02-advance-methods-on-class jdpgs code .pdf

26
What is Constructor?
• Is a special type of subroutine called to
create an object
• Prepares the new object for use
• Is a public method
• Member method/function
• Name must be the class name
Constructors

Example
27
1. Is a public method
2. Name must be the class
name
1 2
Constructors

Multiple constructors
• You can create multiple constructors, each must accept
different parameters.
– Student(){ }
– Student(int age, String Name) { }
– Student (Student stuob) { }
• If you don't write any constructor, the compiler will (in
effect) write one for you:
– classname() {}
– Student()
• If you include any constructors in a class, the compiler
will not create a default constructor!
29
Constructors

Example-Multiple constructors
30
Constructors

Example-Multiple constructors
31
Constructors

32
Multiple Constructors cont.…
• One constructor can call another.
• Use "this", not the classname:
A call to this must be the
first statement in the
constructor!
Constructors

Summary
• Is a special type of public method called to create
an object
• can create multiple constructors, each must accept
different parameters.
– Student(){ }
– Student(int age, String Name) { }
– Student (Student stuob) { }
• One constructor can call another.
33
Constructors

Example
• Create a Java Class “Student” with following
constructors
– Student();
– Student(Name, Age, IndexNo)
• Create a class name “Length” with following
constructors
– Length()
– Length(Feets, Inches )
34

Example
• Complete the class ‘Date’ with following methods
– Date() // 0,0,0
– Date(day, month, year);
– Date(Date d)
– Input()
– print() // 23.5.2015
– printLong() // 23rd may 2015
7/11/2015 Budditha Hettige (budditha@yahoo.com) 35

Java Programming: OOP 36
Destructors
• No!
• There is a finalize() method that is called when
an object is destroyed.
– you don't have control over when the object is
destroyed (it might never be destroyed).
– The JVM garbage collector takes care of
destroying objects automatically (you have limited
control over this process).

toString Method
• Returns a string representation of the object
• Example

Example
• Create a Java class Name with following variables
and methods
– 3 private variables to represent first name, middle
name and last name
– Relevant constructors
– toString method to print full name
– A method name “namewithins() that return name
with initials. example (S. K. SILVA)
– Implement main method to demonstrate name
class capabilities.

Example
• Complete the all previously created class with the
following methods
– 3 Constructors
• Example Date(), Date(int,int,int), Date(date)
– toString method
– Input method
– Print method
• Update main method of the Application class to
demonstrate each class capabilities

Java Programming: OOP 41
Encapsulation
• Information Hiding.
• Don't need to know how some component is
implemented to use it.
• Implementation can change without effecting any
calling code.
• "protects us from ourselves"

Encapsulation
• Is the technique of making the fields in a class
private and providing access to the fields via public
methods
• Is also referred to as data hiding
• Benefit
– is the ability to modify our implemented code
without breaking the code of others who use our
code
• Encapsulation gives maintainability, flexibility and
extensibility to our code
42

Public access
43
public class Test
{
public float money;
public String TV;
public void work ()
{
}

Private access
44
public class Test
{
private float money;
private String TV;
public void work ()
{
}
}

GET Methods
45
public int getData()
{
return data;
}

SET Methods
46
public void setData(int nd)
{
data = nd;
}

Example
public class Encap
{
private String name;
private String idNum;
private int age;
public int getAge()
{
return age;
}
public String getName()
{
return name;
}
47
public String getIdNum()
{
return idNum;
}
public void setAge( int newAge)
{
age = newAge;
}
public void setName(String newName)
{
name = newName;
}
public void setIdNum( String newId)
{
idNum = newId;
}
}

Example
public class RunEncap
{
public static void main(String args[])
{
EncapTest encap = new EncapTest();
encap.setName("James");
encap.setAge(20);
encap.setIdNum("12343ms");
System.out.println("Name : " + encap.getName()+ " Age : "+ encap.getAge());
}
}
48

Benefits of Encapsulation:
• The fields of a class can be made read-only or write-
only.
• A class can have total control over what is stored in
its fields.
• The users of a class do not know how the class
stores its data. A class can change the data type of a
field, and users of the class do not need to change
any of their code.
49

Composition…?
• Way to combine simple objects or data types
into more complex ones
51
Java Primitive data types
– byte
– short
– int
– long
– float
– double
– boolean
– char
Complex Data types/Objects
– Name
– Date
– Student
– Account
– Course
– Results

Composition 52
Composition
• is a way to combine simple objects or data types
into more complex ones
• A composition relation exists between two classes if
one classes’ field(s) consist of another class
• Called as has-a relationship

HAS-A Relationship
• Relationships are mainly based on the usage
• Determines whether a certain class HAS-A certain
thing
53

Example (Date)
• Create a Date class to represent day, month,
and year for a date.
54
Date
Day
Month
Year

Example (Student)
• Create a class Student with student name, date of
birth and group
55
Student has-a Date of birth
class field consist of another class

Example
• Complete the following class

Required Methods for a each class
• Constructor(s)
• Set and Get methods
• toString Method
• Input method (from keyboard)
• Print method(s)
• Other required methods
Compulsory Methods

Example
1. Create a class name customer including following data fields and
methods
– Data fields
• Name
• Full name
• Address
• NIC No
• Date of Birth
• Sex
• Email Address
– Methods
• Get Methods … name, NIC etc.
• Set methods …
• Constructors,
• PrintCustomer
59

Exercise
• Complete the following classes with all relavent methods
– Student
– Employee
– Person
– Vehicle
– Book
• Use Composition as required
• Update main method of the Application class to
demonstrate each class capabilities

02-advance-methods-on-class jdpgs code .pdf

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