Object-Oriented Languages

Computer Technology Series Gerb Objectives: Understand object-oriented languages and their history

Object-Oriented Languages

• Through the 1970s and 1980s, software engineers became concerned with reuse – Developing software is expensive – Reusing an existing piece of software was more expensive that starting from scratch • 3 computer scientists, Booch, Rumbaugh, and Jacobson championed Object-Oriented Languages as a way to improve reuse.

– A program no longer a set of instructions – Describes objects and what you can do to them

Object-Oriented Languages Cont’d

• • Today nearly all serious development is OO.

Three Features of OO Languages

– – –

Encapsulation

- Details of objects can be hidden from the users of the objects

Polymorphism

- The same operation can have different effects on different types of objects.

Inheritance

- An object can be created which has all the characteristics of an existing object to which new characteristics can be added.

Early Object-Oriented Languages • Smalltalk

– Earliest OO Language – Not a commercial Success

• C++

– Created in 1980s by AT&T employee Bjarne Stroustrup – An addition onto the C programming language – Became the most popular OO language

How Object-Oriented Languages Work

• Programmer defines

classes

– A type of object – Program creates

instances

, examples of that type of object • Each instance has information that makes it different from other instances – In C++ called members of the class – In Java called instance variables – In other OO languages, called attributes • Class provides methods that work with instance data – Observers give information about the instance. (AKA accessors) – Modifiers change instance data – In C++, methods are called member functions

How Object-Oriented Languages Work (cont’d)

• Classes can have private instance variables only usable within the class – Public instance variables visible to the rest of the program – This ability is called

encapsulation

• Can create a class which has all the methods and instance variables of the parent class – Called a subclass – Original class is called the superclass – We say the subclass

extends

the superclass – This process is called

inheritance

• Subclasses can have their own version of methods in the superclass – Called specializing these methods – Language knows to call the specialized version – Called

polymorphism