Java Expressions and Assignments

// This class is a recursive-descent parser, // modeled after the programming language's grammar. // It constructs and has-a Scanner for the program // being parsed. import java.util.*; public class Parser { private Scanner scanner; private void match(String s) throws SyntaxException { scanner.match(new Token(s)); } private Token curr() throws SyntaxException { return scanner.curr(); } private int pos() { return scanner.pos(); } private NodeMulop parseMulop() throws SyntaxException { if (curr().equals(new Token("*"))) { match("*"); return new NodeMulop(pos(),"*"); } if (curr().equals(new Token("/"))) { match("/"); return new NodeMulop(pos(),"/"); } return null; } private NodeAddop parseAddop() throws SyntaxException { if (curr().equals(new Token("+"))) { match("+"); return new NodeAddop(pos(),"+"); } if (curr().equals(new Token("-"))) { match("-"); return new NodeAddop(pos(),"-"); } return null; } private NodeFact parseFact() throws SyntaxException { if (curr().equals(new Token("("))) { match("("); NodeExpr expr=parseExpr(); match(")"); return new NodeFactExpr(expr); } if (curr().equals(new Token("id"))) { Token id=curr(); match("id"); return new NodeFactId(pos(),id.lex()); } Token num=curr(); match("num"); return new NodeFactNum(num.lex()); } private NodeTerm parseTerm() throws SyntaxException { NodeFact fact=parseFact(); NodeMulop mulop=parseMulop(); if (mulop==null) return new NodeTerm(fact,null,null); NodeTerm term=parseTerm(); term.append(new NodeTerm(fact,mulop,null)); return term; } private NodeExpr parseExpr() throws SyntaxException { NodeTerm term=parseTerm(); NodeAddop addop=parseAddop(); if (addop==null) return new NodeExpr(term,null,null); NodeExpr expr=parseExpr(); expr.append(new NodeExpr(term,addop,null)); return expr; } private NodeAssn parseAssn() throws SyntaxException { Token id=curr(); match("id"); match("="); NodeExpr expr=parseExpr(); NodeAssn assn=new NodeAssn(id.lex(),expr); return assn; } private NodeStmt parseStmt() throws SyntaxException { NodeAssn assn=parseAssn(); match(";"); NodeStmt stmt=new NodeStmt(assn); return stmt; } public Node parse(String program) throws SyntaxException { scanner=new Scanner(program); scanner.next(); return parseStmt(); } }