DoubleBuffering和NullPointerException

Question

在尝试加倍缓冲区时，我遇到了一个异常：java.lang.NullPointerException。我意识到这意味着指针什么也没看，因此“ g.drawImage（backBuffer，0，0，this）;” （引发异常的区域）不能执行任何操作。但是，浏览完所有内容后，我没有为任何事情分配空值……也许我没有正确使用Graphics对象，但是我做了相当多的研究但无济于事：How do you double buffer in java for a game? and < / em> http://compsci.ca/v3/viewtopic.php?t=25991。对于背景，这是针对使用.png作为背景，子画面等的游戏。而且我确实知道如何实际绘制png只是它遇到的双重缓冲。

package wows;
import java.awt.*;
import java.awt.image.*;
import java.text.AttributedCharacterIterator;
import javax.imageio.*;

class WoWS extends JPanel{

        public static BufferedImage greenSlime;
        public WoWS () {
        super();
        try {
        greenSlime = ImageIO.read(new File ("C:\\Users\\NAME\\Documents\\NetBeansProjects\\WoWS\\src\\imageBank\\Green_Slime.png"));
        } catch (IOException e) {System.out.println("Sprites failed to load...");}
}

            static class Entity{

        String AIType = null;
        int xc = -1;
        int yc = -1;
        String SpriteSet = null;
        int Mh = -1;
        int Ch = -1;
        int Dmg = -1;
        String EffectList = null;
        int hitRad = -1;

            public Entity () {} //constructor

            void cleanSlate() {
            String AIType = null;
            int xc = -1;
            int yc = -1;
            String SpriteSet = null;
            int Mh = -1;
            int Ch = -1;
            int Dmg = -1;
            String EffectList = null;
            int hitRad = -1;
        } //effectively wipes the whole entity to be created anew//

            void genEntity(String AI, int x, int y, String Sprite, int M, int C, int D, String Effect, int hitbox) {
            AI = this.AIType;
            x = this.xc;
            y = this.yc;
            Sprite = this.SpriteSet;
            M = this.Mh;
            C = this.Ch;
            D = this.Dmg;
            Effect = this.EffectList;
            hitbox = this.hitRad;
            } //creates anew
        }

    public static void main(String[] args){
        WoWS game = new WoWS();
        game.run();
        System.exit(0);
        }

        public void run() {
            initialize();
            draw();
            while (isRunning) {
                long time = System.currentTimeMillis();        
                time = (1000/fps) - (System.currentTimeMillis() - time);
                            //game loop logic goes under here//
                //playerIn();
                //entityLog();
                //draw();

                        if (time > 0) {
                            try{Thread.sleep(time);
                        }catch(Exception e) {}
                        } else if (fps == 60){System.out.println("Falling Behind!");fps = 30;}
            }
        }

                boolean isRunning = true; int fps = 60;  //variables

        public void initialize() {
            JFrame f = new JFrame("Window");
            f.add(new WoWS());
            f.setResizable(false);
            f.setSize(1900, 1000);
            f.setBackground(Color.BLACK);
            f.setVisible(true);
            f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
            f.setTitle("The Wizard of West Street");  //JFrame logic
            Entity E1 = new Entity();Entity E2 = new Entity();Entity E3 = new Entity();Entity E4 = new Entity();Entity E5 = new Entity();Entity E6 = new Entity();

        }


        public void draw() {
            BufferedImage backBuffer = new BufferedImage(1900,1000,BufferedImage.TYPE_INT_RGB);
            Graphics g = getGraphics();
            Graphics bg = backBuffer.getGraphics();

            bg.setColor(Color.WHITE);
            bg.fillRect(0,0,500,500);

            bg.setColor(Color.RED);
            bg.fillOval(100,100,200,200);

            paint(bg);

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