Calculadora java

import java.util.Scanner;

import java.util.Stack;

import java.util.StringTokenizer;

import java.util.Vector;

static String expressao;
	public static void main(String args[]){
		
		
		System.out.println(" Digite a expressão matemática" + "\n pode conter até três variáveis (x, y, z):");
	
		Scanner in = new Scanner(System.in);
		expressao = in.next();
		if(!Parenteses()){
			System.out.println("Expressão inválida!");		
		}
		if(expressao.indexOf("x") >= 0){
			System.out.println("Digite um valor para x:");
			String x1 = in.next();	
			expressao = expressao.replace("x", x1);
		}
		if(expressao.indexOf("y") >= 0){
			System.out.println("Digite um valor para y:");
			String y1 = in.next();
			expressao = expressao.replace("y", y1);
		}
		if(expressao.indexOf("z") >= 0){
			System.out.println("Digite um valor para z:");
			String z1 = in.next();
			expressao = expressao.replace("z", z1);
		}
		
		else{
			System.out.println(expressao + " = " + String.valueOf(res(expressao)));
			}
		
	}
	public static boolean Parenteses(){
		int abrepar = expressao.replaceAll("[^(]", "").length();
		int fechapar = expressao.replaceAll("[^)]", "").length();
	
		if(abrepar != fechapar){
			return false;
		}
		return true;
	}
	
	static Vector<String> Tokens(String expressao){
		Vector<String> r = new Vector<String>();
		StringTokenizer exp1;
		
		exp1 = new StringTokenizer(expressao, "()+-*/", true);
		while(exp1.hasMoreTokens()){
			String exp2 = exp1.nextToken();
			r.addElement(exp2);			
		}
		return r;
	}
	static Vector<String> Conv(Vector<String> v){
		Vector<String> r = new Vector<String>();
		Stack<String> p = new Stack<String>();
		for(int i = 0; i < v.size(); i++){
			String t = (String) v.elementAt(i);
			if(t.equals("*") || t.equals("/") || t.equals("+") || t.equals("-")){
				p.push(t);
			}
			else if(t.equals(")")){
				r.addElement(p.pop());
			}
			else if(!t.equals("(")){
				r.addElement(t);
			}
		}
		return r;
	}
	static int res(String expressao){
		Vector<String> v = Tokens(expressao);
		v = Conv(v);
		Stack<String> p = new Stack<String>();
		
		for(int i = 0; i < v.size(); i++){
			String t = (String) v.elementAt(i);
			if(t.equals("+")){
				int e2 = Integer.parseInt(p.pop());
				int e1 = Integer.parseInt(p.pop());
				p.push(String.valueOf(new Integer( e1 + e2)));
			}
			else if(t.equals("-")){
				int e2 = Integer.parseInt(p.pop());
				int e1 = Integer.parseInt(p.pop());
				p.push(String.valueOf(new Integer(e1 - e2)));
			}
			else if(t.equals("*")){
				int e2 = Integer.parseInt(p.pop());
				int e1 = Integer.parseInt(p.pop());
				p.push(String.valueOf(new Integer(e1 * e2)));
			}
			else if(t.equals("/")){
				int e2 = Integer.parseInt(p.pop());
				int e1 = Integer.parseInt(p.pop());
				p.push(String.valueOf(new Integer(e1 / e2)));
			}
			else{
				p.push(String.valueOf(new Integer(t)));
			}
		}
		return (Integer.parseInt(p.pop()));
		
	}
		
}