将变量从一个类传递到paintComponent类

Question

my first time here and total newbie. I have two classes in my program, the first class SwingPaintDemo2 and the second class MyPanel. MyPanel contains my paintComponent(Graphics g) method. I have a boolean variable in my first class called isTrue. I want to make it so that if isTrue = true; then paintComponent executes g.fillRect(l, w, 50, 50). Believe me, I have googled and googled and googled......

import java.awt.*;
import javax.swing.*;

public class SwingPaintDemo2 extends JComponent {

public static boolean isTrue = true;

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            createAndShowGUI(); 
        }
    });
}

private static void createAndShowGUI() {

    JFrame f = new JFrame("Swing Paint Demo");
    JPanel MyPanel = new JPanel();
     MyPanel.setBorder(BorderFactory.createEmptyBorder(1000, 1000, 1000, 1000));
     MyPanel.setPreferredSize(new Dimension(250, 200));
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    f.add(new MyPanel());
    f.pack();
    f.setVisible(true);

}

}

class MyPanel extends JComponent {

public MyPanel() {
    setBorder(BorderFactory.createLineBorder(Color.black));
}

public Dimension getPreferredSize() {
    return new Dimension(250,200);
}

public void paintComponent(Graphics g) {
    super.paintComponent(g); 
    int l = 30;
    int w = 30;

    if (SwingPaintDemo2.isTrue){g.setColor(Color.black); 
  g.fillRect(l, w, 50, 50);}


}  
}

how do I get my isTrue variable to my paintComponent class (getting variable not found error in paintComponent class)? Thanks in advance for any help.

Update: I have just posted my most recent code above after making the changes suggested earlier. Now I am getting "cannot find symbol - variable isTrue", any help would be appreciated, thanks

Answer 1

If you want to access a public static variable, always refer to it using the name of the enclosing class (and not by recreating a new instance of your SwingPaintDemo2 class):

SwingPaintDemo2.isTrue

You should try to avoid static variables.

Now, maybe you meant to declare a constant, then you need to declare it final

public static final boolean isTrue = true;

Finally, I see also a suspicious line:

if (isTrue=true)

which should rather be

if (isTrue)

NB: variables should start with a lower-case letter.

Answer 2

Since isTrue is a static member of SwingPaintDemo2 class, you can access this without instantiating a new object ie SwingPaintDemo2.isTrue

So your code will look like:

public void paintComponent(Graphics g) {
    super.paintComponent(g); 
    int l = 30;
    int w = 30;
   SwingPaintDemo2 PaintDemo = new SwingPaintDemo2();


    if (SwingPaintDemo2.isTrue == true){
        g.setColor(Color.black); 
        g.fillRect(l, w, 50, 50);
    }


}