Object Orientation

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Object Orientation

• Yaodong Bi, Ph.D.
• Department of Computer Sciences
• University of Scranton
• May 26, 2020

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Classes and objects for modeling

• Classes
• Real world concepts
• Student, course, bicycle, etc
• Objects
• Instances of real word concepts
• John Smith, Operating Systems, Mongoose Mountain
  S100, etc

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Classes and objects for modeling
Customers John and Susan entered the
MakeMoney bank and were served by teller
Andy.
Classes
Objects
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Classes and objects in software

• Objects
• Packaging both data and the procedures that
  operate on the data into one
• Operations are the only way to change the data
  items encapsulation
• Classes
• Template of objects of the same type
• Write once and use many times
• Implementation classes/objects
• Some classes/objects do not match to concepts in
  the real world
• List, stack, tree, queue containers
• Exception classes

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Attributes and operations

• Instance attributes (simply attributes)
• Properties (internal state) of an object
• Student name, major, class, GPA
• Course title, of credits, description
• Each object has its own value for an attribute
• Instance operations (simply operations)
• Services of an object
• Stack push, pop, isEmpty, isFull
• Encapsulation
• Attributes (private and protected) are not
  accessible directly by client
• Operations are used to read/write the values of
  attributes

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Attributes and Operations

• Class attributes (static attributes)
• Represent properties that can be applied to all
  objects of the same class
• Only one value for each class attribute shared by
  all objects of the class
• All accounts of the CheckingAccount class have
  the same interest rate
• Can be accessed in both instance and class
  operations
• Class operations (static operations)
• Used to access class attributes
• Cannot access instance attributes and operations

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Class member access modes

• Public
• Public attributes and public operations
• Accessible to all clients or the world
• Protected
• Protected attributes and operations
• Accessible to the class and its subclasses
• Private
• Private attributes and operations
• Only accessible by the operations of the class

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Member access modes in Java
Specifier Class Package Subclass World
Private Y N N N
No Specifier Y Y N N
Protected Y Y Y N
Public Y Y Y Y
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Abstract and concrete classes

• Abstract classes
• Define a common interface for its subclasses
• Defer some or all of its implementation of
  operations to its subclasses
• Cannot be instantiated
• Concrete classes
• That are not abstract classes
• Can be instantiated
• Concrete subclasses implement all the operations

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Interfaces

• Signature of an operation
• Operation name
• Objects/classes it takes as parameters
• Return value and type
• Interface
• The set of signatures of related operations
• Representing a capability or a set of services
• A class may implement multiple interfaces
• Its objects have more than one set of services

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Inheritance

• Specifies is-a or a-kind-of relationship
• Generalization and specialization
• Superclasses and subclasses
• Single and multiple inheritance
• Class and interface inheritance

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Superclasses and subclasses

• Superclass
• Also called base class or parent class
• All of its members are inherited by its
  subclasses
• Subclasses
• Also called child classes
• Inherit all the properties of its superclasses

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Single and multiple inheritance

• Single inheritance
• A subclass cannot have more than one parent
• Multiple inheritance
• A subclass may have more than one parent
• Java does not allow multiple inheritance
• Java allows multiple interface inheritance
• Through interface implementation

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Class inheritance lattice
Faculty office getOffice()
Student major getMajor()
Staff dept getDept()
Graduate PayRate getPayRate()
Underg PayRate getPayRate()
FullTime salary getSalary()
PartTime PayRate getPayRate()
TA labs getLabs()
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Implementation inheritance

• Also called class inheritance
• Define an objects implementation in terms of
  another objects implementation
• Pure class inheritance in C
• Pure class inheritance in Java

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Pure class inheritance with C

• Class BinaryTree
• getRoot()
• setRoot()
• leftTree()
• rightTree()

Class BinSearchTree private BinaryTree insert(
) remove() find()
The operations of BinaryTree are not
accessible/visible to the clients of
BinSearchTree because BinaryTree is private
They use the operations of BinaryTree
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Pure class inheritance with Java

• Java does not allow private parent
• The parent and grandparents are public and
  accessible through the subclass
• Java cannot implement pure implementation
  inheritance.

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Interface inheritance

• Describe when an object can be used in place of
  another
• Pure interface inheritance in C
• Superclasses are pure abstract classes
• Pure interface inheritance in Java
• Superclasses are Java interfaces

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Interface inheritance with C

• Class Stack
• virtual push() 0
• virtual pop() 0
• virtual isEmpty() 0
• virtual isFull() 0

A C Abstract class
Class MyStack public Stack push()
pop() isEmpty() isFull()
Class YourStack public Stack push()
pop() isEmpty() isFull()
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Interface inheritance with Java

• Interface Stack
• push()
• pop()
• isEmpty()
• isFull() 0

A Java Interface
Class MyStack implements Stack push()
pop() isEmpty() isFull()
Class YourStack implements Stack push()
pop() isEmpty() isFull()
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Polymorphism

• When a client sends a request to a reference, the
  method executed depends on the object behind the
  reference
• Polymorphism and inheritance are very powerful
  tool in software design
• So powerful that if they are used properly, they
  can hurt really bad
• If something goes wrong, it is hard to tell which
  class/object caused the problem

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Dynamic (late) binding

• Dynamic binding is a way of implementing
  polymorphism
• It means that a function call is not linked to
  the actual function until execution time
• Since a reference may be used to point to
  different types (subtypes or subclasses) of
  objects at execution time, so the actual function
  to be executed may not be known when the program
  is compiled.
• The opposite of dynamic binding is static binding
  which links a function call to the actual
  function at compilation time.

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Method Overriding

• A subclass overrides the operation(s) defined in
  the parent/grandparent classes.
• This is for specialized subclasses to override
  the behavior defined in the super class(es).
• For example
• Superclass Employees print method prints
  employees SSN, Name, Address.
• Its subclass Engineers print method prints
  engineers type and level in addition to
  employees normal information.

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Function overloading

• The same name is used for more than function
• For example, a function of adding two integers
  may be named the same as the one of adding two
  floats
• Add(int x, int y)
• Add(float x, float y)
• Two functions use the same name for the same
  purpose with different data parameters

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Inheritance vs. Composition

• Two common techniques for reuse
• Class Inheritance
• White-box reuse
• Defined at compile-time, statically
• Cannot change parent at rum-time
• Object Composition
• Black-box reuse
• A reference/pointer to the object
• Can be changed at run-time
• Favor composition over inheritance

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Delegation

• A way to make composition as powerful as class
  inheritance
• A receiving object delegates requests to its
  delegates
• subclass defers request to parent class
• In class inheritance
• parent can use this to access the child
• In delegation
• Receiving object can pass itself to its delegate

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Three techniques for reuse

• Class Inheritance
• can provide default operations and let subclass
  override them
• Cannot be changed at run-time
• Object Composition
• Can change the behavior being composed at
  run-time
• Requires redirection and so becomes less
  efficient
• Generics
• Changes the types that a class can use
• Cannot change at run-time