Jakarta Commons Cookbook

Chapter 4. Functors

Introduction

The American Heritage Dictionary defines a functor as “one that performs an operation or a function,” and, in the context of programming, a functor is an object that encapsulates “functional logic”—a functional object. So with a definition as abstract as “something that does stuff,” you won’t find a lot of satisfying, concise explanations of what a functor can do; in the abstract, anything that performs an operation or function could be considered a functor, and the process of drawing divisions between what should and should not be implemented with functors becomes a matter of personal preference. I’m not going to attempt to give a well-polished, textbook definition of a functor; this chapter simply demonstrates the set of basic functors from Commons Collections. Functors are less of an impressive new technology and are more of an approach to programming. Even if you are unfamiliar with the term, you’ve likely used functors without realizing it; two functors in common usage are Comparator and Iterator.

Both Comparator and Iterator serve to isolate an algorithm; Comparator encapsulates logic to compare two objects, and Iterator encapsulates logic used to iterate over a collection of objects. Functors often lead to more code reuse and a cleaner design because functional logic can be abstracted and extracted out of logic dealing with a specific data structure. For example, to compare two Person beans, you could make the Person class implement Comparable ...