Graphics Giving me NullPointerException in my swings program

Question

hiii
I have used Graphics to draw the histogram of an image on a blank file and then display it. I am getting a NullPointerException at the line where i call getGraphics().
My code is as follows:-
import java.awt.*;
import java.awt.event.*;
import javax.swing.event.*;
import java.awt.image.*;
import javax.swing.*;
public class hist extends JFrame
     JPanel panel = new JPanel(new BorderLayout());
     JSplitPane splitPane = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT);
    JScrollPane PictBox = new JScrollPane();
    JScrollPane HistBox = new JScrollPane();
     String ImageName = "pug.jpg"; //Enter Image to be opened here
     Image image = Toolkit.getDefaultToolkit().getImage(ImageName);
     Image histImage;     
     public hist()
          super("HISTOGRAM OF AN IMAGE");
          getContentPane().add(panel);
          panel.add(splitPane, BorderLayout.CENTER);
        splitPane.setResizeWeight(0.5); // Set the divider in the center
        JLabel originalImage = new JLabel(new ImageIcon(ImageName));
        histImage = createImage(240,320);
        splitPane.setLeftComponent(PictBox);     //this is the original image container
        splitPane.setRightComponent(HistBox);   //this is the Histogram image container
        PictBox.getViewport().add(originalImage);     //Image is added here
        getHistogram();
        displayNewImage();
     public void getHistogram()
          PixelGrabber grabber;
          int width=0,height=0;
          int i,j;
          int sizeofImage=0; //Original Dimensions
          int dataCopy[],data[];
          int ThreeDimImage[][][],ThreeDimImage1[][][];
//STEP 1:- FETCH THE PIXELS TO WORK WITH      
        try
              grabber = new PixelGrabber(image, 0, 0, -1, -1, true);
              if (grabber.grabPixels())
                    width = grabber.getWidth();
                     height = grabber.getHeight();
                     sizeofImage = width * height;
                data = (int[]) grabber.getPixels(); //Pixel values stored in array data
                //Next I make a copy of array data[] as array dataCopy[]
                dataCopy = new int[sizeofImage];
                for(i=0; i<sizeofImage; i++)
                     dataCopy[i] = data;          
//STEP 2:- CONVERT PIXELS FROM 1d ARRAY to 3d ARRAY
     //Create a 3d Array to store Alpha-Red-Green-Blue, storing a byte for each.
     //Remaining 24 bits of each integer set to zero.
     ThreeDimImage = new int[height][width][4];
//      ThreeDimImage1 = new int[height][width][4];
     //Next we perform transfer of actual RGB data from the packed 1d array, to the 3d array.
     int element = 0;
     for(i=0; i<height; i++)
          for(j=0; j<width; j++)
               ThreeDimImage[i][j][0] = (dataCopy[element] >> 24) & 0xFF; //ALPHA COMPONENT          
               ThreeDimImage[i][j][1] = (dataCopy[element] >> 16) & 0xFF; //RED COMPONENT                    
               ThreeDimImage[i][j][2] = (dataCopy[element] >> 8 ) & 0xFF; //GREEN COMPONENT                    
               ThreeDimImage[i][j][3] = (dataCopy[element] ) & 0xFF;     //BLUE COMPONENT          
               element++;
//DRAWING THE HISTOGRAM
               Graphics g = histImage.getGraphics();
               g.setColor(Color.white);
               g.fillRect(0,0,320,240);
               g.setColor(Color.black);
               g.drawRect(10,10,310,230);
               g.setColor(Color.red);
               int xCoord=230;
               int yCoord=10;
               int[] grayLevelImage = new int[height*width];
               int histogram[] = new int[256];
               element=0;
               for(i=0; i<height; i++)
          for(j=0; j<width; j++)
               grayLevelImage[element] = (int)(
                                                       (0.30 * ThreeDimImage[i][j][1]) +
                                                       (0.59 * ThreeDimImage[i][j][2]) +
                                                       (0.11 * ThreeDimImage[i][j][3])
               element++;
     for(i=0;i<256;i++)
          histogram[i]=0;     
     int currentpixel=0;
     for(i=0;i<sizeofImage;i++)
          currentpixel = grayLevelImage[i];
          histogram[currentpixel]++;
     int valueToPlot=0;
     for(i=0;i<256;i++)
          if(histogram[i]==0)
               valueToPlot=0;     //If ther is no pixel of the concernd intensity then print nothn on the histogrm
          else     
               valueToPlot = (histogram[i]/334)+2;     //otherwise print atleast a 2 pixel tall line
          g.drawLine(xCoord,yCoord,xCoord,(yCoord - valueToPlot));     
          xCoord++;
     g.dispose();
     /*element=0;
     for(i=0; i<height; i++)
          for(j=0; j<width; j++)
               ThreeDimImage1[i][j][0] = 255;
               ThreeDimImage1[i][j][1] = grayLevelImage[element];
               ThreeDimImage1[i][j][2] = grayLevelImage[element];
               ThreeDimImage1[i][j][3] = grayLevelImage[element];
               element++;
     //PART 4 - CONVERT PROCESSED 3d ARRAY BACK TO 1d ARRAY
               element = 0;
               for( i=0; i<height; i++)
                    for( j=0; j<width; j++)
                         dataCopy[element] = (
                                                  ((ThreeDimImage1[i][j][0] << 24) & 0xFF000000) |
                         ((ThreeDimImage1[i][j][1] << 16) & 0x00FF0000) |
                         ((ThreeDimImage1[i][j][2] << 8) & 0x0000FF00) |
                         ((ThreeDimImage1[i][j][3]) & 0x000000FF)
                         element++;     
//PART 5:- REGENERATE A MODIFIED IMAGE
               histImage = createImage( new MemoryImageSource(width,height,dataCopy,0,width) );
catch (InterruptedException e1)
     e1.printStackTrace();
//DISPLAY NEW IMAGE     
     public void displayNewImage()
          //FINALLY DISPLAY THE NEW IMAGE
          JLabel histogram = new JLabel(new ImageIcon(histImage));
          HistBox.getViewport().add(histogram);     //Inverted image is added here
     public static void main (String[] args)
          hist h1 = new hist();
          h1.setSize(1000,700);
          h1.setExtendedState(MAXIMIZED_BOTH);
          h1.setVisible(true);

