classes revision
Post on 17-May-2015
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Classes Revision
CST200 – Week 4: Midterm revision
Instructor: Andreea Molnar
Outline
•Classes
•Example
•Constructor
•Encapsulation
Classes
In object oriented programming classes represent objects with different characteristics (attributes, data) and functionality (operations, methods).
Example
Person
Characteristics: name, social security number
Functionality: sleeps, walks
Example
Person
Attributes: name, social security number
Operations: sleeps, walks
Example
Person
Data: name, social security number
Methods: sleeps, walks
Example
Person
Data: define the state of the object
Methods: define the behavior of the object
Example
public class Person {
private String name;
private String socialSecurityNo;
public void sleeps() {
}
public void walks() {
}
}
Data declaration
Method declaration
Examplepublic class Person {
private String name;
private String socialSecurityNo;
public void sleeps() {
}
public void walks() {
}
}
Instance variables – variables created at
the class level
Each instance of a class (object) share the method
definitions
Each instance of a class (object) has its own instance
variables/data space
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).
Constructor
public class Test {
public static void main(String[] args) {
Person p = new Person();
}
}
The default constructor is
called
Constructorpublic class Person {
private String name;
private String socialSecurityNo;
public Person(String name, String socialSecurityNo) {
this.name = name;
this.socialSecurityNo = socialSecurityNo;
}
public void sleeps() {
}
public void walks() {
}
}
Constructor
this is a reference to the current
object
Constructor
public class Test {
public static void main(String[] args) {
Person p = new Person("Mary", "078-05-1120");
}
}
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");
}
}Mary
078-05-1120
John
078-05-1121
Each instance of a class (object)
has its own instance
variables/data space.
name name
socialSecurityNo
socialSecurityNo
Encapsulation
•Hides the data and implementation details of an object
•Protects the data integrity by making it difficult to have unauthorized access
Encapsulation•Uses visibility modifiers (e.g. private) to
deny accessprivate String name;
private String socialSecurityNo;
public class Test {
public static void main(String[] args) {
Person p = new Person("Mary", "078-05-1120");
p.name = "Alice";
}
}
name and socialSecurityNo can be referenced
only within Person class
error
Encapsulation
Allows access to instance variables through methods: accessor method (getter) and mutator method (setter)
Encapsulation
Accessor method (getter) – returns the variable value.
private String name;public String getName() {
return name;}
EncapsulationAccessor 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());}
}Mary
078-05-1120
name
socialSecurityNo
p
Will print Mary
Encapsulation
Mutator method (setter) – changes the variable value
private String name;public void setName(String name) {
this.name = name;}
EncapsulationMutator 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 Maryp.setName("Alice");System.out.println(p.getName()); //will print Alice
}}
Mary Alice
078-05-1120
name
socialSecurityNo
p
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
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