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I was following a tutorial on some basic game design for Java when I ran across an unusual issue. When I run an applet that simply moves an image based character on the screen, it will not let me use the methods to change position of the image. It raises a NullPointerException in located at my robot.update() method which handles the movement of the image. The odd part is that if I choose the "Restart" option from the drop down list in the Applet frame, it works as normal. Any ideas?

Main Class

package Game;

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.net.URL;

public class First extends Applet implements Runnable, KeyListener {

private Robot robot;
private Image image, character;
private Graphics second;
private URL base;

@Override
public void init() {
    setSize(800,480);
    setBackground(Color.BLACK);
    setFocusable(true);
    Frame frame = (Frame) this.getParent().getParent();
    frame.setTitle("First Game");
    addKeyListener(this);

    try {
        base = getDocumentBase();
    } catch (Exception e) {
        e.printStackTrace();
        System.out.println("Image not accessible");

    }
    character = getImage(base, "data/character.png");
    // TODO Auto-generated method stub
    super.init();
}

@Override
public void start() {
    Thread thread = new Thread(this);
    thread.start();
    robot = new Robot();
    // TODO Auto-generated method stub
    super.start();
}

@Override
public void stop() {
    // TODO Auto-generated method stub
    super.stop();
}

@Override
public void destroy() {
    // TODO Auto-generated method stub
    super.destroy();
}

@Override
public void run() {
    // TODO Auto-generated method stub
    while (true) {
        robot.update();
        repaint();
        try {
            Thread.sleep(17);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

public void update(Graphics g) {
    if (image == null) {
        image = createImage(this.getWidth(), this.getHeight());
        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) {
    // TODO Auto-generated method stub
    g.drawImage(character, robot.getCenterX() - 61,
            robot.getCenterY() - 63, this);
    super.paint(g);
}

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

}

@Override
public void keyPressed(KeyEvent e) {
    switch (e.getKeyCode()) {
    case KeyEvent.VK_UP:
        System.out.println("Move UP");
        break;
    case KeyEvent.VK_DOWN:
        System.out.println("Move DOWN");
        break;
    case KeyEvent.VK_LEFT:
        robot.moveLeft();
        break;
    case KeyEvent.VK_RIGHT:
        robot.moveRight();
        break;
    case KeyEvent.VK_SPACE:
        robot.jump();
        break;
    }
    // TODO Auto-generated method stub

}

@Override
public void keyReleased(KeyEvent e) {
    switch (e.getKeyCode()) {
    case KeyEvent.VK_UP:
        System.out.println("Stop UP");
        break;
    case KeyEvent.VK_DOWN:
        System.out.println("Stop DOWN");
        break;
    case KeyEvent.VK_LEFT:
        robot.stop();
        break;
    case KeyEvent.VK_RIGHT:
        robot.stop();
        break;
    case KeyEvent.VK_SPACE:
        robot.stop();
        break;
    // TODO Auto-generated method stub

    }
}
}

Robot Class

package Game;

public class Robot {

private int centerX = 100;
private int centerY = 382;
private boolean jumped = false;

private int speedX = 0;
private int speedY = 1;

public void update() {

    if (speedX < 0) {
        centerX += speedX;
    } else if (speedX == 0){
        System.out.println("Do not scroll the background");
    } else {
        if (centerX <= 300) {
            centerX += speedX;
        } else {
                System.out.println("Scroll background here");
        }
    }

    if (centerY + speedY >= 382) {
        centerY = 382;
    }else{                       
                 centerY += speedY;
            }

    // Handles Jumping
    if (jumped == true) {
        speedY += 1;

        if (centerY + speedY >= 382) {
            centerY = 382;
            speedY = 0;
            jumped = false;
        }

    }

    // Prevents going beyond X coordinate of 0
    if (centerX + speedX <= 60) {
        centerX = 61;
    }
}

public int getCenterX() {
    return centerX;
}

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

public int getCenterY() {
    return centerY;
}

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

public boolean isJumped() {
    return jumped;
}

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

public int getSpeedX() {
    return speedX;
}

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

public int getSpeedY() {
    return speedY;
}

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

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;
    }

}

}

This is the error message I am gettings

Exception in thread "Thread-3" java.lang.NullPointerException
    at Game.First.run(First.java:65)
    at java.lang.Thread.run(Thread.java:722)
4

1 回答 1

1

您应该先初始化机器人对象，而不是在运行线程后，尝试：

robot = new Robot();
Thread thread = new Thread(this);
thread.start();

当您开始执行线程时，它可能以空机器人开始。

当您再次重新加载时，它的工作原理是那时它已被初始化。

于 2013-06-26T04:30:18.560 回答