Identifiers in Java

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:

• MyVariable
• myvariable
• MYVARIABLE
• x
• i
• _myvariable
• $myvariable
• _9pins
• andros
• ανδρος
• OReilly
• This_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_it

The following are not legal variable names:

• My Variable // Contains a space
• 9pins // Begins with a digit
• a+c // The plus sign is not an alphanumeric character
• testing1-2-3 // The hyphen is not an alphanumeric character
• O'Reilly // Apostrophe is not an alphanumeric character
• OReilly_&_Associates // ampersand is not an alphanumeric character

Tip: How to Begin a Variable Name with a Number

If 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.