Convention and Divine Law

In the last few sections, I used the phrase "by convention" several times to indicate design decisions that are arbitrary in the sense that there are no significant reasons to do things one way or another, but dictated by convention.

In these cases, it is to your advantage to be familiar with convention and use it, since it will make your programs easier for others to understand. At the same time, it is important to distinguish between (at least) three kinds of rules:

Divine law

This is my phrase to indicate a rule that is true because of some underlying principle of logic or mathematics, and that is true in any programming language (or other formal system). For example, there is no way to specify the location and size of a bounding box using fewer than four pieces of information. Another example is that adding integers is commutative. That's part of the definition of addition and has nothing to do with Java.

Rules of Java

These are the syntactic and semantic rules of Java that you cannot violate, because the resulting program will not compile or run. Some are arbitrary; for example, the fact that the + symbol represents addition and string concatenation. Others reflect underlying limitations of the compilation or execution process. For example, you have to specify the types of parameters, but not arguments.

Style and convention

There are a lot of rules that are not enforced by the compiler, but that are essential for writing programs that are correct, that you can debug and modify, and that others can read. Examples include indentation and the placement of squiggly braces, as well as conventions for naming variables, methods and classes.

As we go along, I will try to indicate which category various things fall into, but you might want to give it some thought from time to time.

While I am on the topic, you have probably figured out by now that the names of classes always begin with a capital letter, but variables and methods begin with lower case. If a name includes more than one word, you usually capitalize the first letter of each word, as in newLine and printParity. Which category are these rules in?