An identifier is simply the name given to a class, method, variable, etc. in a Java program.
An identifier in Java is a sequence of characters of unlimited length.
The sequence of characters includes all Java letters and Java digits, the first of which must be a Java letter.
For example, A-Z, a-z, _ (underscore), and $ are considered Java letters from the ASCII character set range of Unicode.
Examples of valid identifiers are as follows:
|num1||Can use a-z and 0-9 and start with a letter|
|aLetter||Can mix uppercase and lowercase|
|a1Letter||Can mix letter and digit as long as the first letter is not digit|
|_aaa||Can start with an underscore|
|_||Can have only one letter, which is an underscore|
|sum_of_two_numbers||Can have letters and underscores|
|Outer$Inner$Level||Can have a-z, A-Z and $|
|$var||Can start with $|
Examples of invalid identifiers are as follows:
|2num||Cannot start with a number|
|my name||Cannot have a space|
|num1+num2||Cannot have + sign|
|num1-num2||Cannot have minus sign|