Lazy Evaluation Computer Science 3 Gerb Objective: Understand lazy evaluation in Java

Short-Circuit Evaluation If the first operand of an && is false, the expression is false no matter what the second is. –Pointless even to look at it –Therefore Java won’t look at the second operand of && if the first is false. Example: if (method1() && method2())... –If method1 returns false, method2 won’t even be called. –Important if method2 has side effects like printing, those side effects will not happen. This technique for evaluating && is called short- circuit evaluation or lazy evaluation.

Short-Circuit Evaluation of || If the first operand of || is true, the expression is true no matter what the second is. Therefore Java uses short-circuit evaluation for || as well. If the first operand of || is true, the second is never evaluated.

Why is Short Circuit Evaluation Important When a method does something other than returning a value, we call that a side effect Examples : –Printing something to console –Changing the value of an instance variable –Onscreen graphics If a method is not called, its side effects won’t happen

For Example: What happens if I answer true to the first question? class foo { public static boolean ask(String q){ System.out.println(q+“?”); return sc.nextBoolean(); } public static void main(String args){ if (ask(“Are you over 17”) || ask(“Did you take Driver Ed”)) System.out.println(“Take driving test”);} }

Will not even call ask a second time, so the Driver Ed question won’t be asked class foo { public static boolean ask(String q){ System.out.println(q+”?”); return sc.nextBoolean(); } public static void main(String args){ if (ask(“Are you over 17”) || ask(“Did you take Driver Ed”)) System.out.println(“Take driving test”);} } Are you over 17? true Take driving test

Summary && true if both operands are true, false otherwise. || false if both operands are false, true otherwise. ! true if single operand is false, false otherwise. Short-circuit, or lazy, evaluation –Second operand is only evaluated if necessary.