Slick2d Map NullPointerException

Question

I have been looking for the null pointer for a few hours, and it feels like I've been wasting away staring at the same screen.

So, I'm attempting to open a SpriteSheet and get an image from that sheet, unfortunately at my best attempts it returns an error.

I believe the Map is the problem; I have a Map that documents the title and directory of a sprite sheet. When I try to access the specified SpriteSheet it returns a NullPointerException.

Anyways, getting to the code:

package main;

import java.util.HashMap;
import java.util.Map;

import org.newdawn.slick.Image;
import org.newdawn.slick.SlickException;
import org.newdawn.slick.SpriteSheet;

public class Resources {

    public static Map<String, SpriteSheet> sprites;
    public static Map<String, Image> images;

    public Resources(){
        sprites = new HashMap<String, SpriteSheet>();
        images = new HashMap<String, Image>();

        try {
            sprites.put("Tiles", getSpriteSheet("Tiles.png", 32, 32));
        } catch (SlickException e) {
            e.printStackTrace();
        }

    }

    private static SpriteSheet getSpriteSheet(String image, int w, int h) throws SlickException{
        return new SpriteSheet("/res/" + image, w, h);
    }

    public static Image getImage(String image, int x, int y) throws SlickException{
        Image img = sprites.get("Tiles").getSubImage(x, y);

        if(img != null){
            return img;
        }else{
            System.out.println("ERROR LOADING IMAGE");
            return null;
        }
    }

}

Answer 1

在没有看到堆栈跟踪的情况下，我猜这个问题就出现了：

Image img = sprites.get("Tiles").getSubImage(x, y);

这是因为如果你调用构造函数，你只会为sprites分配一个值，所以如果在碰巧调用之前调用这个静态方法，你会得到一个NullPointerException构造

您可以为静态字段指定一个值，如下所示：

public static Map<String, SpriteSheet> sprites = new HashMap<>();
public static Map<String, Image> images = new HashMap<>();

然后删除构造函数中的赋值。

您可能还需要考虑将静态字段设置为private和final，以便最小化可见性，并分别避免意外重新分配。

如果要将Tiles.png精灵表放入这些静态映射中，可以使用静态初始化块执行此操作：

static {
  try {
    sprites.put("Tiles", getSpriteSheet("Tiles.png", 32, 32));
  } catch (SlickException e) {
    // ...
  }