JFrame and JPanel problems with calling between - jframe

Question

package pozivanjeProzora;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JPanel;
public class MainPanel extends JPanel {
SidePanel panel ;
MainWindow instanca;
public MainPanel()
{
super();
JButton button = new JButton("Pozdrav iz main panela");
add(button);
button.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
instanca.add(panel);
}
});
}
}
So, I extend JFrame in my MainWindow and I created instance of it. I am in MainPanel class which is called in MainWindow right now. But if I come in MainPanel and if I click on this button I want to my MainWindow(which extends JFrame) to add panel from SidePanel class(which has one button and extends JPanel also).
But if I click on button it shows me only NullPointerException. Where is my foult? MainWindow only has couple of lines about location of window and visible and I added MainPanel there as a first panel when I run my program. But, when I call another panel from MainPanel and when I try to add that panel with instance of MainWindow it shows error.

Answer 1

Your variable instanca is never initialised. Perhaps pass it in the constructor.
i.e.
public MainPanel(MainWindow mainWindow)
{
instanca = mainWindow;