I'm cleaning up Java code for someone who starts their functions by declaring all variables up top, and initializing them to null/0/whatever, as opposed to declaring them as they're needed later on.
What are the specific guidelines for this? Are there optimization reasons for one way or the other, or is one way just good practice? Are there any cases where it's acceptable to deviate from whatever the proper way of doing it is?
From the Java Code Conventions, Chapter 6 on Declarations:
6.3 Placement
Put declarations only at the beginning of blocks. (A block is any code surrounded by curly braces "{" and "}".) Don't wait to declare variables until their first use; it can confuse the unwary programmer and hamper code portability within the scope.
void myMethod() { int int1 = 0; // beginning of method block if (condition) { int int2 = 0; // beginning of "if" block ... } }The one exception to the rule is indexes of for loops, which in Java can be declared in the for statement:
for (int i = 0; i < maxLoops; i++) { ... }Avoid local declarations that hide declarations at higher levels. For example, do not declare the same variable name in an inner block:
int count; ... myMethod() { if (condition) { int count = 0; // AVOID! ... } ... }
Defining variable in a wider scope than needed hinders understandability quite a bit. Limited scope signals that this variable has meaning for only this small block of code and you can not think about when reading further. This is a pretty important issue because of the tiny short-term working memory that the brain has (it said that on average you can keep track of only 7 things). One less thing to keep track of is significant.
Similarly you really should try to avoid variables in the literal sense. Try to assign all things once, and declare them final so this is known to the reader. Not having to keep track whether something changes or not really cuts the cognitive load.