97 Things Every Programmer Should Know

Chapter 32. Encapsulate Behavior, Not Just State

Einar Landre

IN SYSTEMS THEORY, containment is one of the most useful constructs when dealing with large and complex system structures. In the software industry, the value of containment or encapsulation is well understood. Containment is supported by programming language constructs such as subroutines and functions, modules and packages, classes, and so on.

Modules and packages address the larger-scale needs for encapsulation, while classes, subroutines, and functions address the more fine-grained aspects of the matter. Over the years, I have discovered that classes seem to be one of the hardest encapsulation constructs for developers to get right. It’s not uncommon to find a class with a single 3,000-line main method, or a class with only set and get methods for its primitive attributes. These examples demonstrate that the developers involved have not fully understood object-oriented thinking, having failed to take advantage of the power of objects as modeling constructs. For developers familiar with the terms POJO (Plain Old Java Object) and POCO (Plain Old C# Object or Plain Old CLR Object), this was the intent in going back to the basics of OO as a modeling paradigm—the objects are plain and simple, but not dumb.

An object encapsulates both state and behavior, where the behavior is defined by the actual state. Consider a door object. ...