there while are more tokens ( variables, operators, constatnts or parentheses)
get next token
if the token is a blank - toss it
else if token is left paren
push it on the operator stack
else if token is a variable
add it to the postfix expression
save variable name
else if token is a constant
add it to the postfix expression
save constant "name" and value
else if token is an operator
while operator stack is not empty and
the next operator on stack is not a left paren and
the next operator has higher priority than token
operator pop from stack and add to postfix
push token on operator stack
else if token is not a right paren
throw IllegalStateException
else
while operator stack is not empty and
top of stack is not a left paren
pop the operator stack and add to postfix
if operator is empty
...