public int evaluatePostFix(String expr) throws Exception{

if(expr == null || expr.isEmpty())

throw new Exception("Null or empty parameter");

Stack operands = new Stack();

for(int i = 0; i < expr.length(); i++){

Character c = expr.charAt(i);

if(isOperand(c))

operands.push(Integer.parseInt(c.toString()));

else{

if(operands.size() < 2)

throw new Exception("Invalid expression");

Integer op2 = operands.pop();

Integer op1 = operands.pop();

if(c.equals('+'))

operands.push(op1 + op2);

else if(c.equals('-'))

operands.push(op1 - op2);

else if(c.equals('*'))

operands.push(op1 * op2);

else if(c.equals('/'))

operands.push(op1 / op2);

else

throw new Exception("Invalid expression");

}

}

return operands.pop();

}