Lazy Evaluation

Computer Science 3Gerb

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?trueTake 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.