Put aside the validity of the rule you cite (which I disagree with, by the way, because I think making the code less readable is a worse sin than any minor performance improvement—if any—that transformation might provide), the resulting code should be:
return !(this.message.length () > ZERO && this.message.charAt(ZERO) == Slash);
You have the parentheses around only the first term of the condition, which is wrong. A slightly more readable version would be:
return this.message.length () <= ZERO || this.message.charAt(ZERO) != Slash;