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我的示例 Java 2D 游戏有点问题。一切都很好，直到出现了一些讨厌的 NullPointerException 错误，我真的不知道为什么会这样。

我试图自己调试它（我通常这样做），我设法找到了一条导致错误的行。

g.drawImage(character, survivor.getCenterX() - 18, survivor.getCenterY() - 18, this);

我知道那character是空的，这会导致错误。但我不知道为什么它是 null 以及如何修复它。

这是错误日志：

Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
at blugame.Blu.paint(Blu.java:97)
at blugame.Blu.update(Blu.java:90)
at sun.awt.RepaintArea.updateComponent(Unknown Source)
at sun.awt.RepaintArea.paint(Unknown Source)
at sun.awt.windows.WComponentPeer.handleEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

我正在使用这里的教程，而不是完全复制粘贴或重写它，所以我想这就是出现错误的原因。我只是将本教程用作一般游戏编程的参考，因为这是我的第一种方法。:)

现在 - 源代码。

蓝光类：

package blugame;

import java.applet.Applet;
import java.awt.Color;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.net.URL;

public class Blu extends Applet implements Runnable, MouseListener, KeyListener {
private final int WIDTH = 800;
private final int HEIGHT = 480;

private Map map;
private Survivor survivor;
private Image image, character;
private Graphics second;
private URL base;

public void init() {
    setSize(WIDTH, HEIGHT);
    setFocusable(true);
    setBackground(Color.BLACK);
    Frame frame = (Frame) this.getParent().getParent();
    frame.setTitle("Blu Alpha");

    frame.addMouseListener(this);
    addMouseListener(this);
    frame.addKeyListener(this);
    addKeyListener(this);

    try {
        base = getDocumentBase();
    } catch (Exception e) {
        e.printStackTrace();
    }

    // Images:
    try {
        character = getImage(base, "data/survivor.png");
    } catch (Exception e) {
        e.printStackTrace();
    }

}

public void start() {
    Thread thread = new Thread(this);
    thread.start();

    new Map(WIDTH, HEIGHT);
    new Survivor(WIDTH / 2, HEIGHT / 2);
}

public void stop() {

}

public void destroy() {

}

@Override
public void run() {
    while (true) {
        repaint();

        try {
            Thread.sleep(17);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

@Override
public void update(Graphics g) {
    if (image == null) {
        image = createImage(this.getHeight(), this.getWidth());
        second = image.getGraphics();
    }

    second.setColor(getBackground());
    second.fillRect(0, 0, getWidth(), getHeight());
    second.setColor(getForeground());
    paint(second);

    g.drawImage(image, 0, 0, this);
}

@Override
public void paint(Graphics g) {
    g.drawImage(character, survivor.getCenterX() - 18,
            survivor.getCenterY() - 18, this);
}

@Override
public void mouseClicked(MouseEvent e) {
    switch (e.getButton()) {
    case MouseEvent.BUTTON1:
        System.out.println("Action log: Mouse clicked at (" + e.getX()
                + ", " + e.getY() + ") with button 1.");
        break;
    case MouseEvent.BUTTON2:
        System.out.println("Action log: Mouse clicked at (" + e.getX()
                + ", " + e.getY() + ") with button 2.");
        break;
    case MouseEvent.BUTTON3:
        System.out.println("Action log: Mouse clicked at (" + e.getX()
                + ", " + e.getY() + ") with button 3.");
        break;
    }

}

@Override
public void mouseEntered(MouseEvent e) {
    System.out.println("Action log: Mouse entered.");

}

@Override
public void mouseExited(MouseEvent e) {
    System.out.println("Action log: Mouse exited.");

}

@Override
public void mousePressed(MouseEvent e) {
    // TODO
}

@Override
public void mouseReleased(MouseEvent e) {
    // TODO Auto-generated method stub

}

@Override
public void keyPressed(KeyEvent e) {
    switch (e.getKeyCode()) {
    case KeyEvent.VK_W:
        System.out.println("Action log: key 'W' pressed.");
        break;
    case KeyEvent.VK_S:
        System.out.println("Action log: key 'S' pressed.");
        break;
    case KeyEvent.VK_A:
        System.out.println("Action log: key 'A' pressed.");
        break;
    }

}

@Override
public void keyReleased(KeyEvent e) {
    switch (e.getKeyCode()) {
    case KeyEvent.VK_W:
        System.out.println("Action log: key 'W' released.");
        break;
    case KeyEvent.VK_S:
        System.out.println("Action log: key 'S' released.");
        break;
    case KeyEvent.VK_A:
        System.out.println("Action log: key 'A' released.");
        break;
    }

