FAQ 5.07 What are the desirable qualities of an object?

An ideal object is a service provider that is alive, responsible, and intelligent.

Each object is an independent entity with its own lifetime, internal state, and set of services. Objects should be alive, responsible, and intelligent agents. Objects are not simply a convenient way to package data structures and functions together. The fundamental purpose of an object is to provide services.

Ideal objects should be alive so that they can take care of themselves. They are born (constructed), live a full and productive life, and die (destroyed). Objects are expected to do something useful and maintain themselves in a coherent state. The opposite of "alive objects" is "dead data." Most data structures in procedural programs are dead in the sense that they just lie there in memory and wait for the functions to manipulate them.

Ideal objects should be responsible so that you can delegate to them. Since they are alive, they have rights and responsibilities. They have the right to look after their own affairs programmers do not reach into their innards and manipulate them, and they are responsible for providing one or more services. This means that other objects ("users") can delegate work to them instead of having to do everything for themselves. This helps produce more modular and reusable software that consists of many objects, each of which does one thing.

Ideal objects should be intelligent so that they can provide services to their users, thus simplifying user code. For example, the knowledge of how to do some task (whether it is simple or complex) resides in only one place, inside the object. Users simply request the service; they concentrate on what needs to be done rather than how it's done. Effectively this moves complexity from the many to the few.

The benefits of moving complexity from the many to the few are all around us. As an analogy, consider how airlines might consider getting rid of all those expensive pilots and let the customers fly the airplanes for a rental fee like the car rental companies do. However, this would require, among other things, that every traveler be trained as a pilot, making air travel both more complicated and more accident prone. Instead, airlines move the intelligence regarding cockpit controls from the many to the few from every traveler using the airlines to the pilots. The result is simpler and safer for travelers (users) and cheaper for the airlines (servers).

As a practical matter, this means that the first step for a new class is to write down its roles and responsibilities. This helps clarify why the class exists, whether or not it is needed, and what services it should provide. Defining the responsibilities of all objects prevents confusion and simplifies the design process.