introduction to objects a way to create our own types
TRANSCRIPT
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 reuseif ((a >= b) && (a>=c)) max = aelse if ((b >= a) && (b>=c)) max = belse 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 it’s size
• Move it
• Ask it for it’s 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 it’s 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 ofeach circle
Circle()
class Circle {
double radius;
Circle(double r) { radius = r; }
double Area() { double thisarea = radius*radius*Math.PI; return thisarea; }
}
A method named Areathat will calculate thearea 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 constructorUsed to initialize the circleLet’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 REFERENCELike having a telephone number for a friend.. a means to find them. Butthis one is a contact without a number.
What’s the difference?
• Circle circle1 = new Circle(10);
• Circle circle1;
Creates the objectand defines the reference to the objectIn this case, circle1 actually refers to a realCircle.
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()); }
}
314.1562..
706.8583..
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 cannotbe 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!