Thread: Need help on simple game for Practice

Hey there.

I am trying to practice my Java and work on this basic game I'm creating.

I am at the stage where I am trying to draw the tiled map, but I'm having two problems. The first problem is a logical one, I am trying to create a for loop that will draw the below image, and assign every 20x20 square a rectangle. I am having problems logically accomplishing this. The second problem is, I started to try a way to do it, but I get this error:

Exception in thread "Thread-2" java.lang.NullPointerException
at bandits.World.draw(World.java:43)
at bandits.GamePanel.draw(GamePanel.java:101)
at bandits.GamePanel.gameRender(GamePanel.java:96)
at bandits.GamePanel.run(GamePanel.java:48)
at java.lang.Thread.run(Thread.java:722)



Tile.jpg

Here is the code for my 3 classes:

package bandits;
 
import javax.swing.JFrame;
 
 
 
public class Bandits extends JFrame {
 
    GamePanel gp;
 
 
    public Bandits(){
        gp = new GamePanel();
        setSize(500, 500);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        setResizable(false);
        add(gp);
 
    }
 
    public static void main(String[] args){
        Bandits b = new Bandits();
    }
}

 
package bandits;
 
import javax.swing.JPanel;
import java.awt.*;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
 
public class GamePanel extends JPanel implements Runnable{
    //Double Buffering
    private Image dbImage;
    private Graphics dbg;
 
    //JPanel Variables
    static final int GWIDTH = 500, GHEIGHT = 400;
    static final Dimension gameDim = new Dimension(GWIDTH, HEIGHT);
 
    //Game Variables
    private Thread game;
    private volatile boolean running = false;
 
    World world;
 
    public GamePanel(){
        world = new World();
        setPreferredSize(gameDim);
        setFocusable(true);
        requestFocus();
 
        addKeyListener(new KeyAdapter(){
            @Override
            public void keyPressed(KeyEvent e){
 
 
            }
            @Override
            public void keyReleased(KeyEvent e){
 
 
            }
      });
      }
    @Override
    public void run(){
        while(running){
 
            gameUpdate();
            gameRender();
            paintScreen();
 
 
        }
    }
 
 
    @Override
    public void addNotify(){
        super.addNotify();
        startGame();
    }
 
    private void startGame(){
        if(game == null || !running){
            game = new Thread(this);
            game.start();
            running = true;
        }
    }
 
    public void stopGame(){
        if(running){
            running = false;
        }
    }
 
    private void gameUpdate() {
        if(running && game !=null){
 
        }
    }
 
    private void gameRender() {
        if(dbImage == null){
            dbImage = createImage(GWIDTH, GHEIGHT);
            if(dbImage == null){
                System.err.println("DBImage is still null!");
                return;
            }else{
                dbg = dbImage.getGraphics();
            }
        }
 
        dbg.setColor(Color.WHITE);
        dbg.fillRect(0, 0, getWidth(), getHeight());
 
        draw(dbg);
    }
 
    /* Everything that is graphics gets drawn here*/
    public void draw(Graphics g){
        world.draw(g);
    }
 
    private void paintScreen(){
        Graphics g;
        try{
            g=this.getGraphics();
            if(dbImage != null && g != null){
                g.drawImage(dbImage,0,0,null);
            }
            Toolkit.getDefaultToolkit().sync();
            g.dispose();
        }catch(Exception e){
            System.err.println(e);
        }
    }
 
