Short-circuit evaluation of booleans

 By now, you may have noticed this already, but it’s so important that it merits an article of its own: there are some instances in which you don’t care—so you don’t even compute—what certain expressions’ values are.

Suppose you have something like:

if(isMonday  || isWednesday || isFriday){
// stuff
}

Then, knowing that || means logical-or (so this means, in English, “if today is Monday, Wednesday, or Friday…”), one might look at the truth table for OR. The truth table contains all the possible inputs and their corresponding outputs:

For “OR”, that looks like:

Var1	Var2	Result
TRUE	TRUE	TRUE
TRUE	FALSE	TRUE
FALSE	TRUE	TRUE
FALSE	FALSE	FALSE

That is, 2-way logical OR is false only if BOTH are false (3-, 4-way, etc., would only be false if ALL are false). Put another way, if ANY are true, then OR is true.

AND, likewise, has a table.

Var1	Var2	Result
TRUE	TRUE	TRUE
TRUE	FALSE	FALSE
FALSE	TRUE	FALSE
FALSE	FALSE	FALSE

For a logical AND to return true, both things being AND-ed must be true (likewise for multi-way AND, ALL must be true for it to be true; any falses cause an overall false). If BOTH are true, then AND is true; if ANY are false, then AND is false.

Back to our day-of-the-week example, once we see that any day matches with today, we don’t have to look at any of the other checks, since the expression is entirely OR-based. Just the fact that one of those is true, provided that they’re all connected by OR, is enough to make the whole expression true, and we can jump into whatever code is prescribed for that case.

Similarly, in a situation where

if(is16OrOlder && hasLicense && noRestrictions && carIsWorking){
// do stuff

}

If any of these are found to be false, we don’t need to check the others. AND works in such a way that, unless all chained ANDs are true, the whole thing is false.

This is a simple and incredibly common logical optimization in Java performed at the compiler level, known as short-circuit evaluation. If you have a sophisticated IDE, it will also probably tell you if it finds any variables that can be short-circuited. IntelliJ, at least in light mode, highlights such expressions in a sepia-like color (which it also uses for infinite loops, or loops that don’t loop).