代码中的Java.lang.NullPointerExpection错误

我正在尝试运行此代码，并生成一个带有变暗图形的图像。但是，我继续遇到此运行时错误：

java.lang.NullPointerException
    at java.lang.String.<init>(Unknown Source)
    at SimplePicture.<init>(SimplePicture.java:96)
    at SimplePicture.explore(SimplePicture.java:364)
    at Prob04Runner.run(Prob04.java:50)
    at Prob04.main(Prob04.java:8)

java.lang.NullPointerException
位于java.lang.String。（未知源）
在SimplePicture。（SimplePicture.java:96）
在SimplePicture.explore（SimplePicture.java:364）
运行（Prob04.java:50）
位于Prob04.main（Prob04.java:8）

我的空值对于代码是必需的

我不明白我做错了什么，这是代码，谢谢你：

import java.awt.Color;
public class Prob04{
  public static void main(String[] args){
    Prob04Runner obj = new Prob04Runner();
    obj.run();
  }//end main
}//end class Prob04
//======================================================//

class Prob04Runner{

  public Prob04Runner(){//constructor
    System.out.println("Display your name here.");
  }//end constructor
  //----------------------------------------------------//

  public void run(){

    Picture skater = new Picture("Prob04a.bmp");
    skater.explore();
    skater = crop(skater,6,59,392,293);

    Picture hole = new Picture("Prob04b.bmp");
    hole.explore();
    hole = crop(hole,6,59,392,293);


    Picture snowScene = new Picture("Prob04c.jpg");
    snowScene.explore();
    snowScene = crop(snowScene,6,59,392,293);

    //Make all the pixels darker in the snow scene except
    // for those in the location of the hole. Make them
    // brighter.
    darkenBackground(hole,snowScene);

    //Apply a red tint to the skater
    redTint(skater);

    //Draw the skater on the snowScene.
    greenScreenDraw(skater,snowScene,0,0);

    //Display students name on the final output and
    // display it.
    snowScene.addMessage("Display your name here.",10,15);

    snowScene.explore();
    System.out.println(snowScene);

  }//end run method
  //----------------------------------------------------//

  //Assumes the source has a pure green background.
  // Applies a red tint to every pixel that is not pure
  // green.
  private void redTint(Picture pic){
    Pixel[] pixels = pic.getPixels();
    Color color = null;
    int red = 0;
    int green = 0;
    int blue = 0;
    for(int cnt = 0;cnt < pixels.length;cnt++){
      color = pixels[cnt].getColor();
      //Apply a red tint to all non-green pixels
      if(!(color.equals(Color.GREEN))){
        //Increase the value of the red component
        red = color.getRed();
        if(red*1.25 < 255){
          red = (int)(red * 1.25);
        }else{
          red = 255;
        }//end else
        //Decrease the value of blue and green
        green = (int)(color.getGreen()*0.8);
        blue = (int)(color.getBlue()*0.8);

        //Apply the new color to the pixel.
        pixels[cnt].setColor(new Color(red,green,blue));
      }//end if
    }//end for loop
  }//end redTint
  //----------------------------------------------------//

  //Assumes the pattern image has a pure green background.
  // Assumes that the pattern and the destination have the
  // same dimensions. Darkens every pixel in the
  // destination that is at the location of a green pixel
  // in the pattern. Applies a red tint to every pixel
  // in the destination that is at the location of a
  // non-green pixel in the pattern
  private void darkenBackground(
                           Picture pattern,
                           Picture dest){

    Pixel[] patternPixels = pattern.getPixels();
    Pixel[] destPixels = dest.getPixels();
    Color color = null;
    int red = 0;
    int green = 0;
    int blue = 0;

    for(int cnt = 0;cnt < patternPixels.length;cnt++){
      color = patternPixels[cnt].getColor();
      if(color.equals(Color.GREEN)){
        //Darken corresponding pixel in the destination.
        color = destPixels[cnt].getColor();
        destPixels[cnt].setColor(color.darker());
      }else{
        //Apply a red tint to the corresponding pixel in
        // the destination.
        color = destPixels[cnt].getColor();
        red = color.getRed();
        if(red*1.25 < 255){
          red = (int)(red * 1.25);
        }else{
          red = 255;
        }//end else
        green = (int)(color.getGreen() * 0.8);
        blue = (int)(color.getBlue() * 0.8);
        destPixels[cnt].setColor(new Color(red,green,blue));
      }//end else
    }//end for loop
  }//end darkenBackground

  private void greenScreenDraw(
                           Picture source,
                           Picture dest,
                           //Place the upper-left corner
                           // of the source image at the
                           // following location in the
                           // destination image.
                           int destX,
                           int destY){
    int width = source.getWidth();
    int height = source.getHeight();
    Pixel pixel = null;
    Color color = null;

    for(int row = 0;row < height;row++){
      for(int col = 0;col < width;col++){
        color = source.getPixel(col,row).getColor();
        if(!(color.equals(Color.GREEN))){
          pixel = dest.getPixel(destX + col,destY + row);
          pixel.setColor(color);
        }//end if
      }//end inner loop
    }//end outer loop

  }//end greenScreenDraw
  //----------------------------------------------------//

  //Crops a Picture object to the given width and height
  // with the upper-left corner located at startCol and
  // startRow.
  private Picture crop(Picture pic,int startCol,
                                   int startRow,
                                   int width,
                                   int height){
    Picture output = new Picture(width,height);

    int colOut = 0;
    int rowOut = 0;
    int col = 0;
    int row = 0;
    Pixel pixel = null;
    Color color = null;
    for(col = startCol;col < startCol+width;col++){
      for(row = startRow;row < startRow+height;row++){
        color = pic.getPixel(col,row).getColor();
        pixel = output.getPixel(colOut,rowOut);
        pixel.setColor(color);
        rowOut++;
      }//end inner loop
      rowOut = 0;
      colOut++;
    }//end outer loop
    return output;
  }//end crop

}//end class Prob04Runner

导入java.awt.Color；
公共类问题04{
公共静态void main（字符串[]args）{
Prob04Runner obj=新的Prob04Runner（）；
obj.run（）；
}//端干管
}//终级Prob04
//======================================================//
类Prob04Runner{
public Prob04Runner（）{//构造函数
System.out.println（“在此处显示您的姓名”）；
}//端构造函数
//----------------------------------------------------//
公开募捐{
Picture skater=新图片（“Prob04a.bmp”）；
溜冰者。探索（）；
skater=作物（skater，6,59392293）；
图片孔=新图片（“Prob04b.bmp”）；
hole.explore（）；
孔=作物（孔，6,59392293）；
图片雪景=新图片（“Prob04c.jpg”）；
雪景。探索（）；
雪景=作物（雪景，659392293）；
//使雪场景中的所有像素变暗，除了
//对于那些在洞的位置上的。制作它们
//更明亮。
黑暗背景（洞穴、雪景）；
//给滑冰者涂上红色
雷丁（滑冰者）；
//在雪景上画滑冰者。
Greenscrendraw（滑冰者，雪景，0,0）；
//在最终输出上显示学生姓名，并
//显示它。
addMessage（“在此处显示您的姓名。”，10,15）；
雪景。探索（）；
System.out.println（雪景）；
}//末端运行法
//----------------------------------------------------//
//假设源具有纯绿色背景。
//对每个非纯像素应用红色色调
//绿色。
私人空白红色调（图片pic）{
像素[]像素=pic.getPixels（）；
颜色=空；
红色整数=0；
绿色整数=0；
int蓝色=0；
对于（int-cnt=0；cnt