我的代码中一直出现错误，但我无法在任何地方找到它，它是在我添加Spritesheet和我的Screen类时开始的，这是堆栈：
Thread [Thread-2] (Suspended (exception NullPointerException))
Screen.render(int[], int, int) line: 51
Game.render() line: 128
Game.run() line: 101
Thread.run() line: not available
`
第51行是这一行：
int colour = tileIndex * 4 + sheet.pixels[sheetPixel++];
这是Screen类：
package ca.vanzeben.game.gfx;
public class Screen {
public static final int MAP_WIDTH = 64;
public static final int MAP_WIDTH_MASK = MAP_WIDTH - 1 ;
public int[] tiles = new int[MAP_WIDTH*MAP_WIDTH];
public int[] colours = new int[MAP_WIDTH*MAP_WIDTH*4];
public int xOffset = 0;
public int yOffset = 0;
public int width;
public int height;
public SpriteSheet sheet;
public Screen(int width, int height, SpriteSheet sheet) {
this.width = width;
this.height = height;
this.sheet = sheet;
for (int i = 0; i<MAP_WIDTH*MAP_WIDTH; i++) {
colours [i*4+0] = 0xff00ff;
colours [i*4+1] = 0x00ffff;
colours [i*4+2] = 0xffff00;
colours [i*4+3] = 0xffffff;
}
}
public void render(int[] pixels, int offset, int row) {
for (int yTile = yOffset >>3;yTile <= (yOffset+height)>>3;yTile++) {
int yMin = yTile * 8 - yOffset;
int yMax = yMin + 8;
if (yMin < 0) yMin = 0;
if (yMax > height) yMax = height;
for (int xTile = xOffset >>3;xTile <= (xOffset+width)>>3;xTile++) {
int xMin = xTile * 8 - xOffset;
int xMax = xMin + 8;
if (xMin < 0) xMin = 0;
if (xMax > width) xMax = width;
int tileIndex = (xTile & (MAP_WIDTH_MASK))+ (yTile &(MAP_WIDTH_MASK))*MAP_WIDTH;
for (int y = yMin; y <yMax; y++) {
int sheetPixel = ((y + yOffset)& 7)* sheet.width + ((xMin + xOffset) & 7);
int tilePixel = offset + xMin + y*row;
for (int x = xMin; x<xMax; x++) {
int colour = tileIndex * 4 + sheet.pixels[sheetPixel++];
pixels [tilePixel++] = colours [colour];
}
}
}
}
}
}
这是Game类：
package ca.vanzeben.game;
import java.awt.BorderLayout;
import java.awt.Canvas;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.image.BufferStrategy;
import java.awt.image.BufferedImage;
import java.awt.image.DataBufferInt;
import javax.swing.JFrame;
public class Game extends Canvas implements Runnable {
private static final long serialVersionUID = 1L;
public static final int WIDTH = 160;
public static final int HEIGHT = WIDTH /12*9;
public static final int SCALE =3;
public static final String NAME = "Wake Me Up";
private JFrame frame;
public boolean running = false;
public int tickCount = 0;
private BufferedImage image = new BufferedImage (WIDTH,HEIGHT,BufferedImage.TYPE_INT_RGB);
private int[] pixels =((DataBufferInt)image.getRaster().getDataBuffer()).getData();
private Screen screen;
public Game() {
setMinimumSize(new Dimension (WIDTH*SCALE,HEIGHT*SCALE));
setMaximumSize(new Dimension (WIDTH*SCALE,HEIGHT*SCALE));
setPreferredSize(new Dimension (WIDTH*SCALE,HEIGHT*SCALE));
frame = new JFrame(NAME);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new BorderLayout());
frame.add(this,BorderLayout.CENTER);
frame.pack();
frame.setResizable(false);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
public void init() {
screen = new Screen (WIDTH,HEIGHT, new SpriteSheet("/sprite_sheet.xcf"));
