The first method transforms the local infix expression to postfix, stores it as the local postfix and returns it as a String. The second method is the inverse: it transforms the local postfix expression to infix, stores it locally, and returns it as a string! The method evaluate() returns the value of the expression.
Your main() should display a prompt to the user, give him/her the opportunity to enter a string in either notation
(hence the two different directions in the constructor) to be transformed to the other, and evaluate the result.
You will probably also need to declare different types of Stack classes, to be used each for a different purpose.
If your program has any fancy features, such as allowing multiple-digit numbers, checking for illegal
characters in the input, adding some extra operators, or others, please make that clear in a comment at
the beginning of your program. Also, when you display the menu of options to the user, make it clear how
they should enter the expressions (i.e. leave spaces around operators etc..)