how to set up a global class in JAVA

knobby67 · 05-10-2011, 05:25 AM

Hi all,
I hope someone can help me out. I've had to do some code in java, a language I'm very much unfamiliarly with so please excuse my incorrect use of terms. The basic outline of my problem is I create a class object as a local within a swing button function it works fine. If I create it as a global ( with I think I need to do ) within main, then prototype it with the other swing objects at the bottom of the file when it is called it causes a host of problems. I think the easiest way is to show it. So I declare the class at the top of my code

Code:

class GameTextData
{
    private String Namestr;

    public void setgamenamestring(String st)
    {

       Namestr = st;

    }

}

I then make the object with in the main loop

Code:

 public static void main(String args[]) {

    
        GameTextData MainVariableList = new GameTextData();

        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new GameDevelopment().setVisible(true);
            }
        });
    }

I then try to call it from my button handler (which netbeans has placed above main) as follows

Code:

  private void jsetupdoneButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                                  
        // TODO add your handling code here:

        


        //what to do with game name entry
        if( jGameNameField.getText().equals("") )
        {
            System.out.println("error please enter game name");
            return;
        }
        else System.out.println(jGameNameField.getText());


        MainVariableList.setgamenamestring( jGameNameField.getText() );

        //test it
        System.out.println( MainVariableList.Namestr );

    }

this reports an error of

Code:

cannot find symbol
symbol  : variable MainVariableList
location: class GameDevelopment
        MainVariableList.setgamenamestring( jGameNameField.getText() );

I can see that netbeans shows prototypes at the bottom of the main file, so I try to copy this and prototype my variable there, if I do this it give a private access for the variable in the class,

Code:

Namestr has private access in GameTextData
        System.out.println( MainVariableList.Namestr );

if I take off private for my string it will compile however when I call the button function it gives a dozen errors.

Can anyone advise? Thanks in advace

JohnGraham · 05-10-2011, 03:10 PM

Quote:

Originally Posted by knobby67

If I create it as a global ( with I think I need to do ) within main

I then make the object with in the main loop

Code:

public static void main(String args[]) {
    GameTextData MainVariableList = new GameTextData();
}

I then try to call it from my button handler (which netbeans has placed above main) as follows

Code:

private void jsetupdoneButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                                  
    MainVariableList.setgamenamestring( jGameNameField.getText() );
}

Code:

cannot find symbol
symbol  : variable MainVariableList

You can't create a "global" variable in Java simply by declaring it in your main function. What you're trying to do is the equivalent of something like this in (say) C:

Code:

int main(void)
{
    int x = 2;
    foo();
}

void foo(void)
{
    printf("%d\n", x);
}

But you can't access `x' from `foo', because it's local to main(). Similarly, the MainVariableList you've created is local to main() and can't be accessed from anywhere else.

Unfortunately, in Java there's no way to create a global variable that's outside of any class. Instead, make a class that represents your program, and make all the data you want "global" as static members of that class, like so:

Code:

public class MyProgram {
    public static GameTextData MainVariableList = new GameTextData();
}

Then you can access it from anywhere in your program like so:

Code:

MyProgram.MainVariableList.setgamenamestring(whatever);

knobby67 · 05-11-2011, 01:55 AM

Thank you Sir that worked! I'm a low level C / Assembler programmer so am finding using JAVA a bit tricky to get my head around. Lots of the bits like what you've helped me out with I can't find in any web documentation or tutorials.