Everyone knows what an object is: a tangible "something" that we can sense, feel, and manipulate. The earliest objects we interact with are typically baby toys. Wooden blocks, plastic shapes, and over-sized puzzle pieces are common first objects. Babies learn quickly that certain objects do certain things. Triangles fit in triangle-shaped holes. Bells ring, buttons press, and levers pull.

The definition of an object in software development is not so very different. Objects are not typically tangible somethings that you can pick up, sense, or feel, but they are models of somethings that can do certain things and have certain things done to them. Formally, an object is a collection of data and associated behaviors.

So knowing what an object is, what does it mean to be object-oriented? Oriented simply means directed toward. So object-oriented simply means, "functionally directed toward modeling objects". It is one of many techniques used for modeling complex systems by describing a collection of interacting objects via their data and behavior.

If you've read any hype, you've probably come across the terms object-oriented analysis, object-oriented design, object-oriented analysis and design, and object-oriented programming. These are all highly related concepts under the general object-oriented umbrella.

In fact, analysis, design, and programming are all stages of software development. Calling them object-oriented simply specifies what style of software development is being pursued.

Object-oriented Analysis (OOA) is the process of looking at a problem, system, or task that somebody wants to turn into an application and identifying the objects and interactions between those objects. The analysis stage is all about what needs to be done. The output of the analysis stage is a set of requirements. If we were to complete the analysis stage in one step, we would have turned a task, such as, "I need a website", into a set of requirements, such as:

Visitors to the website need to be able to (italic represents actions, bold represents objects):

Object-oriented Design (OOD) is the process of converting such requirements into an implementation specification. The designer must name the objects, define the behaviors, and formally specify what objects can activate specific behaviors on other objects. The design stage is all about how things should be done. The output of the design stage is an implementation specification. If we were to complete the design stage in one step, we would have turned the requirements into a set of classes and interfaces that could be implemented in (ideally) any object-oriented programming language.

Object-oriented Programming (OOP) is the process of converting this perfectly defined design into a working program that does exactly what the CEO originally requested.

Yeah, right! It would be lovely if the world met this ideal and we could follow these stages one by one, in perfect order like all the old textbooks told us to. As usual, the real world is much murkier. No matter how hard we try to separate these stages, we'll always find things that need further analysis while we're designing. When we're programming, we find features that need clarification in the design. In the fast-paced modern world, most development happens in an iterative development model. In iterative development, a small part of the task is modeled, designed, and programmed, then the program is reviewed and expanded to improve each feature and include new features in a series of short cycles.

The rest of this book is about object-oriented programming, but in this chapter we will cover the basic object-oriented principles in the context of design. This allows us to understand these rather simple concepts without having to argue with software syntax or interpreters.