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INTRODUCTION
• Java is a true OO language
• the underlying structure of all Java programs is classes.
• Everything must be encapsulated in a class
• that defines the “state” and “behaviour” of each object made from
the class.
• A class essentially serves as a template for an object and
behaves like a basic data type, e.g., “int”.
• We are, in essence, defining a type, along with all the functions
and properties we want associated with the type we’re defining.
• It is therefore important to understand:
• how the properties (fields) and methods are defined in a class
• how they are used to build a Java program
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CLASSES
• Contains definition for all Objects of the same type
• defines the data types and names of properties
• defines methods that define behavior
• the “template” for all Object instances of the same type
Class Name: Circle
A class template
Data Fields:
radius is _______
Methods:
getArea
Circle Object 1
Circle Object 2
Circle Object 3
Data Fields:
radius is 10
Data Fields:
radius is 25
Data Fields:
radius is 125
Three objects of
the Circle class
CLASSES
• The basic syntax for a class definition:
class ClassName {
[fields declaration]
[methods declaration]
}
• Bare bone class – no fields, no methods
public class Circle {
// my circle class
}
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CLASSES
class Circle {
/** The radius of this circle */
double radius = 1.0;
/** Construct a circle object */
Circle() {
}
Data field
Constructors
/** Construct a circle object */
Circle(double newRadius) {
radius = newRadius;
}
/** Return the area of this circle */
double getArea() {
return radius * radius * Math.PI;
}
}
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Method
ADDING PROPERTIES (FIELDS):
• Add fields
public class Circle {
double radius; // radius of the circle
double x, y; // center coordinate
double circ; // circumference
}
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CONSTRUCTORS
Constructors are invoked when we first make
an object (a variable) of the type of the Class.
public class Circle {
double radius;
Circle() { #What happens when the code says new
radius = 1.0;
}
Circle(double newRadius) {
radius = newRadius;
}
}
Notice: no return value – these are not methods!
These are constructors. They make a Circle object.
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ADDING METHODS
• A class with only data fields has no life.
• Objects created from such a class cannot respond to
anything.
• Methods are declared inside the body of the class
and after the declaration of data fields and the
constructors.
• The general form of a method declaration is: (just like
a function, only it belongs to a certain class)
type MethodName (parameter-list)
{
Method-body;
}
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ADDING METHODS TO CLASS CIRCLE
public class Circle {
double radius; // radius of circle
double x, y; // center of the circle
// …Constructors go here
//Methods to return circumference and area
public double circumference() {
return (2*Math.PI*radius);
}
public double area() {
return (Math.PI * Math.pow(radius,2));
}
}
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Method Body
ALL TOGETHER
public class Circle {
double radius;
Circle() {
radius = 1.0;
}
Circle(double d) {
radius = d;
}
public double getArea() {
return(Math.pow(radius,2.0) * Math.PI);
}
public static void main(String[] args) {
Circle c = new Circle(3.2);
System.out.println(c.getArea());
}
}
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Method Body
DATA ABSTRACTION
• Declare the Circle class, have created a new data type –
Data Abstraction
• Can define variables (objects) of that type:
Circle aCircle;
Circle bCircle;
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CREATING CIRCLE OBJECTS CONT.
Circle aCircle;
Circle bCircle;
• aCircle, bCircle simply refers to a Circle object, not an object
itself. (e.g., this is going to be a Circle, but it isn’t yet.)
aCircle
null
Points to nothing (Null Reference)
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bCircle
null
Points to nothing (Null Reference)
CREATING OBJECTS OF A CLASS
• Objects are created using the new keyword.
• aCircle and bCircle refer to Circle objects
aCircle = new Circle() ;
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bCircle = new Circle() ;
CREATING OBJECTS OF A CLASS
aCircle = new Circle();
bCircle = new Circle() ;
bCircle = aCircle;
Before Assignment
aCircle
P
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bCircle
Q
After Assignment
aCircle
P
bCircle
Q
AUTOMATIC GARBAGE COLLECTION
aCircle
P
• The object
•
Q
bCircle
Q
does not have a reference and cannot be used in future.
The object becomes a candidate for automatic garbage collection.
• Java automatically collects garbage periodically and releases the memory used to
be used in the future.
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ACCESSING OBJECT/CIRCLE DATA
ObjectName.VariableName
ObjectName.MethodName(parameter-list)
Circle aCircle = new Circle();
aCircle.x = 2.0; // initialize center and radius
aCircle.y = 2.0;
aCircle.radius = 1.0;
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EXECUTING METHODS IN OBJECT/CIRCLE
• Using Object Methods:
Gets Circle’s method and runs
it for this particular circle
Circle aCircle = new Circle();
aCircle.r = 1.0;
System.out.println(aCircle.area());
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USING CIRCLE CLASS
// Circle.java: Contains both Circle class and its user class
//Add Circle class code here
class MyMain
{
public static void main(String[] args) {
Circle aCircle;
// creating reference
aCircle = new Circle();
// creating object
aCircle.x = 10;
// assigning value to data field
aCircle.y = 20;
aCircle.radius = 5;
double area = aCircle.area(); // invoking method
double circumf = aCircle.circumference();
System.out.println("Radius="+aCircle.r+" Area="+area);
System.out.println("Radius="+aCircle.r+" Circumference ="+circumf);
}
}
Radius=5.0 Area=78.5
Radius=5.0 Circumference =31.40
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ENCAPSULATION : MAKING STUFF PUBLIC AND
PRIVATE
• An object instance owns its state and behavior
• Java provides access modifiers to define what
code can access an object's state and behavior
• public
• all code can access the tagged state or behavior
• private
• only instances of the enclosing class may access this tagged
state or behavior
• Protected (hold that thought for later…)
public class Circle {
private double rad;
private double circ;
private double area;
EXAMPLE OF PUBLIC VS
PRIVATE:
public Circle() {
rad = 1.0;
setValues();
}
public Circle(double r) {
rad = r;
setValues();
}
private void setValues(){
circ = Math.PI * 2 * rad;
area = Math.PI * Math.pow(rad, 2);
}
public void setrad(double r) {
rad = r;
setValues();
}
public String toString(){
String s = "Radius: "+rad+" Cicumference: "+circ+" Area: "+area;
return s;
}
}
public class Circle {
public double rad;
private double circ;
private double area;
public Circle() {
rad = 1.0;
setValues();
}
WHAT
IF? class Hello2 {
public
public static void main (String[] args) {
Circle c = new Circle(3.2);
System.out.println(c);
c.rad = 100;
System.out.println(c);
}
public Circle(double r) {
}
rad = r;
setValues();
>> Radius: 3.2 Cicumference: 20.1 Area: 32.16
}
>> Radius: 100.0 Cicumference: 20.1 Area: 32.16
private void setValues(){
circ = Math.PI * 2 * rad;
area = Math.PI * Math.pow(rad, 2);
}
public void setrad(double r) {
rad = r;
setValues();
}
public String toString(){
String s = "Radius: "+rad+" Cicumference: "+circ+" Area: "+area;
return s;
}
}
THE RULE:
• All Fields should be PRIVATE!!!!
•
Unless there’s a serious concrete reason why you didn’t make it private
• Whenever possible, your methods should be private
•
Always go for the most protected state you can make things
• You’re in control, and you want to control what other people who might be
using your code have access to.
•
You want to keep everything private unless it is something that will be used by
other people/code that is using your code.