RocketMan on 29/4/2009 at 22:29

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Hey all. I'm working on a rather large program that's pretty important to me. I'd like to finish it by mid summer but I have a rather noobish problem. Maybe it's just because I took a break from programming for a while and forgot some stuff I don't know but anyway here's the issue:

I'm compiling my code using the SDK from Sun Microsystems. It uses a dos command line. Right now I have about 14 errors or so and they're almost all related to instance method calls and instance variables.

My main class has a constructor in it which initializes many of my interface variables as well as setting up the interface itself. My main method instantiates the constructor as well as the constructors in a couple of other classes. My main class also implements the Runnable interface for a thread. Within the run() function for the thread I run my core loop, which calls various functions from different classes that do a whole slew of math operations. I also parse some objects defined in my main constructor. All of my errors so far say that the VM cannot find the variables I'm trying to parse because it can't find them. This is obviously an inheritance problem. For some reason I can't seem to get the VM to find stuff defined within the constructor when I call it from the run() method. Here's a hypothetical example of how my program is laid out:


class program implements Runnable {

global variable declarations

program() {

JComboBox example = new JComboBox();
Rest of constructor code....
}



public static void main(String args[]) {

program inst = new program();
class2 inst2 = new class2();
Thread runner = new Thread(this);
runner.start();
etc...
}

public void run() {

double number = Double.parseDouble(example.getString());

}
}





Ok this is a really dirty program i wrote out of my head which probably has errors and isn't anything like the real thing EXCEPT for the basic structure, which is what's important. I don't want to put the actual code because it's huge and confusing. Essentially the problem is that when i try to parse "example", the VM can't find it. Logic would tell me to use the instance "inst" to locate it via "inst.example" but "inst" is defined in a static context and when I tried it it didn't work either. I don't want to call the constructor again by making another instance within the run method because I think that would screw up the program by running the same code again which would be bad.

How do I access instance vars defined in my constructor when i'm doing it outside of the constructor in another method like run() ?? Thanks for the help.

Nameless Voice on 30/4/2009 at 00:56

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Basically, it looks like you're declaring and initialising local variables inside the constructor. Those aren't a part of the class, they're local to the constructor method.

You need to put the variables into the private section of the class and then only assign values to them, instead of declaring them inside your constructor.

Nameless Voice on 30/4/2009 at 01:26

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Static? Only if you want them to be shared across all instances of the class, or if you only plan to have one instance of the class. And they still need to go inside the class, not the constructor.

Volca on 30/4/2009 at 05:58

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The variables will always be visible in the scope defined by the first curly braces enclosing the declaration, and all the scopes included in that one (unless masked). Class member variables should be declared inside the class scope directly (preferably protected or private to avoid unwanted access from outside the class) - basically you have it right in the last code snippet but you *should* include the visibility keyword explicitly (i.e. private/protected keyword). The inst.example.getString() should really just be example.getString() - the run method sees all the variables in the scopes enclosing it.

Also note class names should begin with Upper case letter according to Java coding guides. It is useful because you'll be quickly able to tell package name from a class name.

just my 2 cents :)

RocketMan on 30/4/2009 at 22:13

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I hear ya. Before posting I hopped around from site to site reading pretty much the same thing everywhere. I understand the general rules but when it comes to coding i find it can get a bit confusing without comparable examples. I've made large programs in the past for work involving half a dozen classes or more but I believe the reason I got away with it was because I was lazy and made every variable that needed to be updated static (well..except for local vars in methods and such). This is sort of a cop-out of you don't want to worry about scope. This time around I was actually trying to be a bit more tidy with my code but ran into trouble as you can see.