Problem with queues / stacks for a project

Question

I have to code a calculator that turns an infix expression, where the operator is in the middle, to a postfix expression, where the operators are at the end;
so, for instance, 5 + 5 would really be 5 5 +, and 5 + 5 * 5 / 5 would be 5 5 5 5 + * /.
I've gotten the addition and subtraction working fine, the only problem comes when I have to take into account order of operations (so far).
So, that being said, here is the code:
import java.util.*;
public class iCalc {
      * @param args
     public static void main(String[] args) {
     //     Scanner scan = new Scanner(System.in);
     //     System.out.println("Equation:");
     //     String input = "55+5-10%16."; //might have to store each element in a linked list later
     //     evalExp(input);     //calls evalExp with the input
          String input2 = "55 + 5 * 10 + 16";//declare some dummy input
          //121
          String [] items = input2.split(" ");//split the string's elements into an array
          Queue<String> queue = new LinkedList<String>();//the queue
          Stack<String> stack = new Stack<String>();//make a new stack to store operators
          Stack<String> stackOutput = new Stack<String>();//output stack
          for (String s : items) {//scan the array
              System.out.println("item : " + s);//print out what's being scanned
              if(isNumber(s)) queue.add(s);//add it onto the queue
              if(isOperator(s)) stack.push(s);//add it onto the stack
          //converts to postfix here
         while(!stack.isEmpty()) queue.add(stack.pop());
         evalExp(queue, stackOutput);
     public static void evalExp(Queue<String> exp, Stack<String> outputStack)
          System.out.println("Evaluating the expression...");
          int length = exp.size();
          int temp;
     while(length > 0)
          // 55 5 10 16 + * +
          // 55
          try{
               length--;
               System.out.println("Trying to see if the element is a number...");
               if(isMultiplyOrDivide(exp))//is there a multiply or divide operator
                    int spot = findMultiplyOrDividePosition(exp);//if yes, finds its position in the operators and stores it as an int
                    temp = 0;
                    for(String s : exp)
                    {//PROBLEM: After the first iteration, it suddenly has an empty stack exception
                         temp++;//keeps track of how far you are into the queue
                         outputStack.push(exp.remove());//take the head of the queue, and push it onto the stack
                         if(temp == spot+1)
                              break;     //(if there were enough elements in the queue popped, end this loop
                    outputStack.push(performMath(outputStack.pop(), outputStack.pop(), popTheOperator(exp)));
               Double MyNum = Double.parseDouble(exp.remove());
               String MyStackNum = "" + MyNum;
               outputStack.push(MyStackNum);
                    if(outputStack.size() == 2)
                         length--;
                         String operator = popTheOperator(exp);
                         System.out.println("Two numbers have been popped, time to find the operator...");     
                         outputStack.push(performMath(outputStack.pop(), outputStack.pop(), operator));
                         for(String s : exp)
                              if(s.equals(operator))
                                   exp.remove(s);
                                   break;
          }     catch(Exception ParseException){
               outputStack.push(performMath(outputStack.pop(), outputStack.pop(), exp.remove()));               
          while(!outputStack.isEmpty()) System.out.println("The output stack is:" + outputStack.pop());     
     public static boolean isNumber(String s1)
          char[]arr2 = s1.toCharArray();
          for(int i = 0;i<s1.length();i++)
               System.out.println("checking...." + s1);
               if((!Character.isDigit(arr2))) return false;
          return true;
     public static boolean isOperator(String s2)
          char[]arr3 = s2.toCharArray();
          for(int i = 0;i<arr3.length;i++)
               System.out.println("checking...." + arr3[i]);
               if(findOperator(arr3[i])) return true;
          return false;
     public static boolean findOperator(char op)
                    if(op == '+' ||
                         op == '-' ||
                         op == '/' ||
                         op == '*' ||
                         op == '%' ||
                         op == '^' ||
                         op == '&' ||
                         op == '(' ||
                         op == ')' ||
                         op == '|' ||
                         op == '>' ||
                         op == '<' ||
                         op == '=' ||
                         op == '!' ) return true;          
          return false;          
     // 10 16 + + +
     public static String popTheOperator(Queue<String> myQueue)
          String current = "";
          for(String s: myQueue)
               if(s.equals("*") || s.equals("/"))
                    current = s;
               break;
          if(current == "")
               for(String s: myQueue)
                    if(s.equals("+") || s.equals("-"))
                         current = s;
                         break;
     return current;     
     public static String performMath(String operand1, String operand2, String operator)
          String myString = "";
          double result = 0;
          double temp1 = Double.parseDouble(operand1);
          double temp2 = Double.parseDouble(operand2);
          if(operator.equals("+"))
               result = temp1 + temp2;
          else if(operator.equals("-"))
               result = temp2 - temp1;
          else if(operator.equals("/"))
               result = temp1 / temp2;
          else if(operator.equals("*"))
               result = temp1 * temp2;
          else if(operator.equals("%"))
               result = temp1 % temp2;
          else if(operator.equals("^"))
               result = Math.pow(temp1, temp2);
          else if(operator.equals("&"))
          else if(operator.equals("|"))
               result = temp1 - temp2;
          else if(operator.equals(">"))
               if(temp1 > temp2)
                    result = 1;
               else result = 0;
          else if(operator.equals("<"))
               if(temp1 < temp2)
                    result = 1;
               else result = 0;
          else if(operator.equals("="))
               if(temp1 == temp2)
                    result = 1;
               else result = 0;
          else if(operator.equals("!"))
               if(temp1 != temp2) result = 1;
               else result = 0;
          myString = "" + result;
          return myString;
     public static boolean isMultiplyOrDivide(Queue<String> myQueue)
          for(String s : myQueue)//scan the queue
               if(s.equals("*") || s.equals("/"))
                    return true;//it has found a multiply/divide operator!
          return false;
     public static int findMultiplyOrDividePosition(Queue<String> myQueue)
          int position = 0;
          for(String s: myQueue)
               if(s.equals("+") ||
                              s.equals("-") ||
                              s.equals("%") ||
                              s.equals("^") ||
                              s.equals("&") ||
                              s.equals("|") ||
                              s.equals(">") ||
                              s.equals("<") ||
                              s.equals("=") ||
                              s.equals("!")) position ++;
          return position;
}I think that its trying to remove the head, but because the head was already removed, somehow its not letting it remove it; as if by removing the head, there is no new updated head.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           

Answer 1

               if(isMultiplyOrDivide(exp))//is there a multiply or divide operator
                    int spot = findMultiplyOrDividePosition(exp);//if yes, finds its position in the operators and stores it as an int
                    temp = 0;
                    for(String s : exp)
                    {//PROBLEM: After the first iteration, it suddenly has an empty stack exception
                         temp++;//keeps track of how far you are into the queue
                         outputStack.push(exp.remove());//take the head of the queue, and push it onto the stack
                         if(temp == spot+1)
                              break;     //(if there were enough elements in the queue popped, end this loop
                    outputStack.push(performMath(outputStack.pop(), outputStack.pop(), popTheOperator(exp)));
               }the problem is somewhere in here.