Transcript chap1_class (editedv3)

INTRODUCTION TO CLASS &
OBJECT – PART 1
INSPIRING CREATIVE AND INNOVATIVE MINDS
Questions are KEYS to Knowledge
•
•
•
•
What is a class?
What are the constituents of a class?
How does class diagram look like?
How to define a class?
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INSPIRING CREATIVE AND INNOVATIVE MINDS
What is a class?
• Class is a structure for creating objects.
• The constituents of a class are;
1.
2.
3.
4.
Name
Data attribute
Constructor
Method
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INSPIRING CREATIVE AND INNOVATIVE MINDS
1st Class Component:Name
• As a standard convention, name is a
noun.
– For example: Circle, Student,
Employee.
• Start with a Capital letter.
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2nd
Class Component: Attribute
• Variable is normally declared as scope private.
• Constant is of scope public.
• Attribute can be of the following type;
– Primitive data type like;
int,double,float,char,short,long,
byte,boolean, OR/AND.
– Class for example;
String, Circle, Employee.
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3rd
Class Component: Constructor
• Is needed to create object using new
operator.
• A class may have more than one
constructor(known as constructor
overloading).
– Non-argument also known as default.
– Constructor with parameter.
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Example of Default Constructor
public Circle(){
radius = 0.0;
}
If we forget to declare a constructor in a
class, Java will provide a default
constructor.
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Example of Constructor with Parameter
public Circle(double r){
radius = r;
}
parameter
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4th
Class Component: Methods
• Denotes behaviours of a class.
• Methods serve 3 purposes;
1. public interface to variables of scope
private .
i. Accessor
ii. Mutator
2. Manipulation of attributes to produce
output, may use a formula.
3. Display attributes and output.
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Accessor Method
• The method does not have a parameter
but return a value stored inside a
variable, example;
public double getRadius(){
return radius;
}
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Mutator Method
• The method receives parameter to be
assigned to a variable, but does not
return any value, example;
public void setRadius(double r){
radius = r;
}
parameter
No value is return
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Summary
• Class is a structure for creating objects. It has
4 items;
1.
2.
3.
Name
Attribute
Constructor;
i.
ii.
4.
Default
With parameter
Methods
-
Public Interface (Mutator method is used to set value to attribute,
Accessor method is used to access value of the attribute.
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INTRODUCTION TO CLASS &
OBJECT – PART 2
INSPIRING CREATIVE AND INNOVATIVE MINDS
What is a Class?
• Class is a structure for creating objects.
• The constituents of a class are;
1.
2.
3.
4.
Name
Data attribute
Constructor
Method; 4 types
i.
ii.
iii.
iv.
Mutator
Acessor
manipulate attributes/output producing
display
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INSPIRING CREATIVE AND INNOVATIVE MINDS
Method to Manipulate Attributes
• This method will utilize attributes to
produce a result.
• Normally mathematical formula is used
• Example;
public double calcArea(){
return(PI*radius*radius);
}
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Method to DISPLAY
• Attributes and output must be displayed
to the user.
• Example;
public void display(){
System.out.println("Radius of a
circle:"+radius);
System.out.println("Area of the
circle:"+calcArea());
}
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Class Diagram
Class Name
Attribute
Constructor
Methods
Circle
-radius:double
+Circle()
+Circle(double r)
+getRadius():double
+setRadius(double r):void
+calcArea():double
+display():void
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Class Definition Sequence
1.
2.
3.
4.
Name of Class
Attribute
Constructor
Methods
i. Accessor and Mutator
ii. Output producing Method
iii. Display Method
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Methods
public class Circle {
Name of class
private double radius;
Attributes
public double final PI = 3.13;
public Circle(){
radius = 0.0;
}
Constructor
public Circle(double r){
radius = r;}
public void setRadius(double r){
radius = r;
}
public double getRadius(){
return radius;
}
public double calcArea(){
return(PI*radius*radius);
}
public void display(){
System.out.println("Radius of a circle:"+radius);
System.out.println("Area of the circle:"+calcArea());
}
}
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Save as;
• When we have completed the definition of the
class, we may save it as “Circle.java”.
• We need to compile it to remove syntax error.
• When we run it, no output is generated, WHY
DO YOU THINK?
• Yes, we have not create object from the class
yet.
• HOW to create objects? – we’ll learn it in the
next session.
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Summary
• Sequence to define a class is based on it’s
structure.
• Class Diagram consists of 3 sections;
Name
-Attribute
+Constructor
+Method
-: scope private
+: scope public
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INTRODUCTION TO CLASS &
OBJECT – PART 3
INSPIRING CREATIVE AND INNOVATIVE MINDS
Create Object using New Operator
• We develop another class that consists
of main() method to create object;
public class TestCircle
{
public static void main(String[] args)
{
// use constructor with parameter
Circle obj1 = new Circle(2.5);
// use default constructor
Circle obj2 = new Circle();
}
}
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Invoke Method using (.) Operator
public class TestCircle {
public static void main(String[] args){
Circle obj1 = new Circle(2.3);
Circle obj2 = new Circle();
Call method setRadius()
obj2.setRadius(5.6);
obj2.display();
Call method display()
obj1.display();
}
}
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What Next
• Save the class as “TestCircle.java”.
• Compile using command: javac
TestCircle.java
• Run the class/bytecode using command:
Java TestCircle
Output produced;
Radius of a circle:5.6
Area of the circle:98.4704
Radius of a circle:2.3
Area of the circle:16.610599999999998 25
Summary
• Create object using new operator in a
main() method.
• Call members of class, example method
using dot(.) operator.
• We can create as many objects as we
desired.
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