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Accessing objects made in main method from other classes
public static void main(String[] args) {

    Mainframe mainframe = new Mainframe();
    mainframe.initialiseMF();
    LoginPanel lp = new LoginPanel();
    mainframe.add(lp);
}

public class Mainframe extends JFrame {

public Mainframe () {
    // Set size of mainframe
    setBounds(0, 0, 500, 500);

}

public void initialiseMF (){
    // Get the size of the screen
    Dimension dim = Toolkit.getDefaultToolkit().getScreenSize();

    // Determine the new location of the mainframe
    int w = getSize().width;
    int h = getSize().height;
    int x = (dim.width-w)/2;
    int y = (dim.height-h)/2;

    // Move the mainframe
    setLocation(x, y);
    setVisible(true);
}

Container content = mainframe.getContentPane();

public class Mainframe extends JFrame{
     public Mainframe(){
          initialiseMF ();
     }

     public void initialiseMF (){
          //do ur inits here
     }

}

public class TheOtherClass{

    private Mainframe mainFrame;

    public TheOtherClass(){
        mainFrame = MainFrame.mainFrame; //although I would not suggest this, it will avoid the Main.mainFrame call
    }

    public void otherMethodFromOtherClass(JFrame mainFrame){
        Container content = mainFrame.getConentPane();
    }
}

//local variables are the ones that are declared inside a method
//their life and visibility is limited only within the block in which they are define.
public static void main(String[] args) { // args is also a local variable
    String localVar = "Access modifiers are not allowed for local variables.";
    //the reason that the access modifiers are not allowed is because
    //the local variables are not members of the class
}

class AClass {
    private String memberVar = "A member variable can have access modifier.";
    //the access modifier will determine the visibility of that variable.

    public String getMemberVar() {
        return this.memberVar;
    }
}