Lab4-Software-Construction-BSSE5.pptx ppt

1
Software Construction
Lab 4
Classes and Objects in Java
Object Oriented Design
Implementation in Java

2
Contents
 Implementing classes and objects in Java.
 Defining different variables, constructors and other
methods
 Understand how some of the OO concepts learnt so
far are supported in Java.
 Implementing OO concepts in java.

3
Defining a Constructor: Example
public class Counter {
int CounterIndex;
// Constructor
public Counter() {}
public Counter(int x) {
CounterIndex = x;
}
//Methods to update or access counter
public void increase()
{
// CounterIndex = CounterIndex + 1;
CounterIndex++;
}
public void decrease()
{
CounterIndex = CounterIndex - 1;
}
int getCounterIndex()
{
return CounterIndex;
}
}

4
Trace counter value at each
statement and What is the output ?
class MyClass {
public static void main(String args[])
{
Counter counter1 = new Counter(1);
counter1.increase();
int a = counter1.getCounterIndex();
int b = counter1.getCounterIndex();
if ( a > b )
else
counter1.decrease();
System.out.println(counter1.getCounterIndex());
}
}

5
A Counter with User Supplied Initial
Value ?
 This can be done by adding another
constructor method to the class.
public class Counter {
int CounterIndex;
// Constructor 1
public Counter()
{
CounterIndex = 0;
}
public Counter(int InitValue )
{
CounterIndex = InitValue;
}
}
// A New User Class: Utilising both constructors
Counter counter1 = new Counter();
Counter counter2 = new Counter (10);

6
Adding a Multiple-Parameters
Constructor to our Circle Class
public class Circle {
public double x,y,r;
// Constructor
public Circle(double centreX, double centreY,
double radius)
{
x = centreX;
y = centreY;
r = radius;
}
//Methods to return circumference and area
public double circumference() { return 2*3.14*r; }
public double area() { return 3.14 * r * r; }
}

7
Constructors initialise Objects
 Recall the following OLD Code
Segment:
Circle aCircle = new Circle();
aCircle.x = 10.0; // initialize center and radius
aCircle.y = 20.0
aCircle.r = 5.0;
aCircle = new Circle() ;
At creation time the center and
radius are not defined.
These values are explicitly set
later.

8
Constructors initialise Objects
 With defined constructor
Circle aCircle = new Circle(10.0, 20.0, 5.0);
aCircle = new Circle(10.0, 20.0, 5.0) ;
aCircle is created with center (10, 20)
and radius 5

9
Multiple Constructors
 Sometimes want to initialize in a
number of different ways, depending
on circumstance.
 This can be supported by having
multiple constructors having different
input arguments.

10
Multiple Constructors
public double x,y,r; //instance variables
// Constructors
public Circle(double centreX, double cenreY, double
radius) {
x = centreX; y = centreY; r = radius;
}
public Circle(double radius) { x=0; y=0; r = radius; }
public Circle() { x=0; y=0; r=1.0; }
public double circumference() { return 2*3.14*r; }
public double area() { return 3.14 * r * r; }
}

11
Initializing with constructors
public class TestCircles {
public static void main(String args[]){
Circle circleA = new Circle( 10.0, 12.0,
20.0);
Circle circleB = new Circle(10.0);
Circle circleC = new Circle();
}
}
circleA = new Circle(10, 12, 20) circleB = new Circle(10)
Centre = (0,0)
Radius=10
circleC = new Circle()
Centre = (0,0)
Radius = 1
Centre = (10,12)
Radius = 20

12
Method Overloading
 Constructors all have the same name.
 Methods are distinguished by their
signature:
 name
 number of arguments
 type of arguments
 position of arguments
 That means, a class can also have multiple
usual methods with the same name.
 Not to confuse with method overriding
(coming up), method overloading:

13
Polymorphism
 Allows a single method or operator
associated with different meaning
depending on the type of data passed to it. It
can be realised through:
 Method Overloading
 Operator Overloading (Supported in C++, but not
in Java)
 Defining the same method with different
argument types (method overloading) -
polymorphism.
 The method body can have different logic
depending on the date type of arguments.

14
Scenario
 A Program needs to find a maximum of two
numbers or Strings. Write a separate
function for each operation.

In C:

int max_int(int a, int b)

int max_string(char *s1, char *s2)

max_int (10, 5) or max_string (“melbourne”, “sydney”)

In Java:

int max(int a, int b)

int max(String s1, String s2)

max(10, 5) or max(“melbourne”, “sydney”)

Which is better ? Readability and intuitive wise ?

