1. Lecture 9 Using Objects

2. Remember: 3 Different Kinds of Classes
Application Class – what we've been doing
Has public static void main ( String [] args)
Instantiable Class – use to make objects from
String, Scanner
Classes with only static utility methods
like Math class, Coin class, WordUtil class

3. Today—Instantiable Classes
Instantiable Class – used to make objects from
SO FAR ONLY: String, Scanner
To use an Instantiable class we must
Declare a variable of that class type
String message; Scanner myUserInput;
Instantiate a new object
message = new String("hello"); // message="hello“ is OK
myUserInput = new Scanner(System.in);
Use associated methods (name of object '.' method)
message.trim(); myUserInput.nextInt();

4. Instantiable Classes make Objects
Objects are more complex than primitives
Primitive values are just pieces of data (numbers, characters, or true/false)
For example, 2.3 is a value of type double
Instances of classes (objects), on the other hand, contain both data and ways of acting on that data
For example, a Point object has an x and y coordinate, but also comes with a way to translate the point by giving the amount of change desired in the x and y coordinates

7. Review: Primitives int x; x = 3; int y=x; x = y+2; y = x
Review: Primitives int x; x = 3; int y=x; x = y+2; y = x*3; double z; z = 3.45; z = z + x;

9. Object (reference) variables

12. Demo: Point Class Represents a point in 2D
Used in lots of computer graphics
Stores the x and y value
Has methods to modify
point values

13. Example: Objects of class Point
(download demo file)
Point myPoint = new Point(2,-3);
//This declares the variable origin, and sets it to (0,0)
Point origin = new Point();
//At this point in time, origin is (0,0)
//The next line causes origin to change itself to 7,12
origin.translate(7, 12);
//The next line causes origin to change itself to 10,17
origin.translate(3, 5);
//Prints out: java.awt.Point[x=10,y=17]
System.out.println(origin);

14. Memory Maps – Primitives vs Objects
int x = 24
double y = 34.4
Point p;
p= new Point(14,35);
how to change p to (20,40)?
Point q = new Point(-10,29);
(p contains "null")

15. Recap An instance of a class is called an object
To make a new object, use the new operator
The actions you can invoke on an object are called its methods To use a method, we use dot notation as in the example
To find out the details of how to use a particular class, we use the javadoc documentation

16. Lab: Particle Class Represents a rock or ball subject to gravity
Used in the golf game coming next week
Helps us find out how high ball is after
so many seconds have elapsed
The rock started meters off the ground. After 2.63 seconds, the rock was meters off the ground.