PaintComponent error

Question

i have an error with my code saying:

at javax.swing.JComponent.paintComponent(JComponent.java:783)
at com.game.screen.screen.PaintComponent(screen.java:29)
at com.game.screen.screen.GameScreen(screen.java:39)
at com.game.gamecore.Core.StartGame(Core.java:15)
at com.game.gamecore.Core.main(Core.java:6)

my code: com.game.screen

package com.game.screen;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Ellipse2D;

import javax.swing.*;

public class screen extends JPanel implements ActionListener  {
    /**
     * 
     */
    ActionListener AL = new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {

        }
    };

    Timer timer = new Timer(5, AL);
    double x = 0, y = 0, velY = 0,velX = 0;


    public void PaintComponent(Graphics pixel){
        super.paintComponent(pixel);
        Graphics2D g2 = (Graphics2D) pixel;
        Ellipse2D circle = new Ellipse2D.Double(x,y,40,40);
        g2.fill(circle);
        timer.start();
    }
    public void GameScreen(){

        JFrame GameScreen = new JFrame("THE GAME");
        GameScreen.setSize(600, 600);
         PaintComponent(null);
        GameScreen.setVisible(true);




    }
    @Override
    public void actionPerformed(ActionEvent e) {
        // TODO Auto-generated method stub

    }

}

com.game.gameCore.Core

package com.game.gamecore;

public class Core {
    public static void main(String[] args) {
    //  new Menu();
    new Core().StartGame(); 
    }

    public void StartGame(){

        String GameState = "idle";

        if(GameState=="idle"){
        GameState="Running";
        new com.game.screen.screen().GameScreen();


        } else if(GameState=="Running") {

            //enter process kill code here

        }
    }
}

how do i call my PaintComponent(grahpics G) code?

ineedto type something random else this thing wont let me post it... i will just keep typing untill it stops giving me error...

Answer 1

Problems and some solutions:

1. PaintComponent != paintComponent. String case matters for Java identifiers, so use the right one, here paintComponent.
2. You don't use @Override prior to your paintComponent method override. If you had done this, the compiler would have pointed out that your method wasn't a true override, forcing you to more carefully inspect your method signature to see why, and you'd likely have found the case error yourself.
3. Regarding if(GameState=="idle"){, don't compare Strings using == or !=. Use the equals(...) or the equalsIgnoreCase(...) method instead. Understand that == checks if the two object references are the same which is not what you're interested in. The methods on the other hand check if the two Strings have the same characters in the same order, and that's what matters here. So instead do, if ("idle".equals(GameState)) {, or better yet, use enums for game state as this would be much more foolproof.
4. You post only part of your exception's stacktrace with your question but leave out the crucial beginning part, the part that tells you exactly what exception is being thrown. What is it? A NullPointerException?
5. re, timer.start(); -- you're calling this from within your paintComponent method, and you shouldn't do this. You do not have full control over when or even if paintComponent is called, and so never should have object state changing code within this method.
6. PaintComponent(null); You're calling your painting method directly, something you should never do. Instead call repaint() if you want to suggest to the JVM to do painting for you. This is the reason for your NullPointerException (that we now know is occurring). You're passing in a null reference for the Graphics object, and then trying to call methods on it. Don't do this.