How can I resolve the lex(String) is undefined for the type main?

public class main {

public static void main(String[] args) { //creating file object File file = new File("input.txt"); //scanner object to scan the input file Scanner sc = null; String input = ""; try { sc = new Scanner(file); //traversing the entire input file untile all the lines found while (sc.hasNextLine()) { input += sc.nextLine()+" "; } } catch (FileNotFoundException e) { System.out.println("File Not Found. Please Check Input File"); } // Create tokens and print them ArrayList tokens = lex(input); for (Token token : tokens) System.out.println(token); } }