You may use/not use, or modify these files anyway you want. For the main "truthtable" file, you need to complete the definition of the the abstract syntax tree classes, define a procedure that traverses the tree and collects all the propositional letters in the tree, then write a procedure (presumably in main) that generates truth assignments and evaluates the tree, printing out the truth table in the process.
To generate the truth table, the Integer.toBinaryString(int x) function returns a string of 1's and 0's that corresponds to the binary representation of x. However, leading "0"'s must be added so the length of the string matches the number of propositional letters - this is already done for you by your super-nice professor in the constructor of the "assignment" class of "truthtable.java". If there are n different letters in the propositional sentence, then its truth table will contain (int) Math.pow(2,n) lines.