In this lab we're going to study basics of object-oriented programming:
class declaration, object declaration, and object creation.  Also, we'll
use String and Array types as presented in lecture last period.
Time permitting, we'll do some simple graphics.

For this lab, make a separate directory and keep a file with an entry 
for each exercise.  (It's not necessary to use this file as a template, 
but you may if you wish.)

When lab is over, please mail me your file as a text (ascii) file.
A simple way to do this is: mail -s "lab 2" lorenz <myfile
where "myfile" is the name of your lab file.

If you're working as a pair, please mail two copies of the file from
both accounts.


1) Declaring classes and creating objects.
  After each exercise, make sure your program compiles.

	a) Create a class "Furniture" that extends "Object".  

	b) Extend this class with the subclass "LeggedFurniture".
	
	c) Extend "LeggedFurniture" with two subclasses: "Table" and "Chair"

	d) Enhance the main method in this "Main" class

		class Main {
		     public static void test(LeggedFurniture lf) {
		     }

		     public static void main(String[] args) {
		     }
		}

	   to create an instance of "Furniture", "LeggedFurniture",
	   "Table" and of "Chair".  From main, experiment to find out
           which of the four objects may be passed to the "test" method.

	e) Add a boolean member field to "Furniture" called "canSitOn"
	   initialized to false.

	f) Write a constructor in "Chair" to set "canSitOn" to true.
	   It's signature looks like:
		public Chair() 

	g) After your main method creates the various objects,
	   print out the state of their "canSitOn" fields.

	h) Add an integer field "nLegs" to the "LeggedFurniture" class.
	   Initialize it to 4.

	i) After creating the chair object in main, set its (inherited)
	   "nLegs" field to 3.

	j) Add another constructor to "Chair" that's parameterized by
	   an integer number of legs:
		public Chair(int nLegs)
	   and rewrite main to use this constructor.
	   (Hint: use the "this" keyword to get around name conflicts.)

	k) What happens to a chair's "canSitOn" when it's constructed 
	   via Chair(int) instead of just Chair()?

	l) Declare and create an array of four chairs.  Recall, one must
	   first "new" the array and then "new" the elements in the array.

	m) Add a "static" int field (that is, a class variable) to "Furniture" 
	   called "nPieces" and a constructor to increment it.

	n) After creating the four chairs above, what is "nPieces"?
	  (Recall, class variables are accessed via class names.)
	  What happens if you remove the static declaration?


2) Copy and inspect the following program:

	class X {
	  String str;

	  public X(String s) {
	    str = s;
	  }
    
	  public void doit(X x) {
	    System.out.println("in object "+str+" calling object "+x.str);
	    /** add code here to call x's doit method with the current object
             **/
	  }
	}

	class Main {
	  public static void main(String[] args) {
	    X a = new X("A");
	    X b = new X("B");
	
	    a.doit(b);
	  }
	}


	a) fill in the missing code fragment.  What do you think this program
	   will print out?  Run the program to find out.

	b) ^C interrupts runaway computations...

	c) Add a static variable to X that keeps track of the number of times
	   "doit" has been called and modify "doit" to immediately return
	   when this number exceeds 10.


3) Objects and graphics:
	a) In a new file, create a "Point" class that holds an integer point 
	   (x,y) and a constructor "Point(int x,int y)" to initialize it.

	b) Copy the files from ~lorenz/cs35-2/lab2/ into your directory for
	   this lab.  The file "Draw.java" contains a small program called
	   an Applet that can run in web browsers.  Presuming you've
	   compiled your "Point" class, "Draw.java" should compile to
	   yield "Draw.class".  Load the file "applet.html" into a
	   browser (netscape or hotjava) and you should see the applet
	   draw a line.  Change the line's endpoints (in the "paint" method),
	   recompile, and reload into the browser.  (You may have to restart
	   the browser to reload a class.)

	c) Add a method to "Draw.java" at the comment to draw a rectangle.  
           A rectangle may be specified as a point and a width and height or 
           as two points (upper left and lower right, for example).

	d) Create "Line" and "Rectangle" classes and modify "Draw.java" to
	   use them.