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我正在创建一个游戏。它不会在 Eclipse 中抛出错误，但是当我在调试模式下运行它时，它会抛出一个NullPointerExceptionat 行：

System.out.println("Stone x: " + blocks.get(BlockRectangle.getByID(rectID)).getX() + " y: " + blocks.get(BlockRectangle.getByID(rectID)).getY());

游戏.java：

package lt.projecturanium;

import java.awt.BorderLayout;
import java.awt.Canvas;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.image.BufferStrategy;
import java.util.HashMap;

import javax.swing.JFrame;

import lt.projecturanium.blocks.Block;
import lt.projecturanium.blocks.BlockRectangle;
import lt.projecturanium.entity.Player;
@SuppressWarnings("unused")

public class Game extends Canvas implements Runnable{

    private static final long serialVersionUID = 1L;

    private static JFrame _frame;
    public static Game _instance;

    private static final String TITLE = "Project Uranium";
    private static final int WIDTH = 650;
    private static final int HEIGHT = WIDTH * 3 / 4;

    private static final int UPDATE_RATE = 50;
    private static final int RENDER_RATE = 100;

    public static HashMap<Block, Coordinates> blocks = new HashMap<Block, Coordinates>();

    public int rectx = 0;
    public int recty = 0;
    public int rectID = 0;

    public boolean hitted = false;

    public float interpolation;

    public static final Dimension SIZE = new Dimension(WIDTH, HEIGHT);

    private Thread _thread;

    private boolean _running;

    private int _totalTicks = 0;
    private int _tps = 0;
    private int _fps = 0;

    public Game()
    {
        _instance = this;
        setPreferredSize(SIZE);
        setMinimumSize(SIZE);
        setMaximumSize(SIZE);

        _frame = new JFrame(TITLE);

        _frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        _frame.setLayout(new BorderLayout());
        _frame.add(_instance, BorderLayout.CENTER);
        _frame.pack();

        _frame.setResizable(false);
        _frame.setLocationRelativeTo(null);
        _frame.setVisible(true);
        createBufferStrategy(2);
        blocks.put(new Block(new BlockRectangle(200)), new Coordinates(30, 50));
    }
    public synchronized void start()
    {
        _running = true;
        _thread = new Thread(this, TITLE+"_main");
        _thread.start();
    }
    public synchronized void stop()
    {
        _running = false;
        if (_thread != null)
        {
            try {
                _thread.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    public void paint(Graphics g) {
        super.paint(g);  // fixes the immediate problem.
        Graphics2D g2 = (Graphics2D) g;
        g2.drawString("FPS: " + _fps + "\n TPS: " + _tps, 10, 10);
        if (hitted)
        {
            recty = 0;
            rectx += 21;
            rectID++;
            blocks.put(new Block(new BlockRectangle(rectID)), new Coordinates(rectx, recty));
            hitted = false;
        }
        recty++;
        g2.drawImage(Player.getTexture(), 60, 60, null);
        g2.drawRect(rectx, recty, 20, 20);
        g2.setColor(new Color(101, 67, 33));
        g2.fillRect(0, 430, getWidth(), getHeight());
        g2.setColor(new Color(0, 100, 0));
        g2.fillRect(0, 420, getWidth(), 10);
        g2.setColor(Color.BLACK);
        if (recty == (419 - 20))
        {   
            hitted = true;
        }
    }
    public void run() {
        double lastUpdateTime = System.nanoTime();
        double lastRenderTime = lastUpdateTime;
        final int ns = 1000000000;
        final double nsPerUpdate = (double) ns / UPDATE_RATE;
        final double nsPerRender = (double) ns / RENDER_RATE;
        final int maxUpdatesBeforeRender = 5;

        int lastSecond = (int) (lastUpdateTime / ns);
        int tickCount = 0;
        int renderCount = 0;
        while (_running) {

          long currTime = System.nanoTime();
          int tps = 0;

          while ((currTime - lastUpdateTime) > nsPerUpdate && tps < maxUpdatesBeforeRender) {
            update();
            tickCount++;
            _totalTicks++;
            tps++;
            lastUpdateTime += nsPerUpdate;
            interpolation = Math.min(1.0F, (float) ((currTime - lastUpdateTime) / nsPerUpdate));
            render(interpolation, getGraphics());
          }

          if (currTime - lastUpdateTime > nsPerUpdate) {
            lastUpdateTime = currTime - nsPerUpdate;
          }
          if (currTime - lastRenderTime == maxUpdatesBeforeRender + 1)
          {
              render(interpolation, getGraphics());
          }
          renderCount++;
          lastRenderTime = currTime;

          int currSecond = (int) (lastUpdateTime / ns);
          if (currSecond > lastSecond) {
            _tps = tickCount;
            _fps = renderCount;
            tickCount = 0;
            renderCount = 0;
            lastSecond = currSecond;
            _frame.setTitle(TITLE + " | TPS: " + _tps + " | FPS: "+ _fps);

          }

          while (currTime - lastRenderTime < nsPerRender && currTime - lastUpdateTime < nsPerUpdate) {
            Thread.yield();
            try {
              Thread.sleep(1);
            } catch (InterruptedException e) {
              e.printStackTrace();
            }
            currTime = System.nanoTime();
          }
        }   
      }
    public void update()
    {
        _frame.pack();
    }
    public void render(float interp, Graphics g)
    {
        BufferStrategy myStrategy = getBufferStrategy(); 
        Graphics gra = myStrategy.getDrawGraphics();
        paint(gra);
        g.dispose();
        myStrategy.show();
        //System.out.println("Grass x: " + blocks.get("grass").getX() + " y: " + blocks.get("grass").getY());
        System.out.println("Stone x: " + blocks.get(BlockRectangle.getByID(rectID)).getX() + " y: " + blocks.get(BlockRectangle.getByID(rectID)).getY());
    }
}

