data encapsulation & abstruction

1. 08 Encapsulation and Abstraction

2. 2
Contents
• Defining Abstraction
• Levels of Abstraction
• Class as Abstraction
• Defining a Java Class
• Instantiating a Class
• Class Members
• Class Modifiers
• Member Modifiers
• Accessibility Scope
• Defining Encapsulation
• Principles of
Encapsulation
• Encapsulating a Class
• Setters & Getters
• Constructors

3. 3
Objectives
• Define abstraction
• Identify levels of abstraction
• Understand that a class is a form of abstraction
• Define a Java class
• Learn how to create objects by instantiating their class
• Identify class members
• Identify and define each modifier applicable to a class
• Identify and define each modifier applicable to class
members
• Define access scope for class and its members
• Know the purpose of constructors and how to create one
for a class

4. 4
Objectives (continued)
• Define encapsulation
• Describe the principles of encapsulation
• Learn how to encapsulate a class
• Learn how to use setters and getters

5. 5
Defining Abstraction
• Abstraction is the process of extracting common features
from specific examples
• Abstraction is a process of defining the essential concepts
while ignoring the inessential details

6. 6
Different Types of Abstraction
• Data Abstraction
Programming languages define constructs to simplify the
way information is presented to the programmer.
• Functional Abstraction
Programming languages have constructs that ‘gift wrap’
very complex and low level instructions into instructions
that are much more readable.
• Object Abstraction
OOP languages take the concept even further and
abstract programming constructs as objects.

7. 7
Anything that you can describe can be represented
as an object, and that representation can be created,
manipulated and destroyed to represent how you use
the real object that it models.
Everything is an Object

8. 8
An object is a self-contained entity
with attributes and behaviors
Defining an Object
information an object must know:
 identity – uniqueness
 attributes – structure
 state – current condition
behavior an object must do:
 methods – what it can do
 events – what it responds to

9. 9
Class as Abstraction
• A class is an abstraction of its
instances. It defines all the
attributes and methods that its
instances must also have.
Person
name
sex
age
tellSex()
tellAge()

10. 10
• A Class acts as the template from which an instance of an
object is created. The class defines the properties of the
object and the methods used to control the object's
behavior.
• A Class specifies the structure of data as well as the
methods which manipulate that data. Such data and
methods are contained in each instance of the class.
• A Class is a model or template that can be instantiated to
create objects with a common definition, and therefore
common properties, operations and behavior.
• A Class provides a template for defining the behavior of a
particular type of object. Objects are referred to as
“instances” of a class.
Defining a Class

11. 11
Defining a Java Class
• A Java Class denotes a category of objects, and acts as a blueprint for creating
such objects.
• It defines its members referred to as fields and methods.
• The fields (also known as variables or attributes) refer to the properties of the
class.
• The methods (also known as operations) refer to behaviors that the class
exhibits.
class Person {
String name;
char sex;
int age;
void tellSex() {
if (sex=='M')
System.out.println("I'm Male.");
else if (sex=='F')
System.out.println("I'm Female.");
else System.out.println("I don't know!");
}
void tellAge() {
if (age<10)
System.out.println("I'm just a kid.");
else if (age<20)
System.out.println("I'm a teenager.");
else System.out.println("I'm a grown up.");
}
}

12. 12
Class Members
• A class member refers to one of the fields or methods of a class.
• Static members are variables and methods belonging to a class where only a single copy
of variables and methods are shared by each object.
• Instance members are variables and methods belonging to objects where a copy of each
variable and method is created for each object instantiated.
class Person {
static int maleCount;
static int femaleCount;
String name;
char sex;
int age;
static void showSexDistribution() {
if (maleCount>femaleCount)
System.out.println("Majority are male.");
else if (femaleCount>maleCount)
System.out.println("Majority are female.");
else System.out.println("Equal number of male and female.");
}
void tellSex() {
if (sex=='M')
System.out.println("I'm Male.");
else if (sex=='F')
System.out.println("I'm Female.");
else System.out.println("I don't know!");
}
void tellAge() {
if (age<10)
System.out.println("I'm just a kid.");
else if (age<20)
System.out.println("I'm a teenager.");
else System.out.println("I'm a grown up.");
}
}

13. 13
class Person {
static int maleCount;
static int femaleCount;
String name;
char sex;
int age;
static void showSexDistribution() {
// method body here
}
void tellSex() {
// method body here
}
void tellAge() {
// method body here
}
}
Instantiating a Class &
Accessing its Members
Access class variables
by setting their values
• Instantiating a class means creating objects of its own type.
• The new operator is used to instantiate a class.
class MainProgram {
public static void main(String[] args) {
// instantiating several objects
Person p1 = new Person();
Person p2 = new Person();
Person p3 = new Person();
// accessing instance variables
p1.name = "Vincent"; p1.sex = 'M'; p1.age = 8;
p2.name = "Janice"; p2.sex = 'F'; p2.age = 19;
p3.name = "Ricky"; p3.sex = 'M'; p3.age = 34;
// accessing static variables
Person.maleCount = 2;
Person.femaleCount = 1;
// accesssing instance methods
p1.tellSex(); p1.tellAge();
p2.tellSex(); p2.tellAge();
p3.tellSex(); p3.tellAge();
// accessing static method
Person.showSexDistribution();
}
}
Access class methods
by invoking their names
Create Person objects
using the new operator.
Sample Output:
I'm Male.
I'm just a kid.
I'm Female.
I'm a teenager.
I'm Male.
I'm a grown up.
Majority are male.

14. 14
Class Modifiers
Modifier Description
(no modifier) class is accessible within its package only
public class is accessible by any class in any package
abstract class cannot be instantiated (a class cannot be
abstract and final at the same time)
final class cannot be extended
strictfp class implements strict floating-point arithmetic
• Class modifiers change the way a class can be used.
• Access modifiers describe how a class can be accessed.
• Non-access modifiers describe how a class can be manipulated.

