Swing not drawing on Mac

问题

I am trying to draw an incredibly basic shape using swing in Java, however for some reason it does not seem to be working. This is code that I downloaded from my lecturer that he showed us in a lecture, but when I run it the window opens but nothing is drawn and I have no idea why.

package graphicsEx;

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

public class Lecture1Example extends JPanel{
    // This is where the JPanel gets (re-)painted when the screen is refreshed.
    public void paintComponent(Graphics g) {
        // Cast to Graphics2D for more features.        
        Graphics2D g2D = (Graphics2D) g;

        Rectangle2D rect = new Rectangle2D.Double(20,30,40,50);
        g2D.setColor(Color.red);
        g2D.draw(rect);
        g2D.fill(rect); 
    }

    public static void main(String args[]) {
        JFrame frame = new JFrame("Playing with Graphics");
        frame.setSize(500, 400);
        frame.setVisible(true);
        frame.setContentPane(new Lecture1Example());        
    }
}

I am using the Eclipse IDE.

回答1:

Dear user1821475's lecturer:

• Swing GUI objects should be constructed and manipulated only on the event dispatch thread.

• "Subclasses of Swing components which have a UI delegate (vs. direct subclasses of JComponent), should invoke super.paintComponent() within their paintComponent override.

• "As a convenience add and its variants, remove and setLayout have been overridden to forward to the contentPane as necessary."

• The outermost Container should be setVisible() only after invoking pack() and other methods affecting geometry.

来源：https://stackoverflow.com/questions/14265183/swing-not-drawing-on-mac