Get current size of JPanel,JFrame

I have this code. I need to make this square flexible when I resize JFrame. The size of square should change in percentage. this.getWidth(),this.getHeight() returns (0,0); super.getWidth(), super.getHeight() returns (0,0); getWidth(), getHeight() returns (0,0); No examples found by me. I have nothing more to add. Thank you very much!

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ComponentEvent;
import java.awt.geom.AffineTransform;
import java.awt.geom.Rectangle2D;
import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JSlider;
import javax.swing.SwingConstants;
import javax.swing.UIManager;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

public class Parker {

public static void main(String[] args) {
    new Parker();
}

public Parker() {
    EventQueue.invokeLater(new Runnable() {
        @Override
        public void run() {
            try {
                        UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
            } catch (Exception ex) {
            }

            JFrame frame = new JFrame("Testing");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.add(new ControlPane());
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        }
    });
}

public class ControlPane extends JPanel {

    private JSlider slider;  
    private DrawPane myPanel; 
    public ControlPane() {
        setLayout(new BorderLayout());
        myPanel = new DrawPane();
        myPanel.setBackground(Color.WHITE);  
        Dimension dim =    myPanel.getSize();


        slider = new JSlider(SwingConstants.HORIZONTAL,0,100,20);
        slider.setMajorTickSpacing(20);  
        slider.setPaintTicks(true);  
        slider.setPaintLabels(true);
        slider.setValue(0);


         slider.setBorder( 
          BorderFactory.createCompoundBorder(  
                  BorderFactory.createTitledBorder("Slider"), 
                  BorderFactory.createEmptyBorder(15,10,15,10) 
                          ) 
                  );

        slider.addChangeListener(
                new ChangeListener() {
                    @Override
                    public void stateChanged(ChangeEvent e) {
                        myPanel.setScale(slider.getValue());  
                    }
                }
        );




        add(slider,BorderLayout.SOUTH);
        add(myPanel,BorderLayout.CENTER);


    }

}

public class DrawPane extends JPanel {

    double scale = 1;
    double angle = 0;    
//here i tried different methods to find current size of frame, you can
//set some int values like 10 or 100 to watch that everything works 
//properly     
   int rectWidth =  ;            
   int rectHeight =  ;

    public void componentResized(ComponentEvent e) {
        Dimension newSize = e.getComponent().getBounds().getSize();          
    } 

    @Override
   public void paintComponent(Graphics g) 
    {
        super.paintComponent(g);  

        int originX = getWidth() / 2; 
        int originY = getHeight() / 2;

        int xOffset = -(rectWidth / 2);
        int yOffset = -(rectHeight / 2);

        g.setColor(Color.BLACK);
        Graphics2D g2d = (Graphics2D) g.create();

        g2d.translate(originX, originY);
        g2d.scale(scale, scale);


        g2d.fill(new Rectangle2D.Double(xOffset, yOffset, rectWidth, rectHeight));
        g2d.dispose();

        g.setColor(Color.RED);
        g.drawRect(originX + xOffset, originY + yOffset, rectWidth, rectWidth);
    }



    public void setScale(int scale) {

        this.scale = (scale / 100d);
        repaint();
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(400, 200);
    }

}

}

you should register a ComponentListener to your ControlPanel which is the main content pane of your JFrame for listening resize event's (also listens for initial resizing) and make the rectangle resize,

try this:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ComponentAdapter;
import java.awt.event.ComponentEvent;
import java.awt.geom.Rectangle2D;

import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JSlider;
import javax.swing.SwingConstants;
import javax.swing.UIManager;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

public class Parker {

    public static void main(String[] args) {
        new Parker();
    }

    public Parker() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (Exception ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new ControlPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class ControlPane extends JPanel {

        private JSlider slider;
        private DrawPane myPanel;

        public ControlPane() {
            setLayout(new BorderLayout());
            myPanel = new DrawPane();
            myPanel.setBackground(Color.WHITE);
            Dimension dim = myPanel.getSize();

            addComponentListener(new ComponentAdapter() {

                @Override
                public void componentResized(ComponentEvent e) {
                    Dimension dim = e.getComponent().getSize();
                    myPanel.resizeRectangle(dim.width / 2, dim.height / 2);
                }

            });

            slider = new JSlider(SwingConstants.HORIZONTAL, 0, 100, 20);
            slider.setMajorTickSpacing(20);
            slider.setPaintTicks(true);
            slider.setPaintLabels(true);
            slider.setValue(0);

            slider.setBorder(BorderFactory.createCompoundBorder(BorderFactory.createTitledBorder("Slider"),
                    BorderFactory.createEmptyBorder(15, 10, 15, 10)));

            slider.addChangeListener(new ChangeListener() {
                @Override
                public void stateChanged(ChangeEvent e) {
                    myPanel.setScale(slider.getValue());
                }
            });

            add(slider, BorderLayout.SOUTH);
            add(myPanel, BorderLayout.CENTER);

        }

    }

    public class DrawPane extends JPanel {

        double scale = 1;
        double angle = 0;

        int rectWidth = 50;
        int rectHeight = 50;

        public void componentResized(ComponentEvent e) {
            Dimension newSize = e.getComponent().getBounds().getSize();
        }

        public void resizeRectangle(int width, int height) {
            rectWidth = width;
            rectHeight = height;
            repaint();
        }

        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);

            int originX = getWidth() / 2;
            int originY = getHeight() / 2;

            int xOffset = -(rectWidth / 2);
            int yOffset = -(rectHeight / 2);

            g.setColor(Color.BLACK);
            Graphics2D g2d = (Graphics2D) g.create();

            g2d.translate(originX, originY);
            g2d.scale(scale, scale);

            g2d.fill(new Rectangle2D.Double(xOffset, yOffset, rectWidth, rectHeight));
            g2d.dispose();

            g.setColor(Color.RED);
            g.drawRect(originX + xOffset, originY + yOffset, rectWidth, rectWidth);
        }

        public void setScale(int scale) {
            this.scale = (scale / 100d);
            repaint();
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(400, 200);
        }

    }

}