Java Netbeans - How to draw on a Jpanel inside a Jpanel? (Panel constructor)

Question

Im trying to learn Java and i have now met Jpanels, and since i'm using Netbeans i can use their "constructor" in which i can design the panel i want inside my frame.

With netbeans i made a panel inside my panel, and now i wonder:

How do i draw on a panel inside a panel?

(The panel i want to draw on is EvoPanel)

This is my current design

The dark gray box is the second panel inside the panel, if i try to draw a line from the top left corner to the bottom right i end up with this

My code:

Vindue.java

package vindue;
import javax.swing.*;

/**
 *
 * @author BE56df
 */
public class Vindue {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here

        paneler panel = new paneler();

        JFrame vindue = new JFrame("NEZ - Evolution Simulator v0.1");
        vindue.add(panel);

        vindue.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        vindue.setResizable(false);
        vindue.pack();
        vindue.setVisible(true);

    }

}

Paneler.java

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

/**
 *
 * @author BE56df
 */
public class paneler extends javax.swing.JPanel {

    /**
     * Creates new form paneler
     */
    public paneler() {
        initComponents();
    }

    public void paintComponent(Graphics g){

        super.paintComponent(g);

        g.drawLine(0, 0, 750, 550);

    }

    /**
     * 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() {

        EvoPanel = new javax.swing.JPanel();
        jLabel1 = new javax.swing.JLabel();
        jSeparator1 = new javax.swing.JSeparator();
        jToggleButton1 = new javax.swing.JToggleButton();

        setMaximumSize(new java.awt.Dimension(750, 550));
        setMinimumSize(new java.awt.Dimension(750, 550));
        setPreferredSize(new java.awt.Dimension(750, 550));

        EvoPanel.setBackground(new java.awt.Color(200, 200, 200));
        EvoPanel.setMaximumSize(new java.awt.Dimension(550, 550));
        EvoPanel.setMinimumSize(new java.awt.Dimension(550, 550));

        javax.swing.GroupLayout EvoPanelLayout = new javax.swing.GroupLayout(EvoPanel);
        EvoPanel.setLayout(EvoPanelLayout);
        EvoPanelLayout.setHorizontalGroup(
            EvoPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 550, Short.MAX_VALUE)
        );
        EvoPanelLayout.setVerticalGroup(
            EvoPanelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 550, Short.MAX_VALUE)
        );

        jLabel1.setFont(new java.awt.Font("Tahoma", 1, 11)); // NOI18N
        jLabel1.setText("Control panel");
        jLabel1.setVerticalAlignment(javax.swing.SwingConstants.BOTTOM);
        jLabel1.setVerticalTextPosition(javax.swing.SwingConstants.BOTTOM);

        jToggleButton1.setText("Auto play");

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
        this.setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addComponent(EvoPanel, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
                    .addGroup(javax.swing.GroupLayout.Alignment.LEADING, layout.createSequentialGroup()
                        .addGap(63, 63, 63)
                        .addComponent(jLabel1)
                        .addGap(0, 58, Short.MAX_VALUE))
                    .addGroup(layout.createSequentialGroup()
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                            .addComponent(jSeparator1, javax.swing.GroupLayout.DEFAULT_SIZE, 190, Short.MAX_VALUE)
                            .addComponent(jToggleButton1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))))
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(EvoPanel, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
            .addGroup(layout.createSequentialGroup()
                .addComponent(jLabel1, javax.swing.GroupLayout.PREFERRED_SIZE, 20, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(jSeparator1, javax.swing.GroupLayout.PREFERRED_SIZE, 10, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(jToggleButton1))
        );
    }// </editor-fold>                        


    // Variables declaration - do not modify                     
    private javax.swing.JPanel EvoPanel;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JSeparator jSeparator1;
    private javax.swing.JToggleButton jToggleButton1;
    // End of variables declaration                   
}

Answer 1

Obviously you paint the line on the main JFrame (paneler panel) when you really wants to paint on EvoPanel. You should create a class EvoPanel (with a paintComponent method) and add after paneler#initComponents.

public paneler() {
  initComponents();
  EvoPanel evoPanel = new EvoPanel();
  add(evoPanel) // properly in a layout
}

Advice: for simple GUI's don't use Mantisse (Netbeans GUI builder tool), it won't give you any benefit.