' expression. If it's a class variable it is always initialized, either when an object is constructed (for instance variables) or when the class is loaded (for static variables). However, you and Java may disagree about what "initialization" really means: the variable will acquire a legitimate value of some kind due to Java's own actions, but that's not a guarantee that your own application-specific "initialization" has been performed: /** A mutable integer. */ class Goof { int number; /* Java initializes this to zero */ /** Constructs a new Goof with the given value. */ Goof(int number) { number = number; // intended: `this.number = ...' /* `this.number' is "uninitialized" in the sense * your application intended, even though it is * "initialized" from Java's point of view. */ } } The best way to deal with errors of this sort is, of course, not to commit them If perfection is not an option, you may sometimes be able to "Java-initialize" a variable with a value that makes no sense to your application, so your application can later recognized that value as meaning "somebody goofed." The value `null' can often be used this way for reference variables; the method is more difficult to apply to non-reference types. -- [email]Eric.Sosman (AT) sun (DOT) com[/email] Back to top John C. Bollinger Guest Posted: Mon Apr 26, 2004 8:27 pm    Post subject: Re: Test to see of a variable has been used/initialised Anon wrote: Quote: How do I find out if a variable has been used or initialised yet? To check for user initialization, you can compare the value against the default value or instance initialization value. Of course that fails if the user initializes the variable to its default value, or later sets it to that value. If you *must* reliably determine this then you can make sure that initialization can only be performed via an instance method, and then let the object track whether or not that method has been invoked. As for usage, you'll have to define more precisely what you mean to get any answer. It will in any case involve protecting access to the variable by a method, as there is *no* way in standard Java to determine whether or if a field has been read directly. NOTE WELL, however: if your design depends on being able to determine this sort of thing, then probably it is severely broken. In general, instead of writing your classes in such a way as to have a need to check such things, you should write them so that they only allow permitted operations. And, no, that is in general not a circular requirement. John Bollinger [email]jobollin (AT) indiana (DOT) edu[/email] Back to top Roedy Green Guest Posted: Mon Apr 26, 2004 8:37 pm    Post subject: Re: Test to see of a variable has been used/initialised On Mon, 26 Apr 2004 20:11:30 +0100, "Anon" <voidrubbish (AT) hotmail (DOT) com> wrote or quoted : Quote: How do I find out if a variable has been used or initialised yet? compile the code. Java won't let you write code with uninitialised variables. See http://mindprod.com/jgloss/initialisation.html -- Canadian Mind Products, Roedy Green. Coaching, problem solving, economical contract programming. See http://mindprod.com/jgloss/jgloss.html for The Java Glossary. Back to top Anon Guest Posted: Mon Apr 26, 2004 9:46 pm    Post subject: Re: Test to see of a variable has been used/initialised Hi all, Thanks for the tips. The program I'm developing needs to decide whether or not to do something (buy/sell an equity) based on order types (limit/market/stop). And I was initially planning to test for the presence of the different order types (e.g. stop orders are optional and may not always be present) by testing for initialisation of the different parameters. However, judging by your responses I believe it may be wiser to use boolean variables to flag weather or not the variables have already been set or not. Thanks again! Anon. "Roedy Green" <see (AT) mindprod (DOT) com.invalid> wrote Quote: On Mon, 26 Apr 2004 20:11:30 +0100, "Anon" wrote or quoted : How do I find out if a variable has been used or initialised yet? compile the code. Java won't let you write code with uninitialised variables. See http://mindprod.com/jgloss/initialisation.html -- Canadian Mind Products, Roedy Green. Coaching, problem solving, economical contract programming. See http://mindprod.com/jgloss/jgloss.html for The Java Glossary. Back to top steve Guest Posted: Mon Apr 26, 2004 9:55 pm    Post subject: Re: Test to see of a variable has been used/initialised On Tue, 27 Apr 2004 04:37:02 +0800, Roedy Green wrote (in article <9msq80dc5cv1287uv6rpj9cl464ntatoh9 (AT) 4ax (DOT) com>): Quote: On Mon, 26 Apr 2004 20:11:30 +0100, "Anon" wrote or quoted : How do I find out if a variable has been used or initialised yet? compile the code. Java won't let you write code with uninitialised variables. See http://mindprod.com/jgloss/initialisation.html -- Canadian Mind Products, Roedy Green. Coaching, problem solving, economical contract programming. See http://mindprod.com/jgloss/jgloss.html for The Java