Identifiers are the names that identify elements like classes, methods, and variables in a program.
For example; java
, util
, Scanner
, Fnumout
, main
, String
, args
, System
, out
, println
, Scanner
, in
, fnumber
, input
, nextDouble
, etc. are all identifiers, or the names of things that appear in the program example below:
import java.util.Scanner; public class Fnumout { public static void main(String[] args) { //Prompt the user to enter a floating point number System.out.println("Please enter a floating point number"); //Make a Scanner Object, which is stored in the input variable Scanner input = new Scanner(System.in); //read the floating point number off the keyboard, and store that value in the variable fnumber double fnumber = input.nextDouble(); System.out.println("This is the floating point number that you typed on the keyboard " + fnumber); } }
Rules for Identifiers:
- So, an identifier is allowed to be a sequence of character that consist of letters, digits, underscores
_
and dollar signs$
. - An identifier must start with a letter, underscore
_
or dollar sign$
. It cannot start with a digit. - An identifier cannot be a reserved keyword.
- An identifier cannot be
true
,false
, ornull
. - An identifier can be of any length.
For example, $2
, Compute Area
, area
, radius
, and print
are legal identifiers; whereas 2A
and d+4
are not legal- they do not follow the rules. The Java compiler detects illegal identifiers and reports them as syntax errors.
Note that since Java is case sensitive, area
, Area
, and AREA
are all different identifiers.
Also, you should make identifiers descriptive so that the program is easy to read. ie. use NumberOfStudents
over something vague like numStuds
. However, its conventionally correct to occasionally use variable names such as i
, j
, k
, x
, and y
for brevity.
Conventionally, you should not name identifiers with $
character, because by convention the $
character should only be used in mechanically generated source code.