1. Classes Revision
CST200 – Week 4: Midterm revision
Instructor: Andreea Molnar

2. Outline
• Classes
• Example
• Constructor
• Encapsulation

3. Classes
In object oriented programming classes
represent objects with different
characteristics (attributes, data) and
functionality (operations, methods).

4. Example
Person
Characteristics: name, social security
number
Functionality: sleeps, walks

5. Example
Person
Attributes: name, social security number
Operations: sleeps, walks

6. Example
Person
Data: name, social security number
Methods: sleeps, walks

7. Example
Person
Data: define the state of the object
Methods: define the behavior of the object

8. Example
public class Person {
private String name;
Data declaration
private String socialSecurityNo;
public void sleeps() {
}
public void walks() {
}
}
Method declaration

9. Example
public class Person {
private String name;
private String socialSecurityNo;
Instance variables –
variables created at the
class level
Each instance of a class
(object) has its own instance
variables/data space
public void sleeps() {
}
public void walks() {
}
}
Each instance of a class
(object) share the method
definitions

10. Constructor
•
•
•
Creates (and initializes) an object
Similar to a method that has:
•
•
the same name as the class
no return type
Each class has a default constructor that
accepts no parameters (this is generated
only if no explicit constructor is provided).

11. Constructor
public class Test {
public static void main(String[] args) {
Person p = new Person();
}
}
The default
constructor is called

12. Constructor
public class Person {
private String name;
private String socialSecurityNo;
public Person(String name, String socialSecurityNo) {
this.name = name;
Constructor
this.socialSecurityNo = socialSecurityNo;
}
public void sleeps() {
}
public void walks() {
}
}
this is a reference to the
current object

13. Constructor
public class Test {
public static void main(String[] args) {
Person p = new Person("Mary", "078-05-1120");
}
}

14. Constructor
public class Test {
public static void main(String[] args) {
Person p1 = new Person("Mary", "078-05-1120");
Person p2 = new Person(“John", "078-05-1121");
}
}
name
name
Mary
John
socialSecurityNo
socialSecurityNo
078-05-1120
078-05-1121
Each instance of a
class (object) has its
own instance
variables/data space.

15. Encapsulation
•
•
Hides the data and implementation details
of an object
Protects the data integrity by making it
difficult to have unauthorized access

16. Encapsulation
•
Uses visibility modifiers (e.g. private) to
deny access
private String name;
private String socialSecurityNo;
name and
socialSecurityNo can
be referenced only
within Person class
public class Test {
public static void main(String[] args) {
Person p = new Person("Mary", "078-05-1120");
p.name = "Alice";
}
}
error

17. Encapsulation
Allows access to instance variables through
methods: accessor method (getter) and
mutator method (setter)

18. Encapsulation
Accessor method (getter) – returns the
variable value.
private String name;
public String getName() {
return name;
}

19. Encapsulation
Accessor method (getter) – returns the
variable value.
public class Test {
public static void main(String[] args) {
Person p = new Person("Mary", "078-05-1120");
System.out.println(p.getName());
}
}
Will print Mary
p
name
Mary
socialSecurityNo
078-05-1120

20. Encapsulation
Mutator method (setter) – changes the
variable value
private String name;
public void setName(String name) {
this.name = name;
}

21. Encapsulation
Mutator method (setter) – changes the variable value
public class Test {
public static void main(String[] args) {
Person p = new Person("Mary", "078-05-1120");
System.out.println(p.getName()); //will print Mary
p
p.setName("Alice");
System.out.println(p.getName()); //will print Alice
}
}
name
Mary Alice
socialSecurityNo
078-05-1120

22. Summary
• Initialize instance variables in the
constructor
• Make the instance variables private
unless there is a good reason to do
otherwise
• Allow access to instance variables
through setter and getter methods