Question

我正在制作一款游戏。它不会在Eclipse中引发错误，但是当我在调试模式下运行它会在行中抛出NullPointerException：

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

Game.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());
    }
}

BlockRectangle.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;
    }
}

Block.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

Answer 1

通过查看可能为null的各个值开始调试。您可以使用此块作为示例：

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());

一旦确定了什么是null，就可以开始对代码进行反向跟踪，以确定为什么没有一个值设置在你期望的值。

Answer 2

多么奇怪的编程...

然而，问题似乎就在这里：

1）在课程Game中，您有一个名为 blocks 的HashMap<Block, Coordinates>类型的静态字段。您正在将键的类Block的实例插入到该映射中。对应的行是：

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

2）同时在同一行中创建类型为BlockRectangle的实例时，构造函数将构造中的对象作为键放入HashMap<Integer, BlockRectangle>类型的静态字段中，名为 rects < / em>的。对应的行是：

rects.put(id, this);

3）然后你试图通过调用
获得一个Block
blocks.get(BlockRectangle.getByID(rectID))

内部表达式BlockRectangle.getByID(rectID)将返回在地图 rects 中查找的BlockRectangle实例。现在使用此实例在地图 blocks 中进行查找，但这仅将Block存储为键，而不是BlockRectangle s（请参阅第1点）。

在equals和hashCode中没有任何Block（和BlockRectangle）方法，您就没有进一步了解。但是在任何这些类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。

您应该对软件进行全面的重新设计。

Answer 3

空指针异常是代码尝试使用值执行某些操作的位置，但该值为null。值通常为null，因为它没有实例化Object o = new Object，或者您从数据源中提取，其中所需的记录不存在，它将返回null。见http://docs.oracle.com/javase/7/docs/api/java/lang/NullPointerException.html

在你的情况下，你在System.out.println("Stone x: " + blocks.get(BlockRectangle.getByID(rectID)).getX() + " y: " + blocks.get(BlockRectangle.getByID(rectID)).getY());行获得了一个NPE，因为Map正在返回一个空的BlockRectangle对象。

关于你对@ GamerJosh的答案的评论，你的错误在坐标中，我猜是因为你从getById()返回一个空的BlockRectangle。想一想：你永远不会增加/改变你的rectId字段，但添加第一个对象的Id为200。除此之外，您在paint（）方法中添加ID为0的新块。但是，如果在其他块出现在地图之前到达此日志语句会发生什么？如果ID 0中的唯一记录的ID为200，您的地图会返回什么内容？

话虽如此，我不认为保留所有BlockRectangles的地图是最好的程序设计。相反，请考虑在主类中使用一个跟踪所有内容的变量。

PS。你的英语非常好，而且知道你在年轻时就开始编程是令人兴奋的！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问题，例如游戏设计或游戏制作者可能会更深入了解的编程问题。

Answer 4

无论您在何处运行，您的代码都会抛出NullPointerException。只是在线程的run方法中抛出此异常只能以两种方式捕获：通过使用try {...} catch(NullPointerException e) {...}显式捕获它或者定义线程的uncaughtExceptionHandler()。 / p>

run方法我不一定是您的run，而是定义为调用绘制渲染的方法。

根据您期望的RuntimeExceptions，您可以定义此类异常处理程序，也可以消除异常原因。在您的情况下，其中一个get方法找不到元素并返回null。

在创建游戏时获取NullPointerException

4 个答案: