Intelliji Hallo World sample program error

[fawaz]

I am new to Java and I am writing a simple prg to display msg in intelliji; The code and error is as below. Please advise. thanks.

main.java
--------------
public class Main {


public static void main(String[] args) {

System.out.println("bismillah");

JFrame frame = new JFrame("App");
frame.setContentPane(new App().panelMain);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

frame.pack();
frame.setVisible(true);

}



App.java
-----------

package sample;

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class App {
public JButton buttonMsg;
public JPanel panelMain;

public App() {
buttonMsg.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
JOptionPane.showMessageDialog(null,"HALLO WORLD");
}
});
}

private void createUIComponents() {
// TODO: place custom component creation code here
}


}


The Error msg is:

Hallo World
Exception in thread "main" java.lang.NullPointerException
at sample.App.$$$setupUI$$$(App.java)
at sample.App.<init>(App.java:12)------ buttonMsg.addActionListener(new ActionListener() {
at sample.Main.main(Main.java:53)----------frame.setContentPane(new App().panelMain);

Process finished with exit code 1

 

[xwb]

In Java, objects need to be created, primitive types don't. For instance

// This will work
int x;
x = 20;

// This won't work
JButton noworkies;
noworkies.addActionListener(...);

// It needs to be created first
JButton workies = new JButton(...);  // No idea what the creation params are
workies.addActionListener(...);