Java Lexical Analyzer

I need assistance in wrapping my head around the development of a lexical analyzer in Java.  It has been a little bit since I have taken my last java course, however am fairly familiar with the language.  In the class I am taking we are working with BNF grammars and using recursive descent parsing to traverse these grammars and verify that a user input string is valid according to the rules of the grammar.  I have a decent pseudo code design for the grammar and have pieced most of it together, however the design of the lexical analyzer is eluding me to read the user input string.  
There are only 3 letters I am concerned with a, b and c.  I am trying to stay with the book here and declare the tokens separate as an enumerated type (i.e. {NONE, LETTER, ERROR, END_OF_FILE}).
My initial question:
Is this basic idea correct for the lexer?
•      Start with character NONE
•      Look at next character
    o          if character = a, b, or c
¿              Add character to lexeme
¿              Look at Next character
              •       Continue until end of user input string
    o          If there is no character
¿              Return lexeme
    o          If character != a, b, or c
¿              Return ERROR token
AutkastAsked:
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dpearsonConnect With a Mentor Commented:
Yes that approach basically sounds right, although you don't actually say what the valid lexemes are.  It sounds like you're accepting strings of the form "[abc]+"?

The only part that looks a little odd is here:

>> If there is no character
>>             Return lexeme

Usually this would be "if the next character is white space, then return lexeme".  But there appears to be no white space in your character set, so maybe this is equivalent to what you have?

E.g. If you were parsing java, and the lexical analyzer was reading an identifier, it would normally terminate at the first character not in the valid set for the identifier:

int abc = 10 ;

while parsing 'abc' you want the lexical analyzer to stop at the ' ' (space) after 'abc' - because it's the first character not allowed within an identifier - and return the lexeme at that point.

Anyway, hope that helps,

Doug
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