Parser

package aaa;

import java.io.IOException;

public class parser {

	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub
		System.out.println("Digite uma expresao:" );
		Parser parse = new Parser();
		parse.expr();
	}

	static class Parser{
		char lookahead;
		public Parser() throws IOException {
			lookahead = (char) System.in.read();
		}
		// expr -> term rest
		void expr() throws IOException {
			term();
			rest();
		}
		void rest() throws IOException {
			if(lookahead == '+') {
				match('+');
				term();
				System.out.print('+');
				System.out.print(' ');
				rest();
			} else if(lookahead == '-') {
				match('-');
				term();
				System.out.print('-');
				System.out.print(' ');
				rest();
			} else {

			}

		}
		void term() throws IOException {
			char t = lookahead;
			if (Character.isDigit(t)) { // checa se eh um numero
				match(lookahead);
				System.out.print(t);
				System.out.print(' ');
			}
		}

		void match(char t) throws IOException {
			if(lookahead == t) {
				lookahead = (char) System.in.read();
			} else {
				throw new RuntimeException("Erro de sintaxe");
			}
		}

	}

}