Object-oriented Design and Programming

CS 2210: SW Development Methods

•Reading: Chapter 2 of MSD text– Section 2.3.2 on UML: look at class diagrams but ignore the rest for now

This Unit Overview

• Review OO, objects, classes• Object identity, equality and Java• Intro to abstraction and inheritance• Class identification and modeling• Modeling object interaction• More Java details on classes etc.

– inheritance, abstract classes, types

What’s OO All About?

• At a very high level, programs are:– procedures operating on– data-objects

• The “old” view sometimes called procedural– Procedures are the starting point– Access data through parameters, shared

values– Only approach for languages like C, Pascal

• A system is: a hierarchy of procedure calls

The OO Approach

• Object-oriented:– Emphasis on the data-objects first

• Data encapsulation

– Associate procedures with the data-objects• Procedural encapsulation• Call operations on data-objects, or• Send a data-object a message

• A system is: a group of collaborating objects

What’s an Object?

• Grady Booch’s definition: an object has: state, behavior, identity

• State– encapsulates data (e.g. fields in Java

objects)– contains relationships with other objects

(e.g. references to other objects)

What’s an Object? (2)

• Behavior– Responds to operations, messages (e.g.

Java method calls on that object)– Interacts with other objects to

accomplish a task

What’s an Object? (3)

• Identity– Simple idea, but…

• Are two objects the same? Or are they equal?– Reference variables in Java, C++, etc.– Identity means: refers to the same

object– Book calls this: name equivalence

Object Equality

• Equality: perhaps two things can be different things, but equal according to some problem-defined condition– E.g. two Course objects: equal if same

courseID field (let’s say)– Book’s term: content equivalence

For objects, do you think == tests for:

1. Name equivalence2. Content equivalence3. Neither one4. Both

Do you think equals() returns true for:

1. Name equivalence2. Content equivalence3. Neither one4. Either

Example: x.equals(y) is true means what about x and y?

Java and Object Equivalence

• Java supports both. You must understand them both!– If not: see slides at the end of this deck

• name equivalence: operator ==– E.g. x==y means “do x and y reference

the same (one) object”?• method boolean equals()

– E.g. x.equals(y) means “do x and y stored the same values to make them content equivalent”?

Classes

• So far we’ve just talked about objects• We note that many objects are a

“type of” the same kind of thing– The same “abstraction”– E.g. 1, 2, 7 and 10 – whole numbers– E.g. 1, 3.5, 25, pi – numbers– Bob, Sally, Joe – Students

• But, they are Persons too, aren’t they?

– cs2110, cs2102, engr1620 – Courses

Classes, Type

• Classes define a set of objects with the same properties (state, behavior)

• A class definition serves as a “cookie cutter” for creating new objects– Instantiation of an object of a certain class– In Java, we do this with new and a constructor

is called– Creates an individual object, also called an

instance

• Variables and data objects have a type– What the rules are for that object– An object’s class is one form of object-type