Java NullPointerException碰撞是源，但不确定为什么

Question

I have tried running the code without the collision, and it works fine. But with it, I get a NullPointerException. The root of the problem has something to do with

blocked[xAxis][yAxis] = true;

edit: Sorry for not providing the traces earlier.

console:

Wed Dec 21 20:32:52 PST 2011 INFO:Slick Build #274
Wed Dec 21 20:32:52 PST 2011 INFO:LWJGL Version: 2.8.2
Wed Dec 21 20:32:52 PST 2011 INFO:OriginalDisplayMode: 1440 x 900 x 32 @60Hz
Wed Dec 21 20:32:52 PST 2011 INFO:TargetDisplayMode: 800 x 600 x 0 @0Hz
Wed Dec 21 20:32:53 PST 2011 INFO:Starting display 800x600
Wed Dec 21 20:32:53 PST 2011 INFO:Use Java PNG Loader = true
Loading: net.java.games.input.DirectAndRawInputEnvironmentPlugin
Wed Dec 21 20:32:54 PST 2011 INFO:Found 0 controllers
Exception in thread "main" java.lang.NullPointerException
at RPG.Main.init(Main.java:41)
at org.newdawn.slick.AppGameContainer.setup(AppGameContainer.java:390)
at org.newdawn.slick.AppGameContainer.start(AppGameContainer.java:314)
at RPG.Main.main(Main.java:102)

Properties of the TiledMap

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE map SYSTEM "http://mapeditor.org/dtd/1.0/map.dtd">
<map version="1.0" orientation="orthogonal" width="10" height="10" tilewidth="32"                   <tileheight="32">
<tileset name="wall" firstgid="1" tilewidth="32" tileheight="32">
<image source="wall.png"/>
<tile id="0">
<properties>
<property name="blocked" value="true"/>
</properties>
</tile>
</tileset>
<tileset name="grass" firstgid="2" tilewidth="32" tileheight="32">
<image source="grass.png"/>
</tileset>
<layer name="Layer 0" width="10" height="10">
<data encoding="base64" compression="gzip">
H4sIAAAAAAAAAGNkYGBgJBIzEcCUqBvsYtTwLzEYAHHwHBmQAQAA
</data>
</layer>
</map>

full code.

public class Main extends BasicGame {

Image player = null;
Image land = null;

private float playerX = 32f;
private float playerY = 32f;
private static final int SIZE = 32;

private boolean[][] blocked;

private TiledMap map;


public Main()
{
    super("Classic");
}

@Override
public void init(GameContainer gc)
        throws SlickException 
{
    player = new Image("RPG/player.png");
    map = new TiledMap("RPG/Map.tmx");

    for (int xAxis=0;xAxis<map.getWidth(); xAxis++)
    {
         for (int yAxis=0;yAxis<map.getHeight(); yAxis++)
         {
             int tileID = map.getTileId(xAxis, yAxis, 0);
             String value = map.getTileProperty(tileID, "blocked", "false");
             if ("true".equals(value))
             {
                 blocked[xAxis][yAxis] = true;
             }
         }
     }

}

@Override
public void update(GameContainer gc, int delta)
        throws SlickException
{
    Input input = gc.getInput();

    if (input.isKeyDown(Input.KEY_LEFT)) {
        if (!isBlocked(playerX, playerY - delta * 0.1f))
        {
        playerX-= delta * 0.1f;
        }
    }
    if (input.isKeyDown(Input.KEY_RIGHT)) {
        if (!isBlocked(playerX, playerY - delta * 0.1f))
        {
        playerX+= delta * 0.1f;
        }
    }
    if (input.isKeyDown(Input.KEY_UP)) {
        if (!isBlocked(playerX, playerY - delta * 0.1f))
        {
        playerY-= delta * 0.1f;
        }
    }
    if (input.isKeyDown(Input.KEY_DOWN)) {
        if (!isBlocked(playerX, playerY - delta * 0.1f))
        {
        playerY+= delta * 0.1f;
        }
    }

}

public void render(GameContainer gc, Graphics g)
        throws SlickException
{
    map.render(0, 0);

    player.draw(playerX, playerY);

}

private boolean isBlocked(float playerX, float playerY)
{
    int xBlock = (int)playerX / SIZE;
    int yBlock = (int)playerY / SIZE;
    return blocked[xBlock][yBlock];
}

public static void main(String[] args)
        throws SlickException
{
     AppGameContainer app = new AppGameContainer( new Main() );
     app.setDisplayMode(800, 600, false);
     app.start();
   }

}

Answer 1

blocked is never created. You need to say somewhere:

blocked = new boolean[xsize][ysize];

where xsize and ysize are set appropriately.

Because you haven't done this, blocked is null instead of an array. That's why you're getting the exception when you try to assign something to an element in the array.

In your case, you could add

blocked = new boolean[map.getWidth()][map.getHeight()];

to the top of your init() function.

Answer 2

You didn't initialize the array

private boolean[][] blocked;

that's why you got a NullPointerException in blocked[xAxis][yAxis] = true;

Answer 3

我没有看到被阻止的2D数组在任何地方初始化，你需要将其初始化为地图的大小