Answer 1

As I said! You do your drawing in the paintComponent(Graphics g) method of some component. Something along the lines of this example
import javax.swing.*;
import java.awt.*;
import java.util.*;
public class TestColors extends JFrame
    public TestColors()
        super("Colors");
        final JPanel panel = new JPanel()
                setPreferredSize(new Dimension(640, 480));
                setBackground(new Color(0xffffdf));
            public void paintComponent(Graphics g)
                super.paintComponent(g);
                final int r = 20;
                final int r2 = r/4;
                final int w = getWidth() - r - 1;
                final int h = getHeight() - r - 1;
                final Random random = new Random(12321);
                for (int i = 0; i < 1000; i++)
                    g.setColor(new Color(random.nextFloat(), random.nextFloat(), random.nextFloat()));
                    final int x = random.nextInt(w);
                    final int y = random.nextInt(h);
                    for (int j = 0; j < r2; j++)
                        g.drawRect(x+j, y+j, r-j-j, r-j-j);
        getContentPane().add(new CompassPoint(), BorderLayout.NORTH);
        getContentPane().add(new CompassPoint(), BorderLayout.EAST);
        getContentPane().add(panel, BorderLayout.CENTER);
        getContentPane().add(new CompassPoint(), BorderLayout.WEST);
        getContentPane().add(new CompassPoint(), BorderLayout.SOUTH);
        pack();
    private static class CompassPoint extends JPanel
        private CompassPoint()
            setBackground(Color.WHITE);
            setPreferredSize(new Dimension(100,100));
        public void paintComponent(Graphics g)
            super.paintComponent(g);
            final int h = getHeight();
            final int w = getWidth();
            final double deltac = 1.0/w;
            for (int x = 0; x < w;)
                final float color = (float)(1.0-deltac*x);
                g.setColor(getColor(color));
                for (int j = 0; j < 1; j++)
                    g.drawLine(x,0,x,h);
                    x++;
        protected Color getColor(float color)
            if (color < 0.5)
                color = (float)(1.0-2.0*color);
            else
                color = (float)(2.0*color-1.0);
            return new Color(color, color, 1.0f);
    public static void main(String[] args)
        TestColors test = new TestColors();
        test.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        test.setLocationRelativeTo(null);
        test.setVisible(true);
}The point is that the paintComponent() method gets called whenever a repaint is required. For example, if you re-size the frame or de-iconize the application then the paintComponent() method gets called so that the content is re-drawn.