public class PostfixEvaluator{

         private final static char ADD = '+';

         private final static char SUBTRACT = '-';

         private final static char MULTIPLY = '*';

         private final static char DIVIDE = '/';

         private Stack<Integer> stack;

         public PostfixEvaluator() {

             stack = new Stack<Integer>();

         }

         public int evaluate(String expr) {

             int op1,op2,result=0;

             String token;

             Scanner parser = new Scanner(expr);

             while(parser.hasNext()) {

                 token = parser.next();

                 if(isOperator(token)) {

                     op2 = (stack.pop()).intValue();

                     op1 = (stack.pop()).intValue();

                     result = evaluateSingleOperator(token.charAt(0),op1,op2);

                     stack.push((Integer)result);

                 }else {

                     stack.push(Integer.parseInt(token));

                 }

             }

             return result;

         }

         private boolean isOperator(String token) {

             return (token.equals("+") || token.equals("-") ||

                     token.equals("*") || token.equals("/"));

         }

         private int evaluateSingleOperator(char operation,int op1,int op2) {

             int result = 0;

             switch(operation) {

             case ADD:

                 result = op1+op2;

                 break;

             case SUBTRACT:

                 result = op1-op2;

                 break;

             case MULTIPLY:

                 result = op1*op2;

                 break;

             case DIVIDE:

                 result = op1/op2;

             }

             return result;

         }

     }