Instantiating global object

I've run into a situation that I don't quite understand and was hoping somebody
could help.

I have created a simple class that I'm using in my main class. I tried declaring
a global variable to this class and then instantiate it when Main is called.
Ex:

package myApp;
import myApp.animal;

public class JFrame extends javax.swing.JFrame {

private animal myAnimal;

public static void main(String args[])
{
myAnimal = new animal();
}

private void jButtonMouseClicked()
{
myAnimal.setX(100); // generates an exception error
}
}

In the code above, if you click the button, a null exception is generated.
My guess is because the object is not being instantiated even though I do
so in "main".

If I change the declared variable to read:
private animal myAnimal = new animal();

and delete the instantiation in "main", the code works.

This is confusing to me because I thought you could declare a global class
object and instantiate it elsewhere. For example, I could set "int x;" and
then set its value in a method.

Anyone help on explaining this one is greatly appreciated. Thanks!!!!

Jaime

James,

It's hard to know for sure because you didn't post all your code but I think
it is simply because your main() method is never called. main() methods are
only called when you explicitly do so in your app or when you run a Java
class from the command line.

I would suggest you either:

- put the initialisation in a constructor
- make the variable static if possible and put the initialisation in a static
block
- declare and initialise the variable on the same line

If you need more info let me know...

Regards,
Kent


"James" <jfuhr@longaberger.com> wrote:
>
>I've run into a situation that I don't quite understand and was hoping somebody
>could help.
>
>I have created a simple class that I'm using in my main class. I tried
declaring
>a global variable to this class and then instantiate it when Main is called.
> Ex:
>
>package myApp;
>import myApp.animal;
>
>public class JFrame extends javax.swing.JFrame {
>
> private animal myAnimal;
>
> public static void main(String args[])
> {
> myAnimal = new animal();
> }
>
> private void jButtonMouseClicked()
> {
> myAnimal.setX(100); // generates an exception error
> }
>}
>
>In the code above, if you click the button, a null exception is generated.
> My guess is because the object is not being instantiated even though I
do
>so in "main".
>
>If I change the declared variable to read:
> private animal myAnimal = new animal();
>
>and delete the instantiation in "main", the code works.
>
>This is confusing to me because I thought you could declare a global class
>object and instantiate it elsewhere. For example, I could set "int x;"
and
>then set its value in a method.
>
>Anyone help on explaining this one is greatly appreciated. Thanks!!!!
>
>Jaime

Additional info - if you don't make the variable static (I'm not saying you
should) you will get an error when calling main with your code as is.

While I am at it:
1. animal should be Animal
2. the variable isn't 'Global'.
3. Get rid of the 'my' stuff.
Try instead (including Kents suggestions)

package jf.animal.ui;
import jf.animal;

public class AnimalView extends javax.swing.JPanel {

private Animal animal;
public AnimalView()
{
animal = new Animal();
}
public static void main(String args[])
{
//Create a Jframe and add view
JFrame animalViewContainer = ....
}

private void jButtonMouseClicked()
{
myAnimal.setX(100); // generates an exception error
}
}


This is better but the view shouldn't really be creating a new animal object
- its controller should. But that is an advanced topic.

"Kent" <kb@essential.com.au> wrote:
>
>James,
>
>It's hard to know for sure because you didn't post all your code but I think
>it is simply because your main() method is never called. main() methods
are
>only called when you explicitly do so in your app or when you run a Java
>class from the command line.
>
>I would suggest you either:
>
>- put the initialisation in a constructor
>- make the variable static if possible and put the initialisation in a static
>block
>- declare and initialise the variable on the same line
>
>If you need more info let me know...
>
>Regards,
>Kent
>
>
>"James" <jfuhr@longaberger.com> wrote:
>>
>>I've run into a situation that I don't quite understand and was hoping
somebody
>>could help.
>>
>>I have created a simple class that I'm using in my main class. I tried
>declaring
>>a global variable to this class and then instantiate it when Main is called.
>> Ex:
>>
>>package myApp;
>>import myApp.animal;
>>
>>public class JFrame extends javax.swing.JFrame {
>>
>> private animal myAnimal;
>>
>> public static void main(String args[])
>> {
>> myAnimal = new animal();
>> }
>>
>> private void jButtonMouseClicked()
>> {
>> myAnimal.setX(100); // generates an exception error
>> }
>>}
>>
>>In the code above, if you click the button, a null exception is generated.
>> My guess is because the object is not being instantiated even though I
>do
>>so in "main".
>>
>>If I change the declared variable to read:
>> private animal myAnimal = new animal();
>>
>>and delete the instantiation in "main", the code works.
>>
>>This is confusing to me because I thought you could declare a global class
>>object and instantiate it elsewhere. For example, I could set "int x;"
>and
>>then set its value in a method.
>>
>>Anyone help on explaining this one is greatly appreciated. Thanks!!!!
>>
>>Jaime
>