Java Expressions and Assignments

// This class is a scanner for the program // and programming language being interpreted. import java.util.*; public class Scanner { private String program; // source program being interpreted private int pos; // index of next char in program private Token token; // last/current scanned token // sets of various characters and lexemes private Set<String> whitespace=new HashSet<String>(); private Set<String> digits=new HashSet<String>(); private Set<String> letters=new HashSet<String>(); private Set<String> legits=new HashSet<String>(); private Set<String> keywords=new HashSet<String>(); private Set<String> operators=new HashSet<String>(); // initializers for previous sets private void fill(Set<String> s, char lo, char hi) { for (char c=lo; c<=hi; c++) s.add(c+""); } private void initWhitespace(Set<String> s) { s.add(" "); s.add("\n"); s.add("\t"); } private void initDigits(Set<String> s) { fill(s,'0','9'); } private void initLetters(Set<String> s) { fill(s,'A','Z'); fill(s,'a','z'); } private void initLegits(Set<String> s) { s.addAll(letters); s.addAll(digits); } private void initOperators(Set<String> s) { s.add("="); s.add("+"); s.add("-"); s.add("*"); s.add("/"); s.add("("); s.add(")"); s.add(";"); } private void initKeywords(Set<String> s) { } // constructor: // - squirrel-away source program // - initialize sets public Scanner(String program) { this.program=program; pos=0; token=null; initWhitespace(whitespace); initDigits(digits); initLetters(letters); initLegits(legits); initKeywords(keywords); initOperators(operators); } // handy string-processing methods public boolean done() { return pos>=program.length(); } private void many(Set<String> s) { while (!done() && s.contains(program.charAt(pos)+"")) pos++; } private void past(char c) { while (!done() && c!=program.charAt(pos)) pos++; if (!done() && c==program.charAt(pos)) pos++; } // scan various kinds of lexeme private void nextNumber() { int old=pos; many(digits); token=new Token("num",program.substring(old,pos)); } private void nextKwId() { int old=pos; many(letters); many(legits); String lexeme=program.substring(old,pos); token=new Token((keywords.contains(lexeme) ? lexeme : "id"),lexeme); } private void nextOp() { int old=pos; pos=old+2; if (!done()) { String lexeme=program.substring(old,pos); if (operators.contains(lexeme)) { token=new Token(lexeme); // two-char operator return; } } pos=old+1; String lexeme=program.substring(old,pos); token=new Token(lexeme); // one-char operator } // This method determines the kind of the next token (e.g., "id"), // and calls a method to scan that token's lexeme (e.g., "foo"). public boolean next() { if (done()) return false; many(whitespace); String c=program.charAt(pos)+""; if (digits.contains(c)) nextNumber(); else if (letters.contains(c)) nextKwId(); else if (operators.contains(c)) nextOp(); else { System.err.println("illegal character at position "+pos); pos++; return next(); } return true; } // This method scans the next lexeme, // if the current token is the expected token. public void match(Token t) throws SyntaxException { if (!t.equals(curr())) throw new SyntaxException(pos,t,curr()); next(); } public Token curr() throws SyntaxException { if (token==null) throw new SyntaxException(pos,new Token("ANY"),new Token("EMPTY")); return token; } public int pos() { return pos; } // for unit testing public static void main(String[] args) { try { Scanner scanner=new Scanner(args[0]); while (scanner.next()) System.out.println(scanner.curr()); } catch (SyntaxException e) { System.err.println(e); } } }