Creating an Expression Tree

Hello JPF,

I have just been introduced to trees, and for my assignment, I have to build an expression tree. I have the algorithm and idea down, but I'm not exactly sure where to start. So I have completed the code to change an infix notation to a postfix notation. I know that if I'll have to scan the postfix notation, token by token. If the token is an operand, I need to create a one-node tree and push it onto a stack. If the token is an operator, I have to pop off the two top trees from the stack and create a new tree with the operator as my root and the two operand trees as my children. I'm hoping that someone can look over my codes and help me start off. I have some codes that I have tried, but I'm getting a null error pointer. Anything would be greatly appreciated.

What I have so far:

Code :

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public BinaryTree<String> buildExpressionTree()
   {
      String post=postfixx;
      StringTokenizer tokens = new StringTokenizer(post);
      while (tokens.hasMoreTokens()) {
        String nextToken = tokens.nextToken();
        // Does it start with a digit?
        if (Character.isDigit(nextToken.charAt(0))) 
        {
          // Get the integer value.
          BinaryTree<String> newTree = new BinaryTree<String>(nextToken,null,null);
          tree.push(newTree);[B] //I'm getting an error right here.[/B]
        } // Is it an operator?
        else if (isOperator(nextToken.charAt(0))) 
        {
           BinaryTree<String> first=tree.pop();
           BinaryTree<String> second=tree.pop();
           BinaryTree<String> expression= new BinaryTree<String>(nextToken,first, second);
           eTree=expression;
           tree.push(expression);
 
        }
      } 
      return eTree;
   }

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