Question

I’m starting to learn to create Games in Java, and one of the methods I’m using includes BufferedImage. This is the error I get:

"Exception in thread "main" java.lang.NullPointerException
     at tm.Game.init(Game.java:48)
     at tm.Game.<init>(Game.java:54)"

From this code:

package tm;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.image.BufferedImage;

import javax.swing.JPanel;

@SuppressWarnings("serial")
public class Game extends JPanel implements Runnable {
    private Settings Settings;

    private Thread t;
    private BufferedImage offscreenImage;
    private Graphics offscr;

    public void run() {
        while(true) {
            repaint();

            try {
                Thread.sleep(1000/30);
            } catch (InterruptedException e) { }
        }
    }

    public void paint(Graphics g) {
        offscr.setColor(Color.blue);
        offscr.fillRect(0, 0, Settings.GAME_WIDTH, Settings.GAME_HEIGHT);

        offscr.setColor(Color.white);
        offscr.drawString("Lolz", 10, 10);

        g.drawImage(offscreenImage, 0, 0, this);
    }

    public void update(Graphics g) {
        paint(g);
    }

    public void init() {
        t = new Thread(this);
        t.start();

        offscreenImage = (BufferedImage) createImage(Settings.GAME_WIDTH, Settings.GAME_HEIGHT);
        offscr = offscreenImage.getGraphics();
    }

    public Game() {
        Settings = new Settings();

        init();
    }

}

Settings Class:

package tm;

public class Settings {

    public final int GAME_WIDTH = 500;
    public final int GAME_HEIGHT = 500;

}

Screen Class:

package tm;

import javax.swing.JFrame;

public class Screen extends JFrame {
    private static final long serialVersionUID = 1L;
    private JFrame mainScreen;
    private Game mainGame;
    private Settings Settings;

    public Screen() {
        mainGame = new Game();
        Settings = new Settings();

        mainScreen = new JFrame();
        mainScreen.add(mainGame);
        mainScreen.setSize(Settings.GAME_WIDTH, Settings.GAME_HEIGHT);
        mainScreen.setTitle("Lolz");
        mainScreen.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainScreen.setResizable(false);
        mainScreen.setVisible(true);
    }

    public static void main(String[] args) {
        new Screen();
    }

}

Answer 1

The reason that throw NullPointer exception is that you initialized the offScreenImage and offScr in wrong place.

offscreenImage = (BufferedImage) createImage(Settings.GAME`WIDTH, Settings.GAME_HEIGHT);
offscr = offscreenImage.getGraphics();

This code should be in the function paint. To get the results the Game class should be defined like this. And another tip it is better to declare variables inn Settings class to public static final so that they can be accessed in static way. Make little change to your Game class as defined below. I think this should help you.

package tm;

import javax.swing.JPanel;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.image.BufferedImage;

import tm.Screen.Settings;

public class Game extends JPanel implements Runnable {
    // private Setting Settings;

    private Thread t;
    private BufferedImage offscreenImage;
    private Graphics offscr;

    public void run() {
        while (true) {
            repaint();

            try {
                Thread.sleep(1000 / 30);
            } catch (InterruptedException e) {
            }
        }
    }

    public void paint(Graphics g) {

        if (offscreenImage == null) {

            offscreenImage = (BufferedImage) createImage(Settings.GAME_WIDTH,
                    Settings.GAME_HEIGHT);
        }
        offscr = offscreenImage.getGraphics();
        offscr.setColor(Color.black);
        offscr.fillRect(0, 0, Settings.GAME_WIDTH, Settings.GAME_HEIGHT);
        offscr.setColor(Color.white);
        offscr.drawString("Lolz", 10, 10);

        g.drawImage(offscreenImage, 0, 0, this);
    }

    public void update(Graphics g) {
        paint(g);
    }


    public void init() {
        t = new Thread(this);
        t.start();
    }

    public Game() {
        init();
    }
}