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.