CS 2210: SW Development Methods

CS 2210: SW Development Methods Object-oriented Design and Programming • 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: • Name equivalence • Content equivalence • Neither one • Both

Do you think equals() returns true for: • Name equivalence • Content equivalence • Neither one • 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

CS 2210: SW Development Methods