}

@Override
public void keyTyped(KeyEvent e) {
    // TODO Auto-generated method stub

}

}

班级幸存者：

package blugame;

public class Survivor {
private int centerX;
private int centerY;
private int speedX;
private int speedY;
private boolean jumped;
private boolean alive;

Survivor() {
    this.centerX = 100;
    this.centerY = 100;
    this.speedX = 0;
    this.speedY = 0;
    this.jumped = false;
    this.alive = true;
}

Survivor(int centerX, int centerY) {
    this.centerX = centerX;
    this.centerY = centerY;
    this.speedX = 0;
    this.speedY = 0;
    this.jumped = false;
    this.alive = true;
}

Survivor(int centerX, int centerY, int speedX, int speedY) {
    this.centerX = centerX;
    this.centerY = centerY;
    this.speedX = speedX;
    this.speedY = speedY;
    this.jumped = false;
    this.alive = true;
}

Survivor(int centerX, int centerY, int speedX, int speedY, boolean jumped,
        boolean alive) {
    this.centerX = centerX;
    this.centerY = centerY;
    this.speedX = speedX;
    this.speedY = speedY;
    this.jumped = jumped;
    this.alive = alive;
}

public void update(Map m) {
    // X movement.
    if (speedX < 0) {
        centerX -= speedX;
        if (centerX < m.getScrollLeft() + 18) {
            // Scroll to the left.
        } else {
            // Do not scroll yet.
        }
    } else if (speedX > 0) {
        centerX += speedX;
        if (centerX > m.getScrollRight() - 18) {
            // Scroll the the right.
        } else {
            // Do not scroll yet.
        }
    } else {
        // Do not scroll at all.
    }

    // Y movement.
    if (centerY + speedY >= m.getGround() + 16) {
        centerY = m.getGround();
    } else {
        centerY += speedY;
    }

    if (jumped == true) {
        speedY += 1;
        if (centerY + speedY >= m.getGround()) {
            centerY = m.getGround() + 16;
            speedY = 0;
            jumped = false;
        }
    }

}

public void moveRight() {
    speedX = 6;
}

public void moveLeft() {
    speedX = -6;
}

public void stop() {
    speedX = 0;
}

public void jump() {
    if(jumped == false) {
        speedY = -15;
        jumped = true;
    }
}

public int getCenterX() {
    return centerX;
}

public int getCenterY() {
    return centerY;
}

public int getSpeedX() {
    return speedX;
}

public int getSpeedY() {
    return speedY;
}

public boolean isJumped() {
    return jumped;
}

public boolean isAlive() {
    return alive;
}

public void setCenterX(int centerX) {
    this.centerX = centerX;
}

public void setCenterY(int centerY) {
    this.centerY = centerY;
}

public void setSpeedX(int speedX) {
    this.speedX = speedX;
}

public void setSpeedY(int speedY) {
    this.speedY = speedY;
}

public void setJumped(boolean jumped) {
    this.jumped = jumped;
}

public void setAlive(boolean alive) {
    this.alive = alive;
}


}

最后是类 Map （尚未真正使用）：

package blugame;

public class Map {
private int height;
private int width;
private int ground;
private int scrollLeft;
private int scrollRight;

Map(int height, int width) {
    this.height = height;
    this.width = width;
}

public int getHeight() {
    return this.height - 1;
}

public int getWidth() {
    return this.width - 1;
}

public int getGround() {
    return this.ground - 1;
}

public int getScrollLeft() {
    return this.scrollLeft - 1;
}

public int getScrollRight() {
    return this.scrollRight - 1;
}

public void calcGround() {
    this.ground = this.height - (this.height / 4);
}

public void calcScrollLeft() {
    this.scrollLeft = this.width / 3;
}

public void calcScrollRight() {
    this.scrollRight = this.width - (this.width / 3);
}

}

我知道这个character对象很可能是一个空值（Captain Obvious！），但我不知道如果：

try {
        base = getDocumentBase();
    } catch (Exception e) {
        e.printStackTrace();
    }

    // Images:
    try {
        character = getImage(base, "data/survivor.png");
    } catch (Exception e) {
        e.printStackTrace();
    }

我的survivor.png文件位于Blu/data/survivor.png. 那么有什么问题呢？任何人？

4

1 回答 1

3

在您的start()方法中，您创建了一个新的幸存者，但从未实际分配它。

new Survivor(WIDTH / 2, HEIGHT / 2);

需要是

survivor = new Survivor(WIDTH / 2, HEIGHT / 2);
于 2013-05-25T17:13:13.383 回答