[Java] String Calculator With Parentheses

import java.util.*;import java.lang.String.*;public class Calculator{    private Stack<Double> numbers = new Stack<Double>();    //Stack numbers store the numbers to be calculated.    private Stack<String> operators = new Stack<String>();  //Stack operators store the operators to be used.    String number = ""; //Use the number string to temporarily store the number.    //Compare the priority between two operators,return true if and only if the former is more weighted.(The same priority will return false.)    public boolean prior(String former, String latter){        if((former.equals("*")||former.equals("/")) && (latter.equals("+")||latter.equals("-")||latter.equals("(")))            return true;        else if((former.equals("+") || former.equals("-")) && latter.equals("("))            return true;        else            return false;    }    //Calculate the two numbers.    public double figureout(double former, String operator, double latter){        if(operator.equals("+"))            return former+latter;        else if(operator.equals("-"))            return former-latter;        else if(operator.equals("*"))            return former*latter;        else if(operator.equals("/"))            return former/latter;        else return 0;    }    //identify the char.    public int classifyOperator(String operator){        if(operator.equals("0")||operator.equals("1")||operator.equals("2")||           operator.equals("3")||operator.equals("4")||operator.equals("5")||           operator.equals("6")||operator.equals("7")||operator.equals("8")||           operator.equals("9")||operator.equals("."))            return 0;        if(operator.equals("+")||operator.equals("-")||operator.equals("*")||           operator.equals("/"))            return 1;        if(operator.equals("("))            return 2;        if(operator.equals(")"))            return 3;        else            return 4;    }    //Calculate the String.    public double cal(String sentense){        sentense = "("+sentense+")";    //Add parentheses ahead.        for(int i = 0;i<sentense.length();i++){            if(classifyOperator(""+sentense.charAt(i)) == 0){   //If  we meet a number character we just add it behind the number string.                number = number+sentense.charAt(i);            }            else{                if(!number.equals(""))                    numbers.push(Double.parseDouble(number));   //Whenever it comes to character which isn't a number, parse the whole number string and push it to the stack.                number = "";    //Reset the string to null every time we push the number to the stack.                System.out.println("Now the numbers stack is"+numbers);                if(classifyOperator(""+sentense.charAt(i)) == 2)    //Push the "(" to the stack directly.                    operators.push(""+sentense.charAt(i));                if(classifyOperator(""+sentense.charAt(i)) == 3){   //When it comes to ")".                    while(!operators.peek().equals("(")){   //Do the math with all the operators stored in the operators stack until we meet "("                        double latter = numbers.pop();                        double former = numbers.pop();                        numbers.push(figureout(former,operators.pop(),latter));                    }                    operators.pop();    //Pop the "(" left.            }                if(classifyOperator(""+sentense.charAt(i)) == 1){   //When it comes to +-*/                    if(prior((""+sentense.charAt(i)),operators.peek()))     //If the present char has priority over the one at the top of the stack, just push it in.                        operators.push(""+sentense.charAt(i));                    else{                                                   //else, we repeatedly do the math until we find some operator in the operators stack that is less weighted.                        double latter = numbers.pop();                        double former = numbers.pop();                        do{                            numbers.push(figureout(former,operators.pop(),latter));                        }while(!(prior((""+sentense.charAt(i)),operators.peek())));                        operators.push(""+sentense.charAt(i));              //Push the current char into the stack.                    }                }            }            System.out.println("The number String is "+number);            System.out.println("The numbers stack is "+numbers);            System.out.println("The operators stack is"+operators);            System.out.println("The charAt  is "+sentense.charAt(i));            System.out.println("-----------");        }        while(!operators.empty()){                                      //In the end there may be some numbers left to be calculate.            double latter = numbers.pop();            double former = numbers.pop();            numbers.push(figureout(former,operators.pop(),latter));        }        return numbers.pop();    }    public static void main(String[] args){        String sentense = "((2+3)*7+2)*2+(5-7)*2";        Calculator calculate = new Calculator();        System.out.println(calculate.cal(sentense));    }}