Different position of a variable ......

Question

Hi ,
I have developed a very simple program ... entering in the world of making windows , frames , gui in other words.
The first version of it:
import javax.swing.JFrame;
public class TryWindow
    static JFrame aWindow = new JFrame("This is the Window Title");
    public static void main(String[] args)
    int windowWidth = 400; // Window width in pixels
    int windowHeight = 150; // Window height in pixels
    aWindow.setBounds(50, 100 ,windowWidth, windowHeight);
    aWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    aWindow.setVisible(true);
   }If i leave out the word "static" then the following errors occur:
TryWindow.java:32: non-static variable aWindow cannot be referenced from a static context
    aWindow.setBounds(50, 100 ,windowWidth, windowHeight);
    ^
TryWindow.java:33: non-static variable aWindow cannot be referenced from a static context
    aWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    ^
TryWindow.java:34: non-static variable aWindow cannot be referenced from a static context
    aWindow.setVisible(true);
    ^
3 errors I have a question on this:
Which is the static context...????? The method "setDefaultCloseOperation"...???? However , the signature of this method is : public void setDefaultCloseOperation(int operation)
If i alter the position of this variable.....
2nd version
import javax.swing.JFrame;
public class TryWindow
    public static void main(String[] args)
    int windowWidth = 400; // Window width in pixels
    int windowHeight = 150; // Window height in pixels
    JFrame aWindow = new JFrame("This is the Window Title");
    aWindow.setBounds(50, 100 ,windowWidth, windowHeight);
    aWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    aWindow.setVisible(true);
   }There is no problem... but there would be if i added the word "static" before the declaration of the variable....
Can you please explain why just a modification of this variable's position makes this...????
Thanks....
Sim

Answer 1

You are moving the variable to a static context. That is why it works.
Example:
public class StaticTest {
  //Defined public for ease of showing what I mean
  public static JFrame staticFrame = new JFrame("Static Frame");
  public JFrame nonStaticFrame = new JFrame("Non Static Frame");
  public static void main(String[] args) {
    //You are in a static function, so you can't call variables that aren't instantiated.
    //So you can't access nonStaticFrame
    staticFrame.setVisible(true); //works, your accessing this static variable from a static context.
    nonStaticFrame.setVisible(true); //ERROR Non static variable from static context!
    StaticTest test = new StaticTest();
    test.nonStaticFrame.setVisible(true); //Works since you instantiated an object that holds your nonStaticFrame.
}Edited by: hms on Jan 16, 2008 3:59 PM