Post and Infix expressions in Java

Hello, so I am 2 classes: 1) Infix-expression: I have 2 methods (toString and Evaluate) and I have wrote and tested both methods. However, you will need to write : 1.1) IllegalArgumentsExceptions in the constructor to handle the case of an illegal string examples : " 3 5 +++", "hello", "4 5 6 3 2". A correct input can be "4 9 /", "4 8 3 + /", " a b +". We don't have to handle parenthesis here. 1.2) Another exception in the Evaluate method because for that method we can not evaluate string that contain letters such as " a b +" since we don't know the value of a and b (but it's correct for the constructor). So it's the same logic as the constructor but without letters/variables names. 2) Post-fix expression: I haven't implemented anything yet but: - The logic is different - We have to handle "(" and ")" - There is priority ""+/-" '/ and *". - The methods are the same but we also need to have a method to convert a post-fix to an infix expression