Q:

Java GUI Drawing and Filling a Rectangle when form loads

I have sliders that will create RGB values and then use that to create a box and fill in its color.   When I initialize my program it does not immediate draw the square.   It will draw the square and begin filling it in after moving the slider though. 
The method used to draw the rectangle and fill it with the color is called during the constructor but doesnt do anything, no errors, just doesnt draw the square.   The slider calls the SAME method on state change and it will then draw the square.   It makes no sense. 
Im sure this is something that others may experience with lack of some foundations. 
Here is my code you can copy paste and run in jGrasp or whatever to see what I mean.   Thanks! 
import java.awt. Color;
import java.awt. Graphics;
import java.awt. Graphics2D;
import java.awt. Rectangle;
import javax.swing.event. ChangeEvent;
import javax.swing.event. ChangeListener;

public class Colors extends javax.swing. JFrame {

    public Colors() {
        initComponents();
        addSliderListeners();
        addSliderLabels();
        this.setTitle("Color Mixer");
        sliderRed.setValue(150);
        sliderGreen.setValue(125);
        sliderBlue.setValue(215);
        updateImage();
    }

    private void addSliderListeners(){
        sliderRed.addChangeListener(new ChangeListener() {
            public void stateChanged(ChangeEvent e) {
                updateImage();
            }
        });
        sliderGreen.addChangeListener(new ChangeListener() {
            public void stateChanged(ChangeEvent e) {
                updateImage();
            }
        });
        sliderBlue.addChangeListener(new ChangeListener() {
            public void stateChanged(ChangeEvent e) {
                updateImage();
            }
        });
    }

    private void updateImage(){
        lblRedDec.setText(""+sliderRed.getValue());
        lblRedDec.setForeground(new Color(sliderRed.getValue(),0,0));
        lblRedHex.setText(Integer.toHexString(sliderRed.getValue()));
        lblRedHex.setForeground(lblRedDec.getForeground());

        lblGreenDec.setText(""+sliderGreen.getValue());
        lblGreenDec.setForeground(new Color(0,sliderGreen.getValue(),0));
        lblGreenHex.setText(Integer.toHexString(sliderGreen.getValue()));
        lblGreenHex.setForeground(lblGreenDec.getForeground());

        lblBlueDec.setText(""+sliderBlue.getValue());
        lblBlueHex.setText(Integer.toHexString(sliderBlue.getValue()));
        lblBlueDec.setForeground(new Color(0,0,sliderBlue.getValue()));
        lblBlueHex.setForeground(lblBlueDec.getForeground());

        Graphics2D g2 = (Graphics2D)this.getGraphics();
        g2.setColor(new Color(sliderRed.getValue(),sliderGreen.getValue(),sliderBlue.getValue()));
        g2.fill(new Rectangle(25,125,250,250));
    }

    private void addSliderLabels() {
        sliderRed.setPaintLabels(true);
        sliderGreen.setPaintLabels(true);
        sliderBlue.setPaintLabels(true);
    }
/**
     * This method is called from within the constructor to initialize the form. 
     * WARNING: Do NOT modify this code.  The content of this method is always
     * regenerated by the Form Editor. 
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        sliderRed = new javax.swing. JSlider();
        lblRed = new javax.swing. JLabel();
        lblRedHex = new javax.swing. JLabel();
        lblRedDec = new javax.swing. JLabel();
        sliderGreen = new javax.swing. JSlider();
        lblGreen = new javax.swing. JLabel();
        lblGreenHex = new javax.swing. JLabel();
        lblGreenDec = new javax.swing. JLabel();
        sliderBlue = new javax.swing. JSlider();
        lblBlue = new javax.swing. JLabel();
        lblBlueHex = new javax.swing. JLabel();
        lblBlueDec = new javax.swing. JLabel();
        lblTitle = new java.awt. Label();

        setDefaultCloseOperation(javax.swing. WindowConstants. EXIT_ON_CLOSE);
        setResizable(false);

        sliderRed.setMajorTickSpacing(15);
        sliderRed.setMaximum(255);
        sliderRed.setMinorTickSpacing(5);
        sliderRed.setOrientation(javax.swing. JSlider. VERTICAL);
        sliderRed.setPaintTicks(true);
        sliderRed.setValue(212);
        sliderRed.setCursor(new java.awt. Cursor(java.awt. Cursor. DEFAULT_CURSOR));
        sliderRed.setName(""); // NOI18N

        lblRed.setFont(new java.awt. Font("Tahoma", 1, 11)); // NOI18N
        lblRed.setText("Red");

        lblRedHex.setHorizontalAlignment(javax.swing. SwingConstants. CENTER);
        lblRedHex.setText("hex");

        lblRedDec.setHorizontalAlignment(javax.swing. SwingConstants. CENTER);
        lblRedDec.setText("dec");

