smart calc en java

/**
 *
 * @author amine_err
 */
public class Calc {

    public static boolean isNumber(String str){
        return str.matches("-?(\\d+|\\d+\\.\\d+|\\.\\d+|\\d+\\.)");
    }

    public static double pow(double a, int b){
        double res = 1;
        int i = 0;
        while( i<b ){
            res *= a;
            i++;
        }
        return res;
    }

    public static double round(double n, int a){
        double p = pow(10, a);
        return Math.round(p*n)/p;
    }

    public static boolean check(String exp) throws Exception {
        //if syntax correct
        // not empty
        if ("".equals(exp)) {
            throw new Exception("empty! ");
        }
        // no 2 [*,.,|] in a row
        String ops[] = new String[]{"+", "-", "*", "/", "%", "^"};
        for (String op : ops) {
            for (String op1 : ops) {
                if (exp.contains(op + op1)) {
                    //throw ;
                    throw new Exception("syntax error! " + op + op1);
                }
            }
        }
        // no op)
        for (String op : ops) {
            if (exp.contains(op + ")")) {
                throw new Exception("syntax error! " + op + ")");
            }
        }
        // no (["*", "/", "%", "^"]
        for (int i = 2; i < ops.length; i++) {
            if (exp.contains("(" + ops[i])) {
                throw new Exception("syntax error! " + "(" + ops[i]);
            }
        }
        // no start with ["*", "/", "%", "^"]
        for (int i = 2; i < ops.length; i++) {
            if (exp.charAt(0) == ops[i].charAt(0)) {
                throw new Exception("syntax error! starting with " + ops[i] );
            }
        }
        //
        int k = 0;
        for (int i = 0; i < exp.length(); i++) {
            if (exp.charAt(i) == '(') {
                k++;
            }
            if (exp.charAt(i) == ')') {
                k--;
            }
            if (k < 0) {
                throw new Exception("syntax error! mismatched parentheses ");
            }
        }
        if (k > 0) {
            throw new Exception("syntax error! mismatched parentheses ");
        }
        return true;
    }

    public static double calc(String str) throws Exception{
        // if str is a simple number
        if ( isNumber(str) ) {
            return Double.parseDouble(str);
        }
        else if (!str.equals("")) {
            // (----) => ----
            if (str.charAt(0) == '(' && str.charAt(str.length() - 1) == ')') {
                return calc(str.substring(1, str.length() - 1));
            }
            int k, i;
            // +, -
            for (i = str.length() - 1; i > -1; i--) {
                if (str.charAt(i) == ')') {
                    k = -1;
                    while (!(k == 0 && str.charAt(i) == '(')) {
                        i--;
                        if (str.charAt(i) == ')')
                            k--;
                        else if (str.charAt(i) == '(')
                            k++;
                    }
                    i--;
                }
                if (i > -1) {
                    if (str.charAt(i) == '+') {
                        return calc(str.substring(0, i))+calc(str.substring(i + 1, str.length()));
                    }
                    if (str.charAt(i) == '-') {
                        return calc(str.substring(0, i))-calc(str.substring(i + 1, str.length()));
                    }
                }
            }
            // *, /, %
            for (i = str.length() - 1; i > -1; i--) {
                if (str.charAt(i) == ')') {
                    k = -1;
                    while (!(k == 0 && str.charAt(i) == '(')) {
                        i--;
                        if (str.charAt(i) == ')')
                            k--;
                        else if (str.charAt(i) == '(')
                            k++;
                    }
                    i--;
                }
                if (i > -1) {
                    if (str.charAt(i) == '*') {
                        return calc(str.substring(0, i))*calc(str.substring(i + 1, str.length()));
                    }
                    if (str.charAt(i) == '/') {
                        return calc(str.substring(0, i))/calc(str.substring(i + 1, str.length()));
                    }
                    if (str.charAt(i) == '%') {
                        return calc(str.substring(0, i))%calc(str.substring(i + 1, str.length()));
                    }
                }
            }
            for (i = 0; i < str.length(); i++) {
                if (str.charAt(i) == '(') {
                    k = 1;
                    while (!(k == 0 && str.charAt(i) == ')')) {
                        i++;
                        if (str.charAt(i) == ')')
                            k--;
                        else if (str.charAt(i) == '(')
                            k++;
                    }
                    i++;
                }
                if (i < str.length()) {
                    if (str.charAt(i) == '^') {
                        return pow(calc(str.substring(0, i)),(int)calc(str.substring(i + 1, str.length())));
                    }
                }
            }
            // look if there is a function
            int fi = str.indexOf("(", 1);
            if( fi!=-1 ){
                String function_name = str.substring(0,fi);
                int epi = str.length()-1;
                String parameter = str.substring(fi+1,epi);
                switch (function_name) {
                    case "cos":
                        return Math.cos( calc(parameter+"/180*"+Math.PI) );
                    case "sin":
                        return Math.sin( calc(parameter+"/180*"+Math.PI) );
                    case "tan":
                        return Math.tan( calc(parameter+"/180*"+Math.PI) );
                    default:
                        throw new Exception(function_name+": is not defined");
                }
            }
        }
        return 0;
    }

    public static void resultat(String expr){
        expr = expr.replaceAll(" ", "");
        try {
            if( check(expr) )
                System.out.println(round(calc(expr), 15));
        } catch (Exception ex) {
            System.out.println(ex.getMessage());
        }
    }

    public static void main(String[] args) {
        String expr = "1+2*2 +6*(1.2 -0.2)/3 +  cos(90)";
        resultat(expr);

        String expr1 = "5+2*10+ccos(60)";
        resultat(expr1);

        String expr2 = "(10*5+10)^2/100";
        resultat(expr2);
    }
}