Infix to postfix translator

Postfix.java

package com.example;

import java.io.*;

class Parser {
  
	static int lookahead;
	
	public Parser() throws IOException {
		lookahead = System.in.read();
	}
	
	void expr() throws IOException {
		term() ;
		while(true) {
			if( lookahead == '+' ) {
				match('+') ; term() ; System.out.write('+') ;
			}
			else if( lookahead == '-' ) {
				match('-'); term(); System.out.write('-');
			}
			else return;
		}
	}
	
	void term() throws IOException {
		if (Character.isDigit((char) lookahead)) {
			System.out.write((char) lookahead);
			match(lookahead);
		} 
		else throw new Error("syntax error");
	}
	
	void match(int t) throws IOException {
		if( lookahead == t ) lookahead = System.in.read() ;
		else throw new Error("syntax error");
	}
	
}

public class Postfix {
	public static void main(String[] args) throws IOException {
		Parser parse = new Parser();
		parse.expr();
		System.out.write('\n');
	}
}