        sliderGreen.setMajorTickSpacing(15);
        sliderGreen.setMaximum(255);
        sliderGreen.setMinorTickSpacing(5);
        sliderGreen.setOrientation(javax.swing. JSlider. VERTICAL);
        sliderGreen.setPaintTicks(true);
        sliderGreen.setSnapToTicks(true);
        sliderGreen.setValue(35);
        sliderGreen.setCursor(new java.awt. Cursor(java.awt. Cursor. DEFAULT_CURSOR));
        sliderGreen.setName(""); // NOI18N

        lblGreen.setFont(new java.awt. Font("Tahoma", 1, 11)); // NOI18N
        lblGreen.setText("Green");

        lblGreenHex.setHorizontalAlignment(javax.swing. SwingConstants. CENTER);
        lblGreenHex.setText("hex");

        lblGreenDec.setHorizontalAlignment(javax.swing. SwingConstants. CENTER);
        lblGreenDec.setText("dec");

        sliderBlue.setMajorTickSpacing(15);
        sliderBlue.setMaximum(255);
        sliderBlue.setMinorTickSpacing(5);
        sliderBlue.setOrientation(javax.swing. JSlider. VERTICAL);
        sliderBlue.setPaintTicks(true);
        sliderBlue.setSnapToTicks(true);
        sliderBlue.setValue(218);
        sliderBlue.setCursor(new java.awt. Cursor(java.awt. Cursor. DEFAULT_CURSOR));
        sliderBlue.setName(""); // NOI18N

        lblBlue.setFont(new java.awt. Font("Tahoma", 1, 11)); // NOI18N
        lblBlue.setText("Blue");

        lblBlueHex.setHorizontalAlignment(javax.swing. SwingConstants. CENTER);
        lblBlueHex.setText("hex");

        lblBlueDec.setHorizontalAlignment(javax.swing. SwingConstants. CENTER);
        lblBlueDec.setText("dec");

        lblTitle.setFont(new java.awt. Font("Dialog", 0, 36)); // NOI18N
        lblTitle.setText("Color Mixer");

        javax.swing. GroupLayout layout = new javax.swing. GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing. GroupLayout. Alignment. LEADING)
            .addGroup(javax.swing. GroupLayout. Alignment. TRAILING, layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(lblTitle, javax.swing. GroupLayout. PREFERRED_SIZE, javax.swing. GroupLayout. DEFAULT_SIZE, javax.swing. GroupLayout. PREFERRED_SIZE)
                .addPreferredGap(javax.swing. LayoutStyle. ComponentPlacement. RELATED, 125, Short. MAX_VALUE)
                .addGroup(layout.createParallelGroup(javax.swing. GroupLayout. Alignment. TRAILING)
                    .addGroup(layout.createSequentialGroup()
                        .addGap(27, 27, 27)
                        .addGroup(layout.createParallelGroup(javax.swing. GroupLayout. Alignment. LEADING)
                            .addComponent(lblRedHex)
                            .addComponent(lblRed)
                            .addComponent(lblRedDec))
                        .addGap(25, 25, 25))
                    .addComponent(sliderRed, javax.swing. GroupLayout. PREFERRED_SIZE, 71, javax.swing. GroupLayout. PREFERRED_SIZE))
                .addPreferredGap(javax.swing. LayoutStyle. ComponentPlacement. RELATED)
                .addGroup(layout.createParallelGroup(javax.swing. GroupLayout. Alignment. LEADING)
                    .addComponent(sliderGreen, javax.swing. GroupLayout. Alignment. TRAILING, javax.swing. GroupLayout. PREFERRED_SIZE, 71, javax.swing. GroupLayout. PREFERRED_SIZE)
                    .addGroup(javax.swing. GroupLayout. Alignment. TRAILING, layout.createSequentialGroup()
                        .addGroup(layout.createParallelGroup(javax.swing. GroupLayout. Alignment. LEADING)
                            .addComponent(lblGreenHex)
                            .addComponent(lblGreen)
                            .addComponent(lblGreenDec))
                        .addGap(25, 25, 25)))
                .addPreferredGap(javax.swing. LayoutStyle. ComponentPlacement. RELATED)
                .addGroup(layout.createParallelGroup(javax.swing. GroupLayout. Alignment. LEADING)
                    .addComponent(sliderBlue, javax.swing. GroupLayout. Alignment. TRAILING, javax.swing. GroupLayout. PREFERRED_SIZE, 71, javax.swing. GroupLayout. PREFERRED_SIZE)
                    .addGroup(javax.swing. GroupLayout. Alignment. TRAILING, layout.createSequentialGroup()
                        .addGroup(layout.createParallelGroup(javax.swing. GroupLayout. Alignment. LEADING)
                            .addComponent(lblBlueHex)
                            .addComponent(lblBlue)
                            .addComponent(lblBlueDec))
                        .addGap(25, 25, 25))))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing. GroupLayout. Alignment. LEADING)
            .addGroup(javax.swing. GroupLayout. Alignment. TRAILING, layout.createSequentialGroup()
                .addContainerGap(23, Short. MAX_VALUE)
                .addGroup(layout.createParallelGroup(javax.swing. GroupLayout. Alignment. LEADING)
                    .addGroup(layout.createParallelGroup(javax.swing. GroupLayout. Alignment. TRAILING)
                        .addGroup(layout.createSequentialGroup()
                            .addComponent(lblBlueDec)
                            .addPreferredGap(javax.swing. LayoutStyle. ComponentPlacement. RELATED)
                            .addComponent(lblBlueHex)
                            .addGap(18, 18, 18)
                            .addComponent(lblBlue)
                            .addPreferredGap(javax.swing. LayoutStyle. ComponentPlacement. RELATED)
                            .addComponent(sliderBlue, javax.swing. GroupLayout. PREFERRED_SIZE, 285, javax.swing. GroupLayout. PREFERRED_SIZE))
                        .addGroup(layout.createSequentialGroup()
                            .addComponent(lblGreenDec)
                            .addPreferredGap(javax.swing. LayoutStyle. ComponentPlacement. RELATED)
                            .addComponent(lblGreenHex)
                            .addGap(18, 18, 18)
                            .addComponent(lblGreen)
                            .addPreferredGap(javax.swing. LayoutStyle. ComponentPlacement. RELATED)
                            .addComponent(sliderGreen, javax.swing. GroupLayout. PREFERRED_SIZE, 285, javax.swing. GroupLayout. PREFERRED_SIZE))
                        .addGroup(layout.createSequentialGroup()
                            .addComponent(lblRedDec)
                            .addPreferredGap(javax.swing. LayoutStyle. ComponentPlacement. RELATED)
                            .addComponent(lblRedHex)
                            .addGap(16, 16, 16)
                            .addComponent(lblRed)
                            .addPreferredGap(javax.swing. LayoutStyle. ComponentPlacement. RELATED)
                            .addComponent(sliderRed, javax.swing. GroupLayout. PREFERRED_SIZE, 285, javax.swing. GroupLayout. PREFERRED_SIZE)))
                    .addComponent(lblTitle, javax.swing. GroupLayout. DEFAULT_SIZE, javax.swing. GroupLayout. DEFAULT_SIZE, javax.swing. GroupLayout. PREFERRED_SIZE))
                .addGap(38, 38, 38))
        );

