31. Increment and Decrement Operators

The increment operator increments a value to its next value. When you increment or decrement a variable using ++ or -- the variable is modified to the new value.

The increment and decrement operators come in two flavours, prefix and postfix:

• Prefixes are assessed before the statement is evaluated
  • assert ++5 == 6
• Postfixes are assessed after the statement is evaluated
  • assert 5++ == 5

assert 10++ == 10
assert ++10 == 11
assert --10 == 9
assert 10-- == 10

The increment and decrement behaves differently depending on the type of value being used.

Booleans don’t increment/decrement

Numbers increment/decrement by 1:

def num = 10
num++
assert num == 11

Characters move to the previous (--) or next (++) character:

def ch = 'c'
ch--
assert ch == 'b'

Strings are a little odd and it is the last character in the string that is affected:

def str = 'hello'
str++
assert str == 'hellp'

Enums¹ will cycle through the enum values:

enum Priority {
    LOW, MEDIUM, HIGH
}
def task = Priority.LOW
task++
assert task == Priority.MEDIUM

BUT be aware that you’ll cycle back to the beginning of the value list. The following example is a good example of where you can easily get caught out:

def task = Priority.LOW
task--
assert task == Priority.HIGH

Overloading the Increment and Decrement Operators

By overloading next and previous methods, a custom class can support the increment and decrement operators.

The example below demonstrates a class that increments/decrements by 2:

class StepTwo extends Object {
    def value

    StepTwo(val) {
        this.value = val
    }


    def next() {
        value += 2
        return this
    }

    def previous() {
        value -= 2
        return this
    }

    String toString() {
        return "I have a value of ${this.value}"
    }
}

def two = new StepTwo(3)
println two
two++
println two
two--
println two