15
A Program with Method
Overloading
// Compare.java: a class comparing
different items
class Compare {
static int max(int a, int b)
{
if( a > b)
return a;
else
return b;
}
static String max(String a, String b)
{
if( a.compareTo (b) > 0)
return a;
else
return b;
}
{
String s1 = “Islamabad";
String s2 = “Lahore";
String s3 = “Karachi";
int a = 10;
int b = 20;
System.out.println(max(a, b));
System.out.println(max(s1, s2));
System.out.println(max(s1, s3)); }
}

16
The New this keyword
 this keyword can be used to refer to the object
itself.
It is generally used for accessing class members
(from its own methods) when they have the same
name as those passed as arguments.
// Constructor
public Circle (double x, double y, double r) {
this.x = x;
this.y = y;
this.r = r;
}
}

17
Static Members
 Java supports definition of global methods and
variables that can be accessed without creating
objects of a class. Such members are called Static
members.
 Define a variable by marking with the static
methods.
 This feature is useful when we want to create a
variable common to all instances of a class.
 One of the most common example is to have a
variable that could keep a count of how many
objects of a class have been created.
 Note: Java creates only one copy for a static variable
which can be used even if the class is never
instantiated.

18
Static Variables
 Define using static:
 Access with the class name
(ClassName.StatVarName):
// class variable, one for the Circle class, how many circles
public static int numCircles;
//instance variables,one for each instance of a Circle
// Constructors...
}
nCircles = Circle.numCircles;

19
Static Variables - Example
 Using static variables:
// class variable, one for the Circle class, how many circles
private static int numCircles = 0;
private double x,y,r;
// Constructors...
Circle (double x, double y, double r){
this.x = x;
this.y = y;
this.r = r;
numCircles++;
}
}

20
Class Variables - Example
 Using static variables:
public class CountCircles {
public static void main(String args[]){
Circle circleA = new Circle( 10, 12, 20); //
numCircles = 1
Circle circleB = new Circle( 5, 3, 10); //
numCircles = 2
}
}
circleA = new Circle(10, 12, 20) circleB = new Circle(5, 3, 10)
numCircles

Accessing variables from another class - Example
class AccessVariableFromAnotherClass{
//Instance variable
int a = 10;
//Instance constant
final int c = 30;
//Class variable shared to all objects
static int b = 20;
}
21

Accessing variables from another class - Example
public class MainMethodClassForAcessVariable{
public static void main(String[] args){
AccessVariableFromAnotherClass avfac = new AccessVariableFromAnotherClass();
/* Instance variable can access with object */
System.out.println("Access Instance Variable like this :"+avfac.a);
/* we can access static variable from two types */
/* 1. we can access static variable with the help of object */
System.out.println("Access Static Variable like this :" +" " +avfac.b);
/* 2. we can access static variable with the help of classname */
System.out.println("Access Static Variable like this :" +" " +AccessVariableFromAnotherClass.b);
/* we cannot access static variable directly */
//System.out.println(b);
/* we can access final variable with object */
System.out.println("Access Final Variable like this :" +" " +avfac.c);
/* we cannot access final variable directly */
//System.out.println(c);
}
}
22

23
Instance Vs Static Variables
 Instance variables : One copy per
object. Every object has its own
instance variable.
 E.g. x, y, r (centre and radius in the circle)
 Static variables : One copy per class.
 E.g. numCircles (total number of circle
objects created)

24
Static Methods
 A class can have methods that are defined as
static (e.g., main method).
 Static methods can be accessed without
using objects. Also, there is NO need to
create objects.
 They are prefixed with keyword “static”
 Static methods are generally used to group
related library functions that don’t depend
on data members of its class. For example,
Math library functions.

25
Comparator class with Static
methods
// Comparator.java: A class with static data items comparision methods
class Comparator {
public static int max(int a, int b)
{
if( a > b)
return a;
else
return b;
}
public static String max(String a, String b)
{
if( a.compareTo (b) > 0)
return a;
else
return b;
}
}
class MyClass {
{
String s1 = "Melbourne";
String s2 = "Sydney";
String s3 = "Adelaide";
int a = 10;
int b = 20;
System.out.println(Comparator.max(a, b)); // which number is big
System.out.println(Comparator.max(s1, s2)); // which city is big
System.out.println(Comparator.max(s1, s3)); // which city is big
}
}
Directly accessed using ClassName (NO Objects)

26
Static methods restrictions
 They can only call other static methods.
 They can only access static data.
 They cannot refer to “this” or “super”
(more later) in anyway.

27
Summary
 Constructors allow seamless initialization of
objects.
 Classes can have multiple methods with the
same name [Overloading]
 Classes can have static members, which
serve as global members of all objects of a
class.
 Keywords: constructors, polymorphism,
method overloading, this, static variables,
static methods.

28
Summary
 Classes, objects, and methods are the basic
components used in Java programming.
 We have discussed:
 How to define a class
 How to create objects
 How to add data fields and methods to classes
 How to access data fields and methods to classes

Lab4-Software-Construction-BSSE5.pptx ppt

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