In computer programming, a guard is a boolean expression that must evaluate to true if the program execution is to continue in the branch in question. Regardless of which programming language is used, guard code or a guard clause is a check of integrity preconditions used to avoid errors during execution
https://medium.com/lemon-code/guard-clauses-3bc0cd96a2d3
public void run( Timer timer ) {
if( !timer.isEnabled() )
return;
if( !timer.valid() )
throw new InvalidTimerException();
timer.run();
}