FallingObjectApplet.java - rather than sending print output to the console as in the examples up till this chapter, we send print string output to a text area in the graphical user interface. Algorithm is same as FallingObject_Applet1.java in Chapter 2: Physics.

+ previous classes:
Ch. 6: Java: TextOutputPanel.java, Outputable.java

Java Techniques & Other Programming Notes

• This program illustrates the use of an interface, Outputable in this case, as a callback mechanism. Passing Outputable references to method A allows the method to execute print operations without needing to know where the print output actuallly goes. That depends upon the particular methods that invoke method A.
  
  
• We use a JTextArea for displaying text output.
  
  
• The JTextArea also comes in a JScrollPane to provide scrollbars when the text exceeds the available area.

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

/**
  *  Same as FallingObject_Applet1 class from chapter 2 except
  *  that output goes to a text area displayed on a GUI. We
  *  use Swing components including JApplet and JTextArea.
  *
  *  The TextOutputPanel is a subclass of JPanel. It holds a
  *  JTextArea in a JScrollPane area. It also implements our
  *  Outputable interface that provides print (String) and
  *  println (String) methods similar to those in System.out
  *  but sends the strings to the TextOutputPanel instead of
  *  to the console.
**/
public class FallingObjectApplet extends JApplet
{
  /** Set up the UI panel for displaying the text output of
    * the algorithm. **/
  public void init () {

    TextOutputPanel output_panel = new TextOutputPanel ();

    // Add panel with JTextArea to show output
    add (output_panel);
    compute (output_panel);

  } // init

  void compute (Outputable out) {

    // Variables
    double t, y, vy;
    double total_t;
    int n;

    // Constants
    double y0 =  0.0;
    double v0 =  0.0;
    double g  = -9.80;// meter per sec**2

    // Version 1
    double dt =  0.1;
    int n_steps = 10;
    y = y0;
    vy = v0;

    for (n=0; n < n_steps; n++) {
      y = y + vy * dt;
      vy = vy + g * dt;
    }

    total_t = n * dt;

    out.println ("Version 1, dt=0.1 ");
    out.println ("Time = " + total_t);
    out.println ("y = " + y + ", vy = " + vy);
    out.println ("");

    // Version 2
    dt =  0.01;
    n_steps = 100;
    y = y0;
    vy = v0;

    for (n=0; n< n_steps; n++) {
      y = y + vy * dt;
      vy = vy + g * dt;
    }

    total_t = n * dt;

    out.println ("Version 2, dt=0.01 ");
    out.println ("Time = " + total_t);
    out.println ("y = " + y + ", vy = " + vy);
    out.println ("");

    // Version 3
    dt =  0.001;
    n_steps = 1000;
    y = y0;
    vy = v0;

    for ( n=0; n< n_steps; n++) {
      y = y + vy * dt;
      vy = vy + g * dt;
    }

    total_t = n * dt;

    out.println ("Version 3, dt=0.001 ");
    out.println ("Time = "+ total_t);
    out.println ( + y + ", vy = " + vy);
    out.println ("");

    // Version 40
    dt =  0.0001;
    n_steps = 10000;
    y = y0;
    vy = v0;

    for ( n=0; n< n_steps; n++) {
      y = y + vy * dt;
      vy = vy + g * dt;
    }

    total_t = n * dt;

    out.println ("Version 4, dt=0.0001 ");
    out.println ("Time = "+ total_t);
    out.println ( + y + ", vy = " + vy);
    out.println ("");


    vy = v0 + g;
    y = y0 + 0.5 * g;
    out.println ("Formulas for tTotal = 1.0sec");
    out.println ( + y + ", vy = " + vy);

  } // compute

} // FallingObjectApplet

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