Computer Science 15-100 (Sections T & U), Fall 2007
Class Notes, Day 6:   Tue 18-Sep-2007

Logistics

1. Schedule
   • Quiz2 today.
2. Reading
   • L&L Chapter 3:  Using Classes and Objects
     Sections 3.1 - 3.5

Topic Outline:

1. Using Objects
   We already do this!

      Polygon p1 = new Polygon();
      p1.addPoint(10,10);
      p1.addPoint(40,80);
      p1.addPoint(70,10);
   
      Polygon p2 = new Polygon();
      p2.addPoint(10,110);
      p2.addPoint(40,180);
      p2.addPoint(70,110);
   
      page.setColor(Color.blue);
      p1.fill(page);
   
      page.setColor(Color.red);
      p2.fill(page);

2. Aliases:  Two References to the Same Object

      Polygon p1 = new Polygon();
      p1.addPoint(10,10);
      p1.addPoint(40,80);
      p1.addPoint(70,10);
   
      Polygon p2 = new Polygon();
      p2.addPoint(10,110);
      p2.addPoint(40,180);
      p2.addPoint(70,110);
   
      Polygon p = p1;
      p.addPoint(40,30);
   
      page.setColor(Color.blue);
      p1.fill(page);
   
      page.setColor(Color.red);
      p2.fill(page);

3. More on Aliases

      Polygon p1 = new Polygon();
      p1.addPoint(10,10);
      p1.addPoint(40,80);
      p1.addPoint(70,10);
   
      Polygon p2 = new Polygon();
      p2.addPoint(10,110);
      p2.addPoint(40,180);
      p2.addPoint(70,110);
   
      Polygon p = p1;
      p.addPoint(40,30);
   
      p = p2;
      p.addPoint(40,30);
   
      page.setColor(Color.blue);
      p1.fill(page);
   
      page.setColor(Color.red);
      p2.fill(page);

4. The "==" operator

   a)  For integer values

   class MyCode {
     public static void main(String[] args) {
       int a = 2, b = 2, c;
       c = a;
       System.out.println(c == a); // true
       System.out.println(c == b); // true
     }
   }

   b)  For floating point values
   We have already seen the dangers in using "==" with floating point values.  They are approximate, and "==" will not work right with them.  So:  do not use "==" with floating point values (that is, floats and doubles).
   
   c)  For objects

   class MyCode {
     public static void main(String[] args) {
       String a = new String("2"), b = new String("2"), c;
       c = a;
       System.out.println(c == a);      // true
       System.out.println(c == b);      // false
       // we probably meant to do this:
       System.out.println(c.equals(b)); // true
     }
   }

   d)  For objects cached by the compiler

   class MyCode {
     public static void main(String[] args) {
       String a = "2", b = "2", c;
       c = a;
       System.out.println(c == a);      // true
       System.out.println(c == b);      // true!!!
     }
   }

   e)  The Moral of the Story
   * Use "==" without worries for integers and booleans.
   * Do not use "==" with other types (unless you are certain it is appropriate, and then Be Careful!).
    

5. Garbage Collection

   Polygon p1;
   p1 = new Polygon();
   p1 = new Polygon(); // What happened to our first polygon?

6. The String Class and String Methods

   See Figure 3.1 (p. 119)
    

7. Packages

   See Figure 3.2 (p. 123)
    

8. The Random Class and Random Methods
   
   See Figure 3.3 (p. 125)
    
9. The Math Class and Math Methods

   See Figure 3.4 (p. 128)
   
   Note:  The Math class is static.  You do not create instances.  You call the methods by prefacing them with "Math.".  For example:

   import java.util.Random;
   class MyCode {
     public static void main(String[] args) {
        Random random = new Random();
        int a = random.nextInt(30), b = random.nextInt(5);
        System.out.println("a             = " + a);
        System.out.println("b             = " + b);
        System.out.println("Math.max(a,b) = " + Math.max(a,b));
        System.out.println("Math.min(a,b) = " + Math.min(a,b));
        System.out.println("Math.pow(a,b) = " + Math.pow(a,b));
        System.out.println("Math.sqrt(a)  = " + Math.sqrt(a));
        System.out.println("Math.random() = " + Math.random());
        System.out.println("Math.random() = " + Math.random());
        System.out.println("Math.random() = " + Math.random());
     }
   }

Carpe diem!