The progress of abstraction

All programming languages provide abstractions. It can be argued that the complexity of the
problems you can solve is directly related to the kind and quality of abstraction. By “kind” I
mean: what is it that you are abstracting? Assembly language is a small abstraction of the
underlying machine. Many so-called “imperative” languages that followed (such as
FORTRAN, BASIC, and C) were abstractions of assembly language. These languages are big
improvements over assembly language, but their primary abstraction still requires you to
think in terms of the structure of the computer rather than the structure of the problem you
are trying to solve. The programmer must establish the association between the machine
model (in the “solution space”) and the model of the problem that is actually being solved (in
the “problem space”). The effort required to perform this mapping, and the fact that it is
extrinsic to the programming language, produces programs that are difficult to write and
expensive to maintain, and as a side effect created the entire “programming methods”
industry.
The alternative to modeling the machine is to model the problem you’re trying to solve.
Early languages such as LISP and APL chose particular views of the world (“all problems are
ultimately lists” or “all problems are algorithmic”). PROLOG casts all problems into chains of
decisions. Languages have been created for constraint-based programming and for
programming exclusively by manipulating graphical symbols. (The latter proved to be too
restrictive.) Each of these approaches is a good solution to the particular class of problem
they’re designed to solve, but when you step outside of that domain they become awkward.
The object-oriented approach takes a step farther by providing tools for the programmer to
represent elements in the problem space. This representation is general enough that the
programmer is not constrained to any particular type of problem. We refer to the elements
in the problem space and their representations in the solution space as “objects.” (Of course,
you will also need other objects that don’t have problem-space analogs.) The idea is that the
program is allowed to adapt itself to the lingo of the problem by adding new types of
objects, so when you read the code describing the solution, you’re reading words that also
express the problem. This is a more flexible and powerful language abstraction than what
we’ve had before. Thus OOP allows you to describe the problem in terms of the problem,
rather than in the terms of the solution. There’s still a connection back to the computer,
though. Each object looks quite a bit like a little computer; it has a state, and it has
operations you can ask it to perform. However, this doesn’t seem like such a bad analogy to
objects in the real world; they all have characteristics and behaviors.
Chapter 1: Introduction to Objects 43
Alan Kay summarized five basic characteristics of Smalltalk, the first successful objectoriented
language and one of the languages upon which Java is based. These characteristics
represent a pure approach to object-oriented programming:
1. Everything is an object. Think of an object as a fancy variable; it stores data, but you can
also ask it to perform operations on itself by making requests. In theory, you can take any
conceptual component in the problem you’re trying to solve (dogs, buildings, services, etc.)
and represent it as an object in your program.
2. A program is a bunch of objects telling each other what to do by sending messages. To
make a request of an object, you “send a message” to that object. More concretely, you can
think of a message as a request to call a function that belongs to a particular object.
3. Each object has its own memory made up of other objects. Or, you make a new kind of
object by making a package containing existing objects. Thus, you can build up complexity in
a program while hiding it behind the simplicity of objects.
4. Every object has a type. Using the parlance, each object is an instance of a class, where
“class” is synonymous with “type.” The most important distinguishing characteristic of a
class is “what messages can you send to it?”
5. All objects of a particular type can receive the same messages. This is actually a very
loaded statement, as you will see later. Because an object of type circle is also an object of
type shape, a circle is guaranteed to receive shape messages. This means you can write code
that talks to shapes and automatically handle anything that fits the description of a shape.
This substitutability is one of the most powerful concepts in OOP.
Some language designers have decided that object-oriented programming itself is not
adequate to easily solve all programming problems, and advocate the combination of various
approaches into multiparadigm programming languages.2