1. Lec 10 Using Objects II

2. Agenda Review objects and classes Using static variables of a class
Demo the Island class
moving on an island
declaring several islands
TreasureHunter game design

3. Review: Classes and Objects

4. Object (reference) variables

6. Memory Maps – Primitives vs Objects
int x = 24
double y = 34.4
Point p;
p= new Point(14,35);
Point q = new Point(-10,29);
(p contains "null")

7. Notice Field Summary in Point Class
both x and y are declared int: means they can change. These are called "instance variables"
Point q = new Point(-10,29); // construct a pt
q.translate(20,30); // move a pt

8. Access Restricted! Not allowed direct access to fields x and y
Point q = new Point(-10,29); // construct a pt
q.x = 5; // illegal attempt to access x
System.out.println(q.x); // illegal too.
Users must use method:
q.translate(20,30); // move a pt
q.setX( 5 ); // what we tried above
System.out.println( q.getX() ); // print out pt

9. Unless the fields are declared "static"
Check out Math class: PI and E
Area = Math.PI * r * r; // area of circle radius r
z = Math.pow( Math.E, 4 ); // e raised to 4th pwr
static fields represent unchanging values in a class
refer to them by "Class Name" <dot> Field Name
just like static methods of a Class
tied to Class not instances of the Class

10. More Static Fields Color class holds three values R,G,B
Note the static fields for different Color names
We can declare a color and initialize
Color c = new Color(0,0,255);
We can check if a color is "Blue"
if ( c.equals(Color.BLUE) { // and so on...

11. Today's Lab: Island Class
Island class is a world of fun
Create an island
Island desert = new Island(5); // 5x5 playing grid
Check location:
...println("I'm at: " + desert.currentLocation() );

12. "I'm at 0" ??? Not very helpful. Try:
if ( desert.currentLocation() == Island.PIRATE){ System.out.println( "location is: Pirate! "); }
Note the use of static variables for readability

13. TreatureHunt.java make Lab10 folder, TreasureHunt class
Download Island.class into Lab10 folder
Read problem statement,
Have a design discussion with a partner