Question

i just started learning swings. And thought of trying out a simple program, but i can’t run it.

import java.awt.*;
import javax.swing.*;
class MyDrawPanel extends JPanel 
{
    public void paintComponent(Graphics g) 
    {
        g.setColor(Color.orange);
        g.fillRect(20,50,100,100);
    }
}

I am getting the following error:

Exception in thread "main" java.lang.NoSuchMethodError: main

My Question: Do we need to have a main method in every class we want to run? Can’t JVM run any class which doesnt have a main method. Here i don’t require a main class i think, cuz this paintComponent method should be called by the system, right?

P.S: I am using plain vanilla cmd to compile and run.

Answer 1

java is rather simple, when you give it a class file it will load it and try execute a program. Java programs are defined to start in the “public static void main(String… args)” method. So a class file missing this function has no valid entry point for a program.

To make java call your paintComponent() method you have to add an instance of your class to a toplevel container like JFrame or for web applications an JApplet. (Applets don’t use a main method as they are executed as part of a web page and not a standalone app.)

Example:

import javax.swing.*
public class MyDrawPanel{
     public static void main(String... args)
     {
         JFrame frame = new JFrame(200,200);//A window with 200x200 pixel
         MyDrawPanel mdp = new MyDrawPanel();//Panel instance
         frame.add(mdp);//Add the panel to the window
         frame.setVisible(true);//Display all
         frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);//exit when the window is closed

     }
}