java - How to evaluate a sign in a postfix expression? -

i have completed code postfix expression calculator, i'm unsure how go evaluating expression when come across negative sign.

for example, expression: -1-2-3 comes out in program:

original = -1-2-3

infix tokens = [[-;sign], [1;num], [-;op], [2;num], [-;op], [3;num]]

postfix = [[1;num], [-;sign], [2;num], [-;op], [3;num], [-;op]]

postfix evaluation = -2

obviously answer should -6, can't see logic behind making sign before number apply entire expression. i'm confident postfix method correct , know need create case when see sign, don't know after that.

here code far:

public static string eval(arraylist<tokenize.token> list) {        stack<integer> stack = new stack<integer>();        (int i=0; i<list.size(); i++) {          tokenize.token t = list.get(i);          tokentype type = t.gettype();            if (type==tokentype.num) {                  string x = t.getvalue();                 int x1 = integer.parseint(x);                 stack.push(x1);         }           else if (type==tokentype.op) {                  int y = stack.pop();                 int x = stack.pop();                  if(t.getvalue().equals("*")) {                     int z = y * x;                     stack.push(z);                 }                 else if(t.getvalue().equals("/")) {                     int z = y / x;                     stack.push(z);                  }                 else if(t.getvalue().equals("+")) {                     int z = y + x;                     stack.push(z);                 }                 else if(t.getvalue().equals("-")) {                     int z = y - x;                     stack.push(z);                 }         }           else if(type == tokentype.sign) {          //????                              }       }       return stack.pop() + "";    } 

provided you've tokenized & distinguished sign (unary) minus (binary operator), , in postfix value pushed on stack -- appears case --

it's trivial:

else if (type == tokentype.sign) {      int x = stack.pop();     x = -x;     stack.push( x); }  

you pop number literal off, negate it, , push result on stack. unary operator ==> 1 input off stack, 1 output on.