CSCI 201 Lab 8 -- Using Classes

You'll do this lab with class.

Getting ready

Download the Lab 8 jar file and save it into the directory C:\Files, on Windows, or the directory csci/201, on Linux. To avoid that nasty jGRASP jar bug, type the following commands in the windows labs.

mkdir C:\Files\Lab08
cd C:\Files\Lab08
C:\j2sdk1.4.0_01\bin\jar xfv C:\Files\Lab08.jar

And the following commands in the Linux labs.

mkdir -p ~/csci/201/Lab08
cd ~/csci/201/Lab08
jar xfv ~/csci/201/Lab08.jar

Using the Triangle class

Open the jGRASP project file Forest.gpj that you just extracted. Notice that the project contains an html file Forest.html. This means we'll be developing an applet this week. There's also a Java code file Forest.java. You'll be extending this file. Finally, there a compiled Java bytecode file Triangle.class. You'll use the Triangle class, but you will not modify it. During the course of performing this lab, you'll create a new class called Tree and add it to your project.

The code for the class Triangle of package edu.unca.cs.csci201.LabAids is shown below. Study the comments of this code for a moment. You'll need to use the methods of Triangle to complete this assignment.

package edu.unca.cs.csci201.LabAids;
import java.awt.*;

public class Triangle 
{
  // This class creates an upward-pointing equilateral triangle.
  // Each triangle is specified by the coordinates of its top
  // corner, the length of its sides, and its color.
	
  private Color color;
  private Polygon triangle;
	
  public Triangle(int x, int y, int length, Color c)
  // x and y are the coordinates of the top conner of the triangle,
  // length is the length of its sides, and color is is the fill Color 
  {
    int[] xpoints = {x, x-length/2, x+length/2};
    int[] ypoints = {y, y+(int)(0.866*length), y+(int)(0.866*length)};
    triangle = new Polygon(xpoints, ypoints, 3);
    color = c;
  }
	
  public void draw(Graphics g)
  {
    g.setColor(color);
    g.fillPolygon(triangle);
  }
		
}

Now take a look at the code of Forest, our applet class. The applet, in its incomplete state, contains one instance variable, triangleA. You will want to use that variable when you create the first Triangle object.

In Lab 4 and Lab 6, you designed applets containing a single method paint. The paint method is called whenever the applet needs to display (or redisplay) its windows. The class Forest contains an additional methods init which is called when the applet is first loaded into the browser. Usually init creates objects used by the applet.

Your first task

Now that you are familiar with the setup, create three Triangle objects in your applet and draw them to produce a display similar to the pine tree shown below.

Adding a New Class

Now that you are good at drawing the top of a tree, try drawing a complete tree by adding a tree trunk. Insert a call to the fillRect method of the Graphics object into the applet's Paint() method to do this. The fillRect method should be familiar. You used it in Lab 4. Make the tree trunk grey as shown below.

Now that you can make a tree, why not make a Tree class so that it would be easy to create a whole forest of trees. Add a new class to your project and call it Tree. Make the Tree class public. Since it will use Triangle, you must make sure to import edu.unca.cs.csci201.LabAids.Triangle along with all the classes of the java.awt package.

Now, copy the contents of the Forest class into your Tree class and change the names of its two methods. Make the init method be the Tree class constructor, and change the name of paint to draw. Just making these simple changes should create a Tree class that can be used to draw a single tree. Now modify the Forest applet to create a single Tree object and draw it.

Enhancing the Tree Class

However, what you want to do is draw multiple Tree objects and allow each object to be positioned at a different location and have a different color. In order to do that, we have to introduce additional state variables to the Tree class allowing us to give a unique position, and color to each Tree object. These variables must then be used to position the triangles and rectangle that form the tree, as well as color the triangles.

Make the changes described above to the Tree class, and then draw three trees similar to those shown below.