Introduction to Objects A way to create our own types

Type and Variables Until this point we were only able to create simple types, actually call “primitive” types –integer –double –float –char –String (actually not primitive)

We want to do better Bank Account Math Calculator Stock Car Vending machine Telephone Model “whatever” as software Make a computer act like one

Use type to create a variable Use class to create an object int x; Circle mycircle = new Circle ();

Consider methods Take a known process Package it for reuse if ((a >= b) && (a>=c)) max = a else if ((b >= a) && (b>=c)) max = b else max = c; int maxof3(int a, int b, int c){ int max; if ((a >= b) && (a>=c)) max = a else if ((b >= a) && (b>=c)) max = b else max = c; return max; }

How are object like this? Take a group of methods and data and package those for reuse. a( ) b( ) c( ) int i int k a( ) b( ) c( ) int i int k class Testclass

int i; int j; void a() {…} void b() {…} void c() {…} class TestClass{ int i; int j; void a() {…} void b() {…} void c() {…} }

How do we decide what goes inside? The problem will guide us. The things we put inside will define –What the object will do – How we can interact with it These things will be the “Bank Account”s, “Student”s, etc

Let’s start simple A circle

What are some of the attributes of a circle? Radius (most obvious) Color Border Position

How do we interact with a circle? Change its size Move it Ask it for its area … depending on the problem’s needs

Let’s start with a simple Circle class Just a radius –No borders or colors A means of asking it for its area. This will serve as the basis (a type or class) for creating lots of circles

Circle() class Circle { double radius; Circle(double r) { radius = r; } double Area() { double thisarea = radius*radius*Math.PI; return thisarea; }

Circle() class Circle { double radius; Circle(double r) { radius = r; } double Area() { double this area = radius*radius*Math.PI; } Heading for the class

Circle() class Circle { double radius; Circle(double r) { radius = r; } double Area() { double thisarea = radius*radius*Math.PI; return thisarea; } A property of each circle

Circle() class Circle { double radius; Circle(double r) { radius = r; } double Area() { double thisarea = radius*radius*Math.PI; return thisarea; } A method named Area that will calculate the area of that specific circle

Circle() class Circle { double radius; Circle(double r) { radius = r; } double Area() { double thisarea = radius*radius*Math.PI; return thisarea; } A constructor Used to initialize the circle Let’s see how in the next slide

Creating circles Circle circle1 = new Circle(10); Radius:10 circle1

Creating circles Circle circle1 = new Circle(10); radius:10 circle1 Circle circle2 = new Circle(15); radius:15 circle2

Asking about area Circle circle1 = new Circle(10); System.out.println(circle1.Area()); radius:10 circle1 Circle circle2 = new Circle(15); System.out.println(circle2.Area()); radius:15 circle2 Each circle will return it’s own area

What’s the difference? Circle circle1 = new Circle(10); Circle circle1; Creates a REFERENCE Like having a telephone number for a friend.. a means to find them. But this one is a contact without a number.

What’s the difference? Circle circle1 = new Circle(10); Circle circle1; Creates the object and defines the reference to the object In this case, circle1 actually refers to a real Circle.

Only a reference.. No circle Circle circle1; circle1

A reference with a circle Circle circle1 = new Circle(10); Radius:10 circle1

Put it Together! public class Circle { double radius; Circle(double r) { radius = r; } double Area() { double thisarea = radius*radius*Math.PI; return thisarea; } public static void main(String[ ] args) { Circle circle1 = new Circle(10); System.out.println(circle1.Area()); Circle circle2 = new Circle(15); System.out.println(circle2.Area()); }

Do I need new() ? Can I define a variable and just reference from the main? How about this? … TRY IT… public class Circle() { double radius=5; public static void main(String args[]) { System.out.println(radius); } Error: non-static variable radius cannot be referenced from a static context You never “new()”ed one. No radius exists.

Only a reference.. No circle Circle circle1; circle1

Do I need new() ? Here there is no Area() or radius defined. Because NO new() has occurred! public class Circle() { double radius=5; double Area() { double this area = radius*radius*Math.PI; } public static void main(String args[]) { System.out.println(Area()); } Error : nonstatic method can not be referenced from static method

This one creates the object. Then a radius and Area() exists to use… no errors. public class Circle { double radius; Circle(double r) { radius = r; } double Area() { double thisarea = radius*radius*Math.PI; return thisarea; } public static void main(String args[]) { Circle circle1 = new Circle(10); System.out.println(circle1.Area()); Circle circle2 = new Circle(15); System.out.println(circle2.Area()); } These are created when you new()

public class Circle { double radius; Circle(double r) { radius = r; } double Area() { double thisarea = radius*radius*Math.PI; return thisarea; } public static void main(String args[]) { int i = 5; System.out.println(i); Circle circle1 = new Circle(10); System.out.println(circle1.Area()); } Why can I declare “i” like this in the main, but not radius in the previous example? This is legal!

public class Circle { double radius; Circle(double r) { radius = r; } double Area() { double thisarea = radius*radius*Math.PI; return thisarea; } public static void main(String args[]) { int i = 5; System.out.println(i); Circle circle1 = new Circle(10); System.out.println(circle1.Area()); } Because main is static. Static needs more explanation.

Conclusion: “ new ” before using static is coming!