lA Java identifier
must start with a letter or underscore or dollar sign,
and be followed by zero or more letters (A-Z, a-z), digits (0-9), underscores,
or dollar signs.
lVALID
l age_of_dog
taxRateY2K
l HourlyEmployee
ageOfDog
lNOT VALID (Why?)
l age#
2000TaxRate Age-Of-Dog