infix to postfix

Write a Java application that reads a mathematical expression in a infix form, for example : 38 + 19 * 2 – 4 / 2 and then perform the following : 1. Convert the expression to a postfix form and display it. 2. Calculate and display the result of the expression.  Homework instruction : 1- Use stack class to preform both operations. 2- Your program should be able to handle these operations: + , - , * , / , % , = and round parenthesis ) (. 3- Any Number in the expression could contain more than one digit ( ex : 23 , 4 , 432 , ….. etc ) 4- The program should display an invalid massage if an invalid character was found in the expression ( only digits and operations mentioned earlier ).