Java代码- NullPointerException

Question

我正在做一个游戏，它给了我一个NullPointerException，我相信这意味着变量，或者我试图做的不是返回值？我使用的代码如下：

package OurGame;

import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;

import javax.swing.ImageIcon;
import javax.swing.JPanel;
import javax.swing.Timer;

public class Coin extends JPanel implements ActionListener {
/**
     * 
     */
    private static final long serialVersionUID = 1L;

Image img;

int x,y;
Timer t;
Random r;

        public Coin() {
            x = 50;
            y = 50;

            System.out.println("New coin created: " + x + ", " +y);

            ImageIcon i = new ImageIcon("C:/coin.png");
            img = i.getImage();
            System.out.println(i);

            t = new Timer(3000,this);
            t.start();
        }

        @Override
        public void actionPerformed(ActionEvent arg0) {
            move();

        }

        public void move() {

            setX(r.nextInt(640));
            setY(r.nextInt(480));


        }

        public void setX(int xs) {
            x = xs;
        }

        public void setY(int ys) {
            y = ys;
        }
        public Image getImage(){
            return img;
        }

        public int getX(){
            return x;
        }

        public int getY() {
            return y;
        }


}

错误发生在这里：

        setX(r.nextInt(640));

完整的错误输出如下：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at OurGame.Coin.move(Coin.java:46)
    at OurGame.Coin.actionPerformed(Coin.java:40)
    at javax.swing.Timer.fireActionPerformed(Unknown Source)
    at javax.swing.Timer$DoPostEvent.run(Unknown Source)
    at java.awt.event.InvocationEvent.dispatch(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)

我看不出发生这种事的任何理由，但也许你能帮上忙。

谢谢。

Answer 1

您需要初始化变量r。查看java.util.Random的构造函数，例如：

Random r = new Random();

Answer 2

您在没有说r = new ...的情况下使用r。

Answer 3

r为空。首先初始化它。把这个放到你的构造函数中：

r = new Random();