Java getGraphics() returning null

javaswing

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();
    }

}

Best Answer

It is not getGraphics() that returns null but rather the previous function createImage(). From the Component documentation for createImage():

returns an off-screen drawable image, which can be used for double buffering. The return value may be null if the component is not displayable. This will always happen if GraphicsEnvironment.isHeadless() returns true.

You then get a NullPointerException when calling getGraphics() on offscreenImage which is null.

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