}
public synchronized void start() {
running = true;
new Thread(this).start();
}
public synchronized void stop() {
}
public void run() {
long lastTime = System.nanoTime();
double nsPerTick = 1000000000D/60D;
int frames = 0;
int ticks = 0;
long lastTimer = System.currentTimeMillis();
double delta = 0;
init();
while (running) {
long now = System.nanoTime();
delta += (now-lastTime) / nsPerTick;
lastTime = now;
boolean shouldRender = true;
while (delta >=1) {
ticks++;
tick();
delta -= 1;
shouldRender = true;
}
try {
Thread.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (shouldRender) {
frames++;
render();
}
if (System.currentTimeMillis()-lastTimer>=1000) {
lastTimer += 1000;
System.out.println(ticks+ "," +frames);
frames = 0;
ticks = 0;
}
}
}
public void tick() {
tickCount++;
for (int i=0; i < pixels.length;i++) {
pixels[i] = i+tickCount;
}
}
public void render() {
BufferStrategy bs = getBufferStrategy();
if (bs == null) {
createBufferStrategy(3);
return;
}
screen.render(pixels, 0, WIDTH);
Graphics g = bs.getDrawGraphics();
g.setColor(Color.BLACK);
g.fillRect(0, 0, getWidth(), getHeight());
g.drawImage(image, 0, 0, getWidth(), getHeight(), null);
g.dispose();
bs.show();
}
public static void main(String[]args) {
new Game().start();
}
}
这是SpriteSheet类：
package ca.vanzeben.game.gfx;
import java.awt.image.BufferedImage;
import java.io.IOException;
import javax.imageio.ImageIO;
public class SpriteSheet {
public String path;
public int width;
public int height;
public int [] pixels;
public SpriteSheet(String path) {
BufferedImage image = null;
try {
image = ImageIO.read(SpriteSheet.class.getResourceAsStream(path));
} catch (IOException e) {
e.printStackTrace();
}
if (image == null) {
return;
}
this.path = path;
this.width = image.getWidth();
this.height = image.getHeight();
pixels = image.getRGB(0, 0, width, height, null, 0, width);
for(int i = 0; i< pixels.length;i++) {
pixels[i] = (pixels[i] & 0xff)/64;
}
for(int i = 0; i<8;i++) {
System.out.println(pixels[i]);
}
}
}
通常我不求助，但我可以在任何地方找到错误。谢谢！
答案 0 :(得分：2)
最可能的解释是sheet.pixels为null（如果sheet本身为null，则您会在较早的行上获得NPE。）
答案 1 :(得分：0)
我可以找到sheet.pixels在指定行为空的唯一解释是SpriteSheet构造函数中的ImageIO.read(...)已经触发了IOException，因此将图像保留为null。
public SpriteSheet(String path) {
BufferedImage image = null;
try {
image = ImageIO.read(SpriteSheet.class.getResourceAsStream(path)); //<-- has an IOException problem
} catch (IOException e) {
e.printStackTrace();
}
if (image == null) { //<--image remains null
return; //<--- rest of constructor is skipped
}
this.path = path;
this.width = image.getWidth();
this.height = image.getHeight();
pixels = image.getRGB(0, 0, width, height, null, 0, width);
.... rest of constructor
如果image = ImageIO.read(SpriteSheet.class.getResourceAsStream(path));遇到IOException并且图像保持为null，那么下一个if语句只会终止SpriteSheet构造函数而无法初始化包括像素在内的多个变量;留空。
如果遇到问题，让构造函数返回一个可怕的未正确初始化的对象是一个非常糟糕的主意，除非你有特定的理由这样做;如果需要传播错误或者（如果你不想正确处理它;也许是因为你相信它永远不会出现）至少抛出运行时异常，所以如果所有其他方法都失败了你至少知道出了什么问题。像这样保留一个未完全初始化的对象比异常更糟糕，因为它仍会导致问题（可能会在以后出现异常），但异常将远离问题的实际原因，如您所发现的那样;或者更糟糕的是，它可能不会在以后引起异常，只是给出一个“疯狂”的结果，这个结果更难以诊断。