java.lang.NullPointerException at frame.add(Object)

Question

I'm trying to do a simple software that will draws a rectangle and some rows but i have a unexpected java.lang.NullPointerException when i try to add my panel (extended to JPanel) at my JFrame. The code is:

import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JPanel;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.JMenuBar;
import javax.swing.JMenu;
import javax.swing.JMenuItem;

import com.Entity.robot.Map;




import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;



public class MainWindow {
private JFrame frame;
private JFileChooser fileChooser;
private MapGUI panel;


/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                MainWindow window = new MainWindow();
                window.frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

/**
 * Create the application.
 */
public MainWindow() {
    initialize();
}

/**
 * Initialize the contents of the frame.
 */
private void initialize() {

    panel= new MapGUI();
    panel.setBounds(200, 100, 500, 250);
    frame.getContentPane().add(panel); <---- THE EXCEPTION IS HERE**
    frame = new JFrame();
    frame.getContentPane().setLayout(null);
    frame.setResizable(false);
    frame.setBounds(100, 100, 1500, 900);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JMenuBar menuBar = new JMenuBar();
        frame.setJMenuBar(menuBar);

         JMenu mnFile = new JMenu("File");
        menuBar.add(mnFile);

        JMenuItem mntmOpenMapFrom = new JMenuItem("Open map from...");
        mnFile.add(mntmOpenMapFrom);
    }

}



class MapGUI extends JPanel {

    public MapGUI(){
        setPreferredSize(new Dimension(300, 300));
    }

    public void paint (Graphics g){
        g.setColor(Color.white);
        g.drawRect(1, 1, 500, 250);


    }

}

How can i fix it?

Cat · Accepted Answer · 2013-11-03 18:04:45Z

2

frame.getContentPane().add(panel); <---- THE EXCEPTION IS HERE**
frame = new JFrame();

In the above code, you haven't defined what frame is until the line after the exception. Switch their order:

frame = new JFrame();
frame.getContentPane().add(panel);

answered Nov 3, 2013 at 18:04

Cat

67.6k24 gold badges137 silver badges144 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Juned Ahsan · Accepted Answer · 2013-11-03 18:15:37Z

2

The reason of NullPointerException is that you are trying to call a method on uninitialized/null frame object. In your code you need to initalize your frame object prior using it.

Just reverse these statements:

  frame.getContentPane().add(panel); <---- THE EXCEPTION IS HERE**
  frame = new JFrame();

to

   frame = new JFrame();
   frame.getContentPane().add(panel); <---- THE EXCEPTION IS HERE**

edited Nov 3, 2013 at 18:15

answered Nov 3, 2013 at 18:05

Juned Ahsan

68.9k12 gold badges101 silver badges138 bronze badges

Collectives™ on Stack Overflow

java.lang.NullPointerException at frame.add(Object)

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related