java - Throwing and catching an invalid token -

i'm creating program turns postfix expression infix expression. i've done coming blank on how throw , catch exception of invalid token , in case, invalid token consists of besides number or operator ie: " & ". required throw , catch exception in main class. can catch exception if user enters " & " first input not if isn't entered first. or tips great. thanks.

this actionlistener main class:

    public void actionperformed(actionevent arg0) {         tree t = new tree();     string postfixexp = inputfield.gettext();     t.buildtree(postfixexp);     node root = t.buildtree(postfixexp);     answerfield.settext(root.inorderwalk());         } 

this postfix -> infix method class:

node buildtree(string postfixexp) {     stk = new stack<node>();     string[] tokens = postfixexp.split(" ");      (string token : tokens) {         if (isnumeric(token)) {             double d = double.parsedouble(token);             node n = new operandnode(d);             stk.push(n);         } else if (token.equals("+")) {             node n1 = stk.pop();             node n2 = stk.pop();             node add = new operatornode(new addoperator(), n2, n1);             stk.push(add);         } else if (token.equals("-")) {             node n1 = stk.pop();             node n2 = stk.pop();             node sub = new operatornode(new suboperator(), n2, n1);             stk.push(sub);         } else if (token.equals("*")) {             node n1 = stk.pop();             node n2 = stk.pop();             node mul = new operatornode(new muloperator(), n2, n1);             stk.push(mul);         } else if (token.equals("/")) {             node n1 = stk.pop();             node n2 = stk.pop();             node div = new operatornode(new divoperator(), n2, n1);             stk.push(div);         }      }     node root = stk.pop();     return root; }