Glossary. So you are saying a null variable is initialised ? Back to top Tony Morris Guest Posted: Mon Apr 26, 2004 10:37 pm    Post subject: Re: Test to see of a variable has been used/initialised Quote: The compiler will sometimes issue bogus complaints, Care to back this up with a bug report ? Sounds like baloney to me. -- Tony Morris (BInfTech, Cert 3 I.T.) Software Engineer (2003 VTR1000F) Sun Certified Programmer for the Java 2 Platform (1.4) Sun Certified Developer for the Java 2 Platform Back to top Virgil Green Guest Posted: Mon Apr 26, 2004 10:51 pm    Post subject: Re: Test to see of a variable has been used/initialised "steve" <me (AT) me (DOT) com> wrote Quote: On Tue, 27 Apr 2004 04:37:02 +0800, Roedy Green wrote (in article <9msq80dc5cv1287uv6rpj9cl464ntatoh9 (AT) 4ax (DOT) com>): On Mon, 26 Apr 2004 20:11:30 +0100, "Anon" wrote or quoted : How do I find out if a variable has been used or initialised yet? compile the code. Java won't let you write code with uninitialised variables. See http://mindprod.com/jgloss/initialisation.html -- Canadian Mind Products, Roedy Green. Coaching, problem solving, economical contract programming. See http://mindprod.com/jgloss/jgloss.html for The Java Glossary. So you are saying a null variable is initialised ? Absolutely. An unitialized variable would contatin whatever garbage was previously hanging around in that memory location. Null, in this case, is definitely the result of initialization. - Virgil Back to top Tony Morris Guest Posted: Mon Apr 26, 2004 11:06 pm    Post subject: Re: Test to see of a variable has been used/initialised Quote: So you are saying a null variable is initialised ? No. The concept of a "null variable" is undefined in this context (the Java Programming Language). The suggestions that the compiler will complain if a variable may not have been explicitly initialized apply to local variables only. For example: void m() { // local variable int x; if(something == 42) { x = 7; } // Compile error - x may not have been initialised. System.out.println(x); } A member variable is always implicitly initialized (AND (not OR - this can be demonstrated if need be) optionally explicitly initialized). The default value that a member variable is explicitly initialized to is dependant on the type of the varaible - I suggest reading the Java Language Specification or an appropriate tutorial to learn these values. class C { // member variable int x; void m() { // fine - x has been implicitly initialized to 0 System.out.println(x); } } -- Tony Morris (BInfTech, Cert 3 I.T.) Software Engineer (2003 VTR1000F) Sun Certified Programmer for the Java 2 Platform (1.4) Sun Certified Developer for the Java 2 Platform Back to top Tony Morris Guest Posted: Mon Apr 26, 2004 11:39 pm    Post subject: Re: Test to see of a variable has been used/initialised Quote: Absolutely. An unitialized variable would contatin whatever garbage was previously hanging around in that memory location. Null, in this case, is definitely the result of initialization. No, it wouldn't contain anything - it would simpyly be uninitialized and prevented from use. Java != C -- Tony Morris (BInfTech, Cert 3 I.T.) Software Engineer (2003 VTR1000F) Sun Certified Programmer for the Java 2 Platform (1.4) Sun Certified Developer for the Java 2 Platform Back to top Roedy Green Guest Posted: Mon Apr 26, 2004 11:56 pm    Post subject: Re: Test to see of a variable has been used/initialised On Mon, 26 Apr 2004 22:46:26 +0100, "Anon" <voidrubbish (AT) hotmail (DOT) com> wrote or quoted : Quote: However, judging by your responses I believe it may be wiser to use boolean variables to flag weather or not the variables have already been set or not. If at all possible never allow objects to exist that are in an dangerous half-baked state. Make the constructor them at least to a non-dangerous minimalist state. No one should have to be testing fields before touching an object. Another way to do it, which unfortunately is exceedingly clumsy in java is to have a set of classes like nested dolls. An object starts out small, and as you discover more, it adds layers with new fields to hold the additional information. You know how complete it is by its type. You CAN'T write code that uses the object improperly! The catch is there is no easy way to covert a Dog to a Dalmatian and then add the Dalmatian-specific fields. You need a copy constructor that takes a Dog, and then fishes the fields out of Dog one by one. PHHT! Very error prone, hard to maintain, and can't clone private fields. I wish there were some shortcut to do this. In assembler, it could be implemented with single move instruction, to copy over the common part. -- Canadian Mind Products, Roedy Green. Coaching, problem solving, economical contract programming. See http://mindprod.com/jgloss/jgloss.html for The Java Glossary. Back to top Roedy Green Guest Posted: Mon Apr 26, 2004 11:57 pm    Post subject: Re: Test to see of a variable has been used/initialised On Tue, 27 Apr 2004 05:55:05 +0800, steve <me (AT) me (DOT) com> wrote or quoted : Quote: So you are saying a null variable is initialised ? Yes, null is a value. Uninitialised variables is C have a random value in them left around from the last guy who used the RAM. That never happens in Java. Either Java nulls/zeroes for you, or forces you to set locals to something, possibly null/0. -- Canadian Mind Products, Roedy Green. Coaching, problem solving, economical contract programming. See http://mindprod.com/jgloss/jgloss.html for The Java Glossary. Back to top Larry Barowski Guest Posted: Tue Apr 27, 2004 5:20 am    Post subject: Re: Test to see of a variable has been used/initialised "Tony Morris" <dibblego (AT) optusnet (DOT) com.au> wrote Quote: The compiler will sometimes issue bogus complaints, Care to back this up with a bug report ? Sounds like baloney to me. It's not a bug. The problem of determining if a variable is initialized is not computable (for those not familiar with computational theory, this basically means "impossible to solve in the general case using an algorithm"). Here is a simple example: int j; int i = 5; if(i == 5) j = 0; j++; Here is an example you could not expect any compiler to catch (assume you have a method to compute the nth digit of pi): int j; if(digitOfPI(98000009) == 3) j = 0; j++; -Larry Barowski Back to top Chris Uppal Guest Posted: Tue Apr 27, 2004 8:21 am    Post subject: Re: Test to see of a variable has been used/initialised Eric Sosman wrote: Quote: If it's a local variable in a method, the compiler will complain if it thinks the variable might be used before being initialized. The compiler will sometimes issue bogus complaints I think the wording you've choosen is a bit misleading. The complaints are "bogus" in the sense that a different analysis with different rules might have recognised the case. However they are not "bogus" in the sense of being wrong. There are definite rules for deciding whether a variable has been intialised before use, those rules are part of the definition of the Java language, and the compiler follows them -- it is not allowed any choice. There may be bugs, of course, but I, personally, don't know of any. -- chris Back to top Tony Morris Guest Posted: Tue Apr 27, 2004 9:44 am    Post subject: Re: Test to see of a variable has been used/initialised "Larry Barowski" <larrybarATengDOTauburnDOTeduANDthatISall> wrote Quote: "Tony Morris" <dibblego (AT) optusnet (DOT) com.au> wrote in message news:c6k2ud$mho$1 (AT) news (DOT) btv.ibm.com... The compiler will sometimes issue bogus complaints, Care to back this up with a bug report ? Sounds like baloney to me. It's not a bug. The problem of determining if a variable is initialized is not computable (for those not familiar with computational theory, this basically means "impossible to solve in the general case using an algorithm"). Here is a simple example: int j; int i = 5; if(i == 5) j = 0; j++; Here is an example you could not expect any compiler to catch (assume you have a method to compute the nth digit of pi): int j; if(digitOfPI(98000009) == 3) j = 0; j++; -Larry Barowski When the compiler complains about those cases, it is correct. Consider the following example: class S { String s; } // omitted myS.s = "blah"; // point X if(myS.s == "blah") { // this is NOT guaranteed to execute } If a thread pre-empts at point X and changes the object referred to by myS.s then the boolean expression may evaluate to false (otherwise, it will evaluate to true). It is unreasonable to expect that the compiler should *know that*, in your case, the int can never be altered by another thread (since it is a primitive type and cannot be dereferenced), and therefore will evaluate always to true. Most JIT compilers will determine that the condition always evaluates to true (unless it is declared with the volatile keyword to indicate otherwise), and so will not bother evaluating the expression. Either way, I'm not convinced that this case was the intended reference. Unless proven otherwise, I'll maintain that the comment itself was bogus. -- Tony Morris (BInfTech, Cert 3 I.T., SCJP[1.4], SCJD) Software Engineer IBM Australia - Tivoli Security Software (2003 VTR1000F) Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         AppletTalk.com Forum Index -> Java Language Programming All times are GMT Goto page 1, 2, 3  Next Page 1 of 3   Jump to: Select a forum Java Newsgroups----------------Java DiscussionsWeb-based java games comp.lang.java 3D Graphics API's for Java Java: Support and criticism Java System Announcements comp.lang.java.api JavaBeans Java and CORBA Java and Databases comp.lang.java.developer Java GUI Toolkits Java Help comp.lang.java.javascript JVM, native methods and hardware comp.lang.java.misc Java Language Programming Security and Java comp.lang.java.setup Java Software Tools comp.lang.java.tech ibm.software.java.linux  Non-English Java Newsgroups----------------Java Language (German)Language Java (Français)Java (Italian)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.