java - Evaluate Postfix Using Stack -

i trying create program evaluates postfix expression.for example "3500 43 12 * 47 2 / + -" .here code

 public static int evaluatepostfixexpression(string postfixexpr){      stack s = new stack();      int result = 0;      string operand = null;        for(int = 0; i<postfixexpr.length();i++){          if(character.isdigit(postfixexpr.charat(i)) == true){               operand = operand + postfixexpr.charat(i);              if(character.isdigit(postfixexpr.charat(i+1)) == false){                  s.push(operand);                  operand = null;                   }           }          if(postfixexpr.charat(i) == '+'){             result = result + integer.parseint((string) s.pop()) + integer.parseint((string) s.pop()) ;          }          if(postfixexpr.charat(i) == '-'){                 result = result + integer.parseint((string) s.pop()) - integer.parseint((string) s.pop()) ;              }          if(postfixexpr.charat(i) == '*'){                 result = result + integer.parseint((string) s.pop()) * integer.parseint((string) s.pop()) ;              }          if(postfixexpr.charat(i) == '/'){                 result = result + integer.parseint((string) s.pop()) / integer.parseint((string) s.pop()) ;              }       }      return result;   } //end-evaluatepostfixexpression 

when try run it, there occurs error.

exception in thread "main" java.lang.numberformatexception: input string: "null12"     @ java.lang.numberformatexception.forinputstring(unknown source)     @ java.lang.integer.parseint(unknown source)     @ java.lang.integer.parseint(unknown source) 

i can't find solution.it great if help.

edit: handled errors , code works;

 public static int evaluatepostfixexpression(string postfixexpr){      stack s = new stack();      int result = 0;      string operand = "";        for(int = 0; i<postfixexpr.length();i++){          if(character.isdigit(postfixexpr.charat(i)) == true){               operand = operand + postfixexpr.charat(i);              if(character.isdigit(postfixexpr.charat(i+1)) == false){                  s.push(operand);                  operand = "";                   }           }          if(postfixexpr.charat(i) == '+'){                 int x = integer.parseint((string) s.pop()) + integer.parseint((string) s.pop());                 result = result + x ;                 s.push(string.valueof(x));          }          if(postfixexpr.charat(i) == '-'){                 int x = integer.parseint((string) s.pop()) - integer.parseint((string) s.pop());                     result = result + x ;                 s.push(string.valueof(x));              }          if(postfixexpr.charat(i) == '*'){                 int x = integer.parseint("" + s.pop()) * integer.parseint("" + s.pop());                     result = result + x ;                 s.push(string.valueof(x));              }          if(postfixexpr.charat(i) == '/'){                 int x = integer.parseint((string) s.pop()) / integer.parseint((string) s.pop());                     result = result + x ;                 s.push(string.valueof(x));              }       }      return result;   }  

but result wrong.it should 2961 getting -1952.

try one:

 string operand = ""; 

change when

if(character.isdigit(postfixexpr.charat(i+1)) == false){     s.push(operand);     operand = ""; }