Help on a Postfix conversion method

Question

I have run into problems with one function for an assignment where I need to make a Postfix (aka Polish) calculator to takes a standard expression and changes it into postfix notation then computes.
I'm fairly new to Java programming although I am a fairly experienced C and C++ programmer.
My main problem is the function seems to simply echo the input data, instead of converting it like i'd like it to. I've tried two different ways of extracting the input string into the seperate components: first using stringtokenizer, which I comented out, and second converting it to a char array and looking at each individual element. neither way seems to work. here is the function I wrote ...
public String ConvertToPostFix( String norm )
     String convertedstring = "";
     String holder;
     MyStack stack = new MyStack();
     /*java.util.StringTokenizer st = new java.util.StringTokenizer(norm);     while(st.hasMoreTokens()){holder = st.nextToken();*/
     for(int i=0; i < norm.length(); i++ )
          holder = norm.substring(i);
          if(holder == "(" || holder == " ")
          else if(holder == "+" || holder == "-")
               stack.push(holder);
          else if (holder == ")")
               {holder=stack.pop();convertedstring+=holder;}
               else
               convertedstring += holder;
          return convertedstring;
any help I receive would be appritated. Thanks

Answer 1

The StringTokenizer way is better...
Use .equals() when comparing Strings. holder == "+" compares the references, not the strings. It will always be false in this case, since the '+' character in the norm string and the string literal "+" are definitely not at the same location in memory.if(holder.equals("(") || holder.equals(" "))
else if(holder.equals("+") || holder.equals("-"))
  stack.push(holder);
else if (holder.equals(")")) {
  holder=stack.pop();
  convertedstring+=holder;
}That should get you started..