The unary operator NOT, !, works on a single value. This operator evaluates to the opposite boolean to which it’s applied:

!false // true !true // false

NOT is useful for expressing our intent clearly in programs. For example, sometimes we need the opposite behavior of an if-then: run a code block only if the condition is false.

boolean hasPrerequisite = false; if (hasPrerequisite) { // do nothing } else { System.out.println("Must complete prerequisite course!"); }

This code does what we want but it’s strange to have a code block that does nothing!

The logical NOT operator cleans up our example:

boolean hasPrerequisite = false; if (!hasPrerequisite) { System.out.println("Must complete prerequisite course!"); }

We can write a succinct conditional statement without an empty code block.



Let’s make informUser() more informative. If their reservation is not confirmed, they should know!

Write an if-then-else statement and use ! with isConfirmed as the condition.

If their reservation is not confirmed, write Unable to confirm reservation, please contact restaurant.

Else write: Please enjoy your meal!

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