}

package bandits;
 
import java.awt.*;
import javax.swing.ImageIcon;
 
 
public class World {
 
    private Rectangle[] blocks;
    private Image[] blockImg;
    private final int arrayNum = 625;
 
    private Image BLOCK_FLOOR, BLOCK_WALL;
 
    private int x= 0;
    private int y= 0;
 
    public World(){
        BLOCK_FLOOR = new ImageIcon("C:/Users/Thomas/Documents/NetBeansProjects/Images/Floor.gif").getImage();
        BLOCK_WALL = new ImageIcon("C:/Users/Thomas/Documents/NetBeansProjects/Images/Wall.gif").getImage();
 
        blocks = new Rectangle[625];
        blockImg = new Image[625];
 
        loadArrays();
    }
 
    private void loadArrays(){
        for(int i = 0; i < arrayNum; i++){
          if(x > 500){
              x = 0;
              y += 20;
          }
          if(x <= 20){
              blockImg[i] = BLOCK_WALL;
              blocks[i] = new Rectangle(x,y,20,20);
          }
          x += 20;
        }
    }
    public void draw(Graphics g){
        for(int i=0; i < arrayNum; i++){
            g.drawImage(blockImg[i], blocks[i].x, blocks[i].y, null);
        }
    }
}

Any help would be appreciated, this is not a homework assignment, just trying to refine, practice, and improve my Java skills.

Thanks for the help you have given so far! I really appreciate this community, hopefully one day I'll be answering these types of questions!

Also thank you for showing me a way to debug, didn't think of that. My knowledge is nothing but one intro to Java class and a few youtube tutorials. I have got more advanced ones on the way, but I want to stay frosty.

I have come to the conclusion that it must be something in the loadArrays(); Because it works just fine, well it only paints one rectangle in the topright corner, when I remove the x += 20. You two were mentioning null values, so I think I'm off...

I'm stumped. I have been staring at this the whole week. I cannot figure it out. I did a test loop and I didn't run into any problems, I just have the null pointer problem. Any further advice would be greatly appreciated. Thanks!

For some reason dbImage is still Null!... The if statement says if dbImage is null, dbImage = createImage(500, 500);. but for some reason it skips this step and goes to the next step. The next step makes sure dbImage isnt null, if it is, it returns the statement "dbImage is still Null!".

There's the test loop I did to make sure the way I cycle through x's and y's worked.

int x=0;
        int y=0;
 
        for(int i =0; i < 500; i++){
            if(x >500){
                x=0;
                y+=20;
            }
            System.out.println("i is = " + i + " x is = " + x + " y is = " + y);
            x += 20;
     }

I assigned it too dbImage=createImage(500, 500). Is that a wrong type to assign it to?

run:
in loadArrays(): i = 0
DBImage is still null!
in loadArrays(): i = 1
java.lang.NullPointerException
DBImage is still null!
java.lang.NullPointerException
DBImage is still null!
java.lang.NullPointerException
DBImage is still null!
java.lang.NullPointerException
DBImage is still null!
java.lang.NullPointerException
DBImage is still null!
java.lang.NullPointerException
DBImage is still null!
"
"
java.lang.NullPointerException
DBImage is still null!
Exception in thread "Thread-1" java.lang.NullPointerException
at bandits.World.draw(World.java:46)
at bandits.GamePanel.draw(GamePanel.java:102)
at bandits.GamePanel.gameRender(GamePanel.java:97)
at bandits.GamePanel.run(GamePanel.java:49)
at java.lang.Thread.run(Thread.java:722)
BUILD SUCCESSFUL (total time: 2 seconds)

Hope this helps.

Here is the code for my 3 classes:

package bandits;
 
import javax.swing.JFrame;
 
 
 
public class Bandits extends JFrame {
 
    GamePanel gp;
 
 
    public Bandits(){
        gp = new GamePanel();
        setSize(500, 500);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        setResizable(false);
        add(gp);
 
    }
 
    public static void main(String[] args){
        Bandits b = new Bandits();
    }
}

package bandits;
 
import javax.swing.JPanel;
import java.awt.*;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
 
public class GamePanel extends JPanel implements Runnable{
    //Double Buffering
    private Image dbImage;
    private Graphics dbg;
 
    //JPanel Variables
    static final int GWIDTH = 500, GHEIGHT = 400;
    static final Dimension gameDim = new Dimension(GWIDTH, HEIGHT);
 
