java - 创建游戏时出现 NullPointerException

Question

我正在创建一个游戏。它不会在 Eclipse 中抛出错误，但是当我在 Debug模式下运行它时，它会在以下行抛出 NullPointerException:

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());
    }
}

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

通过查看可能为空的各个值开始调试。您可以使用此 block 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());

确定什么是 null 后，您可以开始回溯代码，以确定为什么没有在您期望的位置设置值。