ObjectOriented Programming Part II

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Object-Oriented ProgrammingPart II

• COMP114
• Thursday January 31

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Announcements

• Read Section 4.2
• Study for test (Thursday, Feb. 7)
• All material up to today
• Book Ch. 1, 2.1-2.4, 3.1, 3.2, 3.4, 3.5, 4.1
• Extra office hours
• Tuesday 2/5 400 500
• Ill be out of office on 2/6-7
• TA office hours on web page

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Topics

• Last Time
• Review of References
• Object-Oriented Programming
• Encapsulation

• This Class
• Little more on Overloading
• Inheritance
• Protection

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Overloading

• Cant overload by return type in Java
• Example
• Compiler will complain.

int getLength( ) ... double getLength( ) ...
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Inheritance

• Idea is to extend functionality of an object
• Specialize
• Car specialized from Vehicle
• Toyota specialized from Car
• Each level of specialization typically adds new
  functionality

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Inheritance

• Derive new class from an existing class.

Is-A Relationship
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Nomenclature

• Superclass
• Subclass
• Parent class
• Child class
• Base class
• Derived class

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Superclass Person

• public class Person
• public Person(String theName)
• name theName
• private String name null

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Subclass Employee

• public class Employee extends Person
• public void setSalary(double sal)
• salary sal
• public double getSalary()
• return salary
• // Add a salary
• private double salary 0

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Why Not Just Copy/Paste?

• If you need to make change, you have to change
  all
• You can't work with any of the objects as if it
  were the base class (Person)
• Example say you just need to send a security
  email. Does it matter whether it's Student,
  Staff, or Faculty?

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Derived Class

• Can add methods
• Can add new instance variables
• Can also override methods
• Cannot remove methods or instance variables

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Employee New Constructor

• public class Employee extends Person
• Employee(String theName, double sal)
• super(theName)
• salary sal
• private double salary 0

Constructor of superclass
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Rules for Contructors

• If no constructor specified
• Automatically constructed with class variables
  set to 0 or null
• If constructor specified
• If super() is not first command,
• then constructor of superclass automatically
  called with no arguments

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Remember Superclass

• public class Person
• public Person(String theName)
• name theName

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Subclass Overrides Constructor
Overrides method in class Person

• public class Employee extends Person
• Employee(String theName)
• super(theName)
• employeeID EmployeeIDGen.newID()
• Employee(String theName, int id)
• super(theName)
• employeeID id
• private int employeeID 0

Not just for constructor
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Access

• public class Person
• public Person(String theName)
• name theName
• private String name null

Subclasses cant modify directly
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Why Not Allow Subclass Access

• Wed be back to same problem
• If we changed variable in superclass, wed have
  to find (and change) all the places it was
  manipulated.

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Private Methods not Inherited

• If a class has a private method, its meant to be
  used locally
• As a helper for something
• Not accessible from subclass.

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Dynamic Binding

• A reference variable of the superclass can also
  reference subclasses

Person p p new Employee(Lastra, 1234)
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Example From Book

• Person p new Person 4
• p0 new Person( "joe", 25, "New York",
  "212-555-1212" )
• p1 new Student( "becky", 27, "Chicago",
  "312-555-1212", 4.0 )
• p3 new Employee( "bob", 29, "Boston",
  "617-555-1212", 100000.0 )

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Polymorphism

• From Websters
• the quality or state of being able to assume
  different forms
• Reference variable that is polymorphic can
  reference objects of several types

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Parameters

• Very similar when object is used as parameter

In class bookStore public void debit(Person p,
double c) Student s
new Student(John Doe) bookStore.debit(s,
123.45)
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Multiple Types

• An object is in some sense
• of its own type and
• also of its ancestors types.
• Is there an Adam/Eve?

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Object Class

• Object is the top of the inheritance hierarchy.
• If class doesnt specify inheritance, its a
  subclass of Object
• So youve all been using OOP anyway
• Object has some useful methods.
• More on that later.

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Inheritance Hierarchies

• Inheritance proceeds along generations
• So
• Faculty, which is derived from Employee, which is
  derived from Person
• Can use the Person methods

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Has-A

• I said that inheritance exhibits the Is-A
  relationship
• Student Is-A Person
• Not a Has-A relationship
• Person Has-A address
• Has-A should be implemented using class
  variables, not inheritance

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Summary

• Derived class extends base class
• Can add data and methods
• Can override superclass methods
• If not overridden, methods are inherited

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Next Time

• More on Objects
• Abstract methods
• Review for test