The OO Technology - "a way of thinking about a problem using real world concepts rather than computer concepts."

"Object-Oriented ___"

means that we organize software as a collection of discrete objects that incorporate both data structures and behavior.

At a higher scale - distributed objects "the network is the computer"

The OO Technique

New?

No.

Began around 1965 with the development of SIMULA

(an extension to Algol)

SIMULA was released in 1967

Now, Smalltalk, LOOPS, CLOS, C++, Object Cobol, Eiffel, etc. and Java!

But WHY?

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The Case for Object-Orientation

Software crisis:

• High percentage of code is
  • undeliverable
  • unusable
  • abandoned
  • late
• Maintenance costs
  • 40-80%
• Cost of employee turnover is high
  • Unreadable spagetti code
  • Non-scalable methods and tools
  • Build from scratch
• Production of software
  • hacking
  • science vs. art

Previously, No emphasis on iteration, reuse, no unifying model to integrate phases

Object-Oriented Analysis, Design and Implementation are not separable

A clear mapping enhances trace- ability and maintenance/error control

An approach which is

• natural
  closely identified with real-world concepts
• and flexible
  quickly adapts to changes in the problem specifications

• higher reusability
• higher maintainability
• greater complexity

Note:

• the methodology must be applied "correctly"
• OO methodologies are no easier than other methodologies
• making the transition to OO from other methodologies is difficult

Paradigm shift...new world view which is incompatible with the previous

There is more to OOP than syntax -

there is also more than just terms!

Pure OO languages force one to see and use the OO concepts to a greater extent...thus the use of Java.

Nonetheless, terms:

Typically four important aspects:

• identity
• classification
• polymorphism
• inheritance

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Identity

discrete, distinguishable entities called objects. E.G.:

Interesting choice of classes to show. I thought I would investigate classes (left panel) in the Java 1.5 API
Some classes: Number , Integer , NoSuchMethodException , Throwable, PrintStream

An object is a grouping of information (its properties) and a description of its manipulation (its possible actions). (ADT's)

• Objects can be concrete or abstract.

• Each object has a unique handle (sorta like pointers)

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Classification

objects with the same data structure (attributes) and behaviors (operations or methods) are grouped in the same class.

They should be thought of as a holder of "typical instance" type information, similar to a frame-like idea.

They are related to each other by their location in a class hierarchy.

Classes must be instantiated giving an "instance" of the class.

Each object is an instance of some class. (or a class itself)

Storage and information:

Class Name

• Attributes (class, variables)
• Methods

CVs and methods are declared and exist in the class and may be accessed by all instances of the class

Instance

• Inherit Class Values
• Attributes (Instance variables)
  (declared in class but exist here)
• Inherit methods

CVs are class variables which are declared in a class, but which can be accesses in either the class or any instance of the class. (One memory location)

The IV value that is set in the class definition (default) is returned unless there is a local setting in the specific instance

Example:

Class - csuProfessor

would salary be CV or IV ?

would maxSalary be CV or IV ?

(age, maxAge)

Polymorphism

the same operation may behave differently on different classes

The language selects the correct method based on the name of the operation and the class of the object being operated on. (not overload like C) (discuss)

Inheritance:

sharing of attributes and operations

OO Themes:

• abstraction
  • essential, inherent aspects
    (ignore accidental)
  • levels of abstraction?
    "best" level for a class
    • if too abstract - create sub
      Specialize
    • if too detailed - look to super
      Generalize

  Notice - design thus becomes neither top-down nor bottom up, but "middle out"

• encapsulation
  • separate external aspects from internal implementation details
    (change data for instances but not methods...unless specialize...)
    (allows for different implementations)
  • (send messages to change data)
• sharing
  • inheritance (internal) AND
  • sharing of classes (reuse/external)

Advantages of Object Orientation

• data abstraction
• compatibility (combine software components)
• flexibility (task allocation in software development)
• reuse
• extensibility
• maintenance

Disadvantages of Object Orientation

• finding the objects
• traditional methods do not work

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How about in this class...

There are three very common relationships between classes:

• inheritence (is-a)
• aggregation (part-of)
• association (uses...)

Most relationships are uses (or instantiation of classes)

Not a lot of inheritence - and often used incorrectly.

Consider lab 1. Timer... it uses Date, but is it a Date?

Of course not.

Where would we use inheritence? (Reusable things)

• Trees, binary trees, heaps
• Graphs, bi-directional graphs
• Applets, my-special applet
• Threads, my-special thread
• Buttons, my-special button