DrawHistApplet.java - create an instance of the Histogram class and fill it with values chosen from a Gaussian random number generator. Then create an instance of HistPanel and add it to the content pane.

+ New classes:

HistPanel.java - PlotPanel subclass that displays instances of Histogram. It draws retangles to represent the bin values as columns. The PlotPanel methods provide the scale information, axes labeling. title, etc.

Histogram.java - our new histogram base class combines the previous BasicHist and BetterHist classes. It includes the basics of histograms such as bin array, over/under flow variables, etc. In subsequent chapters we will create subclasses of this class with additional capabilities.

+ Previous classes:
Chapter 6: Tech: PlotPanel.java, PlotFormat.java

Java Techniques & Other Programming Notes

• The Graphics class methods use integer values for the drawing positions and dimensions.When coordinates depend on floating point calculations there will be  round off errors. These errors can build up in a series of drawing commands that sum rounded off values. In the paintBars() method in HistPanel we reduce the effect by using floating point values to do the position computations before casting to the integer values.
  
  
• Java2D methods offer the option of using floating-point values to reduce the round off problem.
  
  
• The HistPanel class properties include a number of variables for the display settings such as the colors of the frame and so forth. Additional getter/setter methods could be added to allow for access and modifications to these variables.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;

/**
  *  Create an instance of Histogram, add a Gaussian generated
  *  distribution of data to it and display it.
 **/
public class DrawHistApplet extends JApplet
{
  Histogram fHistogram = null;

  public void init ()  {

    Container content_pane = getContentPane ();

    // Create a histogram with Gaussian distribution.
    makeHist ();

    // Create an instance of a JPanel sub-class
    HistPanel hist_panel = new HistPanel (fHistogram);

    // And add one or more panels to the JApplet panel.
    content_pane.add (hist_panel);

  } // init

  /**
    *  Create the histogram and a set of data drawn
    *  from a Gaussian random number generator.
   **/
  void makeHist () {
    // Create an instance of the Random class for
    // producing our random values.
    java.util.Random r = new java.util.Random ();

    // Them method nextGaussian in the class Random produces
    // a value centered at 0.0 and a standard deviation
    // of 1.0.

    // Create an instance of our basic gHistogram class.
    // Make it wide enough enough to include most of the
    // gaussian values.
    fHistogram = new Histogram ("Gaussian Distribution", "random values",
                             10,-2.0,2.0);

    // Fill histogram with Gaussian distribution
    for (int i=0; i<100000; i++) {
         double val = r.nextGaussian ();
         fHistogram.add (val);
    }
  } // makeHist

} // class DrawHistApplet

import java.awt.*;
import javax.swing.*;

/**  Display histogram data on a PlotPanel subclass. **/
public class HistPanel extends PlotPanel {

  // Histogram reference
  protected Histogram fHistogram;

  // Histogram bar parameters.
  protected Color fBarLineColor = Color.DARK_GRAY;
  protected Color fBarFillColor = Color.PINK;
  int fGap = 2;  // 2 pixels between bars

  // Fractional margin between highest bar and top frame
  double fTopMargin  = 0.05;

  // Fractional margnin between side bars and frame
  double fSideMargin = 0.01;

  // Arrays to hold numbers for axes scale values
  double [] fXScaleValue;
  double [] fYScaleValue;

  // Number of values on each axis
  int fNumYScaleValues = 2;
  int fNumXScaleValues = 5;

  /** Create the panel with the histogram. **/
  public HistPanel (Histogram histogram) {
    fHistogram = histogram;
    getScaling ();
  }

  /** Switch to a new histogram. **/
  public void setHistogram (Histogram histogram){
    fHistogram = histogram;
    getScaling ();
    repaint ();
  }

  /**
    * Get the values for the scale values on
    * the plot axes.
   **/
  void getScaling () {

    fYScaleValue = new double[fNumYScaleValues];
    // Use lowest value of 0;
    fYScaleValue[0] = 0.0;

    fXScaleValue = new double[fNumXScaleValues];
    // First get the low and high values;
    fXScaleValue[0] = fHistogram.getLo ();
    fXScaleValue[fNumXScaleValues-1] = fHistogram.getHi ();

    // Then calculate the difference between the values
    //  (assumes linear scale)
    double range = fXScaleValue[fNumXScaleValues-1] -
                        fXScaleValue[0];
    double dx = range/(fNumXScaleValues-1);

    // Now set the intermediate scale values.
    for (int i=1; i < (fNumXScaleValues-1); i++) {
        fXScaleValue[i] = i*dx + fXScaleValue[0];
    }

  } // getScaling

  /** Optional bar color settings. **/
  public void setBarColors (Color line, Color fill) {
    if ( line != null) fBarLineColor = line;
    if ( fill != null) fBarFillColor = fill;
  }


  /**
    * Overrides the abstract method in PlotPanel superclass.
    * Draw the vertical bars that represent the bin contents.
    * Draw also the numbers for the scales on the axes.
   **/
  void paintContents (Graphics g) {

    // Get the histogram max value and bin data
    int max_data_value = fHistogram.getMax ();
    int [] bins = fHistogram.getBins ();

    if (max_data_value == 0) return;

    Color old_color=g.getColor ();  // remember color for later

    // Dimensions of the drawing area for the bars
    int side_space =  (int) (fFrameWidth*fSideMargin);
    int draw_width= fFrameWidth - 2 * side_space- (bins.length-1)*fGap;

    int draw_height=  (int)(fFrameHeight* (1.0 - fTopMargin));

    // To avoid build up of round off errors, the bar
    // positions will be calculated from a FP value.
    float step_width= (float)draw_width/(float)bins.length;
    int bar_width = Math.round (step_width);
    step_width += fGap;

    // Scale the bars to the maximum bin value.
    float scale_factor= (float)draw_height/max_data_value;

    int start_x = fFrameX + side_space;
    int start_y = fFrameY + fFrameHeight;

    for (int i=0; i < bins.length; i++) {
        int bar_height =  (int)(bins[i] * scale_factor);

        int bar_x =  (int)(i * step_width) + start_x;

        // Bar drawn from top left corner
        int bar_y= start_y-bar_height;

        g.setColor (fBarLineColor);
        g.drawRect (bar_x,bar_y, bar_width ,bar_height);

        g.setColor (fBarFillColor);
        g.fillRect (bar_x+1, bar_y+1, bar_width-2, bar_height-1);
    }

    // Find the scale value of the full frame height.
    fYScaleValue[1] =  (double) (fFrameHeight/scale_factor);

    // Draw the numbers along the axes.
    drawAxesNumbers (g, fXScaleValue, fYScaleValue);

    g.setColor (old_color); //reset original color

  } // paintContents

  // Methods overriding those in PlotPanel
  String getTitle () {
    return fHistogram.getTitle ();
  }

  String getXLabel () {
    return fHistogram.getXLabel ();
  }

} // class HistPanel

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