块矩形.java：

package lt.projecturanium.blocks;

import java.awt.Image;
import java.io.IOException;
import java.util.HashMap;

import javax.imageio.ImageIO;

import lt.projecturanium.Game;

public class BlockRectangle extends Block{
    private int id;
    private static HashMap<Integer, BlockRectangle> rects = new HashMap<Integer, BlockRectangle>();
    public BlockRectangle(int id)
    {
        super();
        this.id = id;
        rects.put(id, this);
    }
    public int getID()
    {
        return this.id;
    }
    public static BlockRectangle getByID(int id)
    {
        return rects.get(id);
    }
    public static Image getTexture()
    {
        try{        
            return ImageIO.read(Game._instance.getClass().getClassLoader().getResource("../res/player.png"));   
        }
        catch(IOException e)
        {
            e.printStackTrace();
        }
        return null;
    }
}

块.java：

包 lt.projecturanium.blocks；

public class Block {
    private Block block;
    public Block (Block block){
        this.block = block;
    }
    public Block getBlock() {
        return block;
    }
    public Block getBlockById(int id)
    {
        return block;
    }
    public Block()
    {

    }
}

错误：

Thread [Project Uranium_main] (Suspended (exception NullPointerException))  
    Game.render(float, Graphics) line: 186  
    Game.run() line: 139    
    Thread.run() line: not available
4

4 回答 4

1

通过查看可能为空的单个值开始调试。您可以将此块用作示例：

BlockRectangle blockRectangle = BlockRectangle.getByID(rectID);
System.out.println("BlockRectangle: " + blockRectangle);
Coordinates coordinates= blocks.get(blockRectangle);
System.out.println("Coordinates: " + coordinates);
System.out.println("X: " + coordinates.getX());
System.out.println("Y: " + coordinates.getY());

一旦确定什么是空值，您就可以开始回溯代码，以确定为什么没有设置您期望的值。

于 2013-09-16T13:42:33.227 回答
1

多么奇怪的编程...

但是，问题似乎就在这里：

1）在课堂Game上，你有一个HashMap<Block, Coordinates>名为blocks的静态字段。您正在将键的类实例插入Block到该映射中。对应的行是：

blocks.put(new Block(new BlockRectangle(rectID)), new Coordinates(rectx, recty));

2) 在BlockRectangle同一行中创建类型实例的同时，构造函数将构造中的对象作为键放入HashMap<Integer, BlockRectangle>名为rects的类型的静态字段中。对应的行是：

rects.put(id, this);

3）然后你试图通过调用来获得一个块

blocks.get(BlockRectangle.getByID(rectID))

内部表达式BlockRectangle.getByID(rectID)将返回在地图rectsBlockRectangle中查找的实例。有了这个实例，现在在 map blocks中进行了查找，但这仅将s 存储为键，而不是s（参见第 1 点）。BlockBlockRectangle

如果在and中没有任何equals(and hashCode) 方法，您将不会有任何进展。但是在这些类中的任何一个中实现这些方法会使情况变得更糟，因为将块与块矩形进行比较并不是一个好主意。BlockBlockRectangle

您应该彻底重新设计您的软件。

于 2013-09-16T14:41:46.517 回答
0

空指针异常是您的代码尝试对某个值执行某些操作，但该值为空。一个值通常为空，因为要么它没有被实例化Object o = new Object，要么你从数据源中提取所需的记录不存在，它将返回空。请参阅http://docs.oracle.com/javase/7/docs/api/java/lang/NullPointerException.html

在您的情况下，您在该行中得到一个 NPE，System.out.println("Stone x: " + blocks.get(BlockRectangle.getByID(rectID)).getX() + " y: " + blocks.get(BlockRectangle.getByID(rectID)).getY());因为 Map 返回一个空 BlockRectangle 对象。

关于您对@GamerJosh 答案的评论，您的错误在坐标中，我猜这是因为您从getById(). 可以这样想：您永远不会增加/更改您的 rectId 字段，而是将第一个对象添加为具有200. 除此之外，您可以0在您的 paint() 方法中添加 ID 为的新块。但是，如果您在其他块出现在您的地图中之前到达此日志记录语句会发生什么？0如果那里唯一的记录的 id 为 ，您的地图会返回什么 Id 200？

话虽如此，我不认为保留所有 BlockRectangle 的地图是最伟大的程序设计。相反，请考虑在您的主类中使用一个变量来跟踪所有这些。

PS。你的英语很好，很高兴知道你在很小的时候就开始编程了！此外，由于您正在制作游戏，我想指出Game Development Stack Exchange网站，您可以在其中提出游戏特定的问题，例如游戏设计或游戏制作者可能更深入了解的编程问题。

于 2013-09-16T13:42:42.547 回答
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无论您在哪里运行，您的代码都会引发 NullPointerException。只是在线程的方法中抛出这个异常run只能通过两种方式捕获：要么通过显式捕获它，要么通过try {...} catch(NullPointerException e) {...}定义uncaughtExceptionHandler()线程的an。

run我不一定是您的方法，run而是定义为调用绘制渲染的方法。

根据您期望的 RuntimeExceptions，您可以定义这样的异常处理程序，也可以消除异常的原因。在您的情况下，其中一种get方法找不到元素并返回 null。

于 2013-09-16T13:48:38.657 回答