        lblTitle.getAccessibleContext().setAccessibleName("");
        lblTitle.getAccessibleContext().setAccessibleDescription("");

        pack();
    }// </editor-fold>                        

    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel. 
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing. UIManager. LookAndFeelInfo info : javax.swing. UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing. UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging. Logger.getLogger(Colors.class.getName()).log(java.util.logging. Level. SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging. Logger.getLogger(Colors.class.getName()).log(java.util.logging. Level. SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging. Logger.getLogger(Colors.class.getName()).log(java.util.logging. Level. SEVERE, null, ex);
        } catch (javax.swing. UnsupportedLookAndFeelException ex) {
            java.util.logging. Logger.getLogger(Colors.class.getName()).log(java.util.logging. Level. SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt. EventQueue.invokeLater(new Runnable() {
            public void run() {
                new Colors().setVisible(true);
            }
        });
    }
    // Variables declaration - do not modify                     
    private javax.swing. JLabel lblBlue;
    private javax.swing. JLabel lblBlueDec;
    private javax.swing. JLabel lblBlueHex;
    private javax.swing. JLabel lblGreen;
    private javax.swing. JLabel lblGreenDec;
    private javax.swing. JLabel lblGreenHex;
    private javax.swing. JLabel lblRed;
    private javax.swing. JLabel lblRedDec;
    private javax.swing. JLabel lblRedHex;
    private java.awt. Label lblTitle;
    private javax.swing. JSlider sliderBlue;
    private javax.swing. JSlider sliderGreen;
    private javax.swing. JSlider sliderRed;
    // End of variables declaration                   

}

A:

The reason that the color square doesn't show up initially is due to the fact that GroupLayout causes a repaint of all of its child components which effectively 'paints over' the square.  
To get your square to show up you could either:

Make a custom JComponent color square & add to the GroupLayout. 
Extract all current components to a new JComponent and override the paintComponent method so that all GroupLayout component are drawn first.  This option would allow you not to add the color square to the GroupLayout. 

Code:
@Override
public void paintComponent(Graphics g) {
    super.paintComponent(g);
    updateImage();
}