15. 15
Access Modifiers
Modifier Description
(no
modifier)
member is accessible within its package only
public member is accessible from any class of any
package
protected member is accessible in its class package and by
its subclasses
private member is accessible only from its class
• Member modifiers change the way class members can be used
• Access modifiers describe how a member can be accessed

16. 16
Sample
Package
ClassClass
Class
Class
Private features of the
Sample class can only be
accessed from within the
class itself.
Private features of the
Sample class can only be
accessed from within the
class itself.
private
Classes that are in the
package and all its
subclasses may access
protected features of the
Sample class.
Classes that are in the
package and all its
subclasses may access
protected features of the
Sample class.
protected
Only classes that are in the
package may access
default features of classes
that are in the package
Only classes that are in the
package may access
default features of classes
that are in the package
default
All classes may access
public features of the
Sample class.
All classes may access
public features of the
Sample class.
public
Class
Class
* Default is not a modifier; it is just the name of the access level if no access modifier is specified.
Access Modifiers

17. 17
Member Modifiers
• Member modifiers change the way class members can be used
• Non-access modifiers describe how a member can be manipulated
Modifier Description
static member belongs to a class
final declares a constant variable or method
abstract method is declared with no implementation (applied to methods, cannot
be combined with other non-access modifiers )
strictfp method implements strict floating-point arithmetic (applied to methods)
synchronized method is executed by only one thread at a time (applied only to
methods)
native method implementation is written in other language (applied only to
methods)
transient an instance variable is not saved when its object is persisted or
serialized (applied only to variables)
volatile variable is modified asynchronously by concurrently running threads
(applied only to variables)

18. 18
Accessibility Scope
Scope Access
static static code can access static members but not instance
members
non-static non-static code can access both static members and
instance members
package a class and its members can be accessed within the
package they are declared
class class members can be accessed within the class
block local variables can be accessed only within a block
• Accessibility scope defines the boundary of access to a
class and its members

19. 19
Defining Encapsulation
• Encapsulation is the process of hiding an
object’s implementation from another
object, while presenting only the interfaces
that should be visible.

20. 20
Principles of Encapsulation
“Don’t ask how I do it, but this is what I
can do”
- The encapsulated object
“I don’t care how, just do your job, and I’ll
do mine”
- One encapsulated object to another

21. 21
Encapsulating a Class
• Members of a class must always be declared with the
minimum level of visibility.
• Provide setters and getters (also known as
accessors/mutators) to allow controlled access to private
data.
• Provide other public methods (known as interfaces ) that
other objects must adhere to in order to interact with the
object.

22. 22
Setters and Getters
private char sex;
public void setSex(char s) {
// validate here
sex = s;
}
public char getSex() {
// format here
return sex;
}
• Setters and Getters allow controlled access to class data
• Setters are methods that (only) alter the state of an object
• Use setters to validate data before changing the object
state
• Getters are methods that (only) return information about the
state of an object
• Use getters to format data before returning the object’s
state

23. 23
Encapsulation Example
public static void main(String[] args) {
// instantiate several objects
Person p1 = new Person();
Person p2 = new Person();
Person p3 = new Person();
// access instance variables using setters
p1.setName("Vincent"); p1.setSex('M');
p1.setAge(8);
p2.setName("Janice"); p2.setSex('F');
p1.setAge(19);
p3.setName("Ricky"); p3.setSex('M');
p3.setAge(34);
// access static variables directly
Person.maleCount=2;
Person.femaleCount=1;
// access instance methods
p1.tellSex(); p1.tellAge();
p2.tellSex(); p2.tellAge();
p3.tellSex(); p3.tellAge();
// access static method
Person.showSexDistribution();
}
class Person {
// set variables to private
private static int maleCount;
private static int femaleCount;
private String name;
private char sex;
private int age;
/*
* setters & getters, set to public
*/
public int getAge() { return age;}
public void setAge(int a) { age = a;}
public String getName() { return name;}
public void setName(String n) { name = n;}
public char getSex() { return sex;}
public void setSex(char s) { sex = s;}
/*
* set other methods as interfaces
*/
public static void showSexDistribution() {
// implementation here
}
public void tellSex() {
// implementation here
}
public void tellAge() {
// implementation here
}
}
I'm Male.
I'm just a kid.
I'm Female.
I'm a teenager.
I'm Male.
I'm a grown up.
Majority are male.

24. 24
Constructors
• Constructors are methods which set the initial state of an
object
• Constructors are called when an object is created using
the new operator
• A default constructor is a constructor with no parameters,
it initializes the instance variables to default values
• Restrictions on constructors
• constructor name must be the same as the class name
• constructor cannot return a value, not even void
• only an access modifier is allowed

25. 25
Key Points
• Abstraction is the process of formulating general concepts by
extracting common properties of instances.
• A class is an abstraction of its instances.
• A Java Class denotes a category of objects.
• Class members refer to its fields and methods.
• Static members are variables and methods belonging to a class.
• Instance members are variables and methods belonging to
objects.
• Instantiating a class means creating objects of its own type.
• Class modifiers include: (no modifier), public, abstract,
final and strictfp.
• Member modifiers include: (no modifier), public, protected,
private, static, final, abstract, strictfp,
synchronized, native, transient and volatile.

26. 26
Key Points (Continued)
• Encapsulation hides implementation details of a class.
• Encapsulating a class means declaring members with
minimum level of visibility.
• Setters are methods whose only function is to alter the
state of an object in a controlled manner.
• Getters are methods which only function is to return
information about the state of an object.
• Constructors are methods which set the initial state of an
object upon creation of the object.