Identifiers are the names of variables, methods,
classes, packages and interfaces. Unlike literals they are not the
things themselves, just ways of referring to them. In the
HelloWorld program, HelloWorld, String,
args, main and println are identifiers.
Identifiers must be composed of letters, numbers, the underscore
_ and the dollar sign $. Identifiers may
only begin with a letter, the underscore or a dollar sign.
Each variable has a name by which it is identified in the
program. It's a good idea to give your variables mnemonic names
that are closely related to the values they hold. Variable names
can include any alphabetic character or digit and the underscore _.
The main restriction on the names you can give your variables is
that they cannot contain any white space. You cannot begin a
variable name with a number. It is important to note that as in C
but not as in Fortran or Basic, all variable names are
case-sensitive. MyVariable is not the same as
myVariable. There is no limit to the length of a Java
variable name. The following are legal variable names:
MyVariablemyvariableMYVARIABLExi_myvariable$myvariable_9pinsandrosανδροςOReillyThis_is_an_insanely_long_variable_name_that_just_keeps_going_and_going_and_going_and_well_you_get_the_idea_The_line_breaks_arent_really_part_of_the_variable_name_Its_just_that_this_variable_name_is_so_ridiculously_long_that_it_won't_fit_on_the_page_I_cant_imagine_why_you_would_need_such_a_long_variable_name_but_if_you_do_you_can_have_itThe following are not legal variable names:
My Variable // Contains a space9pins // Begins with a digita+c // The plus sign is not an alphanumeric
charactertesting1-2-3 // The hyphen is not an alphanumeric
characterO'Reilly // Apostrophe is not an alphanumeric
characterOReilly_&_Associates // ampersand is not an
alphanumeric characterIf you want to begin a variable name with a digit, prefix the name you'd like to have (e.g. 8ball) with an underscore, e.g. _8ball. You can also use the underscore to act like a space in long variable names.