    //Game Variables
    private Thread game;
    private volatile boolean running = false;
 
    World world;
 
    public GamePanel(){
        world = new World();
        setPreferredSize(gameDim);
        setFocusable(true);
        requestFocus();
 
        addKeyListener(new KeyAdapter(){
            @Override
            public void keyPressed(KeyEvent e){
 
 
            }
            @Override
            public void keyReleased(KeyEvent e){
 
 
            }
      });
      }
    @Override
    public void run(){
        while(running){
 
            gameUpdate();
            gameRender();
            paintScreen();
 
 
        }
    }
 
 
    @Override
    public void addNotify(){
        super.addNotify();
        startGame();
    }
 
    private void startGame(){
        if(game == null || !running){
            game = new Thread(this);
            game.start();
            running = true;
        }
    }
 
    public void stopGame(){
        if(running){
            running = false;
        }
    }
 
    private void gameUpdate() {
        if(running && game !=null){
 
        }
    }
 
    private void gameRender() {
        if(dbImage == null){
            dbImage = createImage(GWIDTH, GHEIGHT);
            if(dbImage == null){
                System.err.println("DBImage is still null!");
                return;
            }else{
                dbg = dbImage.getGraphics();
            }
        }
 
        dbg.setColor(Color.WHITE);
        dbg.fillRect(0, 0, getWidth(), getHeight());
 
        draw(dbg);
    }
 
    /* Everything that is graphics gets drawn here*/
    public void draw(Graphics g){
        world.draw(g);
    }
 
    private void paintScreen(){
        Graphics g;
        try{
            g=this.getGraphics();
            if(dbImage != null && g != null){
                g.drawImage(dbImage,0,0,null);
            }
            Toolkit.getDefaultToolkit().sync();
            g.dispose();
        }catch(Exception e){
            System.err.println(e);
        }
    }
 
}

package bandits;
 
import java.awt.*;
import javax.swing.ImageIcon;
 
 
public class World {
 
    private Rectangle[] blocks;
    private Image[] blockImg;
    private final int arrayNum = 625;
 
    private Image BLOCK_FLOOR, BLOCK_WALL;
 
    private int x= 0;
    private int y= 0;
 
    public World(){
        BLOCK_FLOOR = new ImageIcon("C:/Users/Thomas/Documents/NetBeansProjects/Images/Floor.gif").getImage();
        BLOCK_WALL = new ImageIcon("C:/Users/Thomas/Documents/NetBeansProjects/Images/Wall.gif").getImage();
 
        blocks = new Rectangle[625];
        blockImg = new Image[625];
 
        loadArrays();
    }
 
    private void loadArrays(){
        for(int i = 0; i < arrayNum; i++){
          if(x > 500){
              x = 0;
              y += 20;
          }
          if(x <= 20){
              blockImg[i] = BLOCK_WALL;
              blocks[i] = new Rectangle(x,y,20,20);
          }
          x += 20;
        }
    }
    public void draw(Graphics g){
        for(int i=0; i < arrayNum; i++){
            g.drawImage(blockImg[i], blocks[i].x, blocks[i].y, null);
        }
    }
}

dbImage is the variable that is null, even right after the assign statement it is still null.

The logic worked the same in both I thought, just have problems with dbImage. Am I mising something?

Initially, but I assign it too createImage(500,500). I am starting to think that createImage is null. I'm not sure how createImage works totally, I navigated to source but it is kind of confusing.

I'm still terribly new to java, hence I'm working on this simple top down game to maybe learn somethings and refine what I already know.

The code hasn't changed from page 1, I just copied and pasted it unto page 2.

"The logic worked the same in both I thought, just have problems with dbImage. Am I mising something?"
Yes I saw it, and it is basically exactly the same minus the fact that I add the arrays into it. But the x and y coordinate increases are the same.