Prototyped a little bit the structure of the program.
Implemented the parsing of the simpleFactor and also added the parseAdditionArithmetic to solve all the arithmetic symbols at only one place - instead of having a separate function for each.
Changed hardcoded arithmetic symbols to use constants.
Implemented and refactored expression parsing code.
Added a method (toStringRecursive()) in ASTNode to easily get the result of the parsing.