Help with a node tree

Question

hi guys here's the problem i'm having:
I have a class called Node that I am building a tree out of. Node is a very simple structure, has three parts: element, left and right, with element being the top. Then I have a class called converterTree. This class will take in a postfix (polish notation) string and convert it into a tree through tokenizing etc. for an ultimate calculator class. The code I will post below uses a stack implementation to do so. It should: add all operands to the stack until it hits an operator, then make a new node with the operator as the element, and the left and right are the top and second stack objects respectively. then it pushes this entire tree back onto the stack and keeps going, at each step creating a new tree and linking it to the old one. The problem is that for some reason when I get the top node back in the return, it comes back as only that node, as if all the things it referenced are gone. You wont be able to run the code below since it needs some other classes, but in my main method where all those printlns are know that the return comes back as -, null, null, null, null, null, null with - being the thing on the stack that is SUPPOSED to link to the rest of them, yet for some reason they all come back null? any suggestions? heres the code:
import java.util.StringTokenizer;
public class ConverterTree
     String stringToConvert;
     static QueueLi returnQueue = new QueueLi();
     public ConverterTree (String toConvert)
          stringToConvert = toConvert;
     public static boolean isOperator(String op)
          char c = op.charAt(0);
          return (c == '*'|| c =='/' || c == '+' || c =='-');
     public static boolean isLParen (String op )
          char c = op.charAt(0);
          return c== '(';
     public static boolean isRParen (String op)
          char c = op.charAt(0);
          return c == ')';
     public static boolean isSpace(String op)
          char c = op.charAt(0);
          return c == ' ';
     public static boolean isOperand (String op)
          return (!( isOperator(op) || isLParen(op) || isRParen(op) || isSpace(op)));
     public Node toTree (String toConvert)
          String delims = new String("+-/* ");
          StringTokenizer tokenizer = new StringTokenizer(stringToConvert, delims, true);
          String token = new String();
          StackLi stack = new StackLi();
          while (tokenizer.hasMoreTokens())
               token = tokenizer.nextToken();
               System.out.println("token: " + token);
               if (isOperand(token))
                    ListNode node1 = new ListNode(token);
                    System.out.println("This operand is being pushed: " + token);
                    stack.push(token);
               else if (isOperator(token))
                    Node op2 = new Node(stack.topAndPop());
                    Node op1 = new Node(stack.topAndPop());
                    Node opTree = new Node(token, op1, op2);
                    System.out.println("node and left and right: " + opTree.element + opTree.left + opTree.right);
                    stack.push(opTree);
                    System.out.println("node and left and right: " + opTree.element + opTree.left + opTree.right);
          Node returnNode = new Node(stack.topAndPop());
          return returnNode;
     public static void main (String [] args)
          String toConvert1 = "2 5 + 6 8 + -";
          ConverterTree treeTest = new ConverterTree(toConvert1);
          Node testNode = treeTest.toTree(toConvert1);
          Node right = new Node(testNode.right);
          Node left = new Node(testNode.left);
          System.out.println(testNode);
          System.out.println(left);
          System.out.println(right);
          System.out.println(left.left);
          System.out.println(left.right);
          System.out.println(right.left);
          System.out.println(right.right);
}

Answer 1

hi guys here's the problem i'm having:
I have a class called Node that I am building a tree out of. Node is a very simple structure, has three parts: element, left and right, with element being the top. Then I have a class called converterTree. This class will take in a postfix (polish notation) string and convert it into a tree through tokenizing etc. for an ultimate calculator class. The code I will post below uses a stack implementation to do so. It should: add all operands to the stack until it hits an operator, then make a new node with the operator as the element, and the left and right are the top and second stack objects respectively. then it pushes this entire tree back onto the stack and keeps going, at each step creating a new tree and linking it to the old one. The problem is that for some reason when I get the top node back in the return, it comes back as only that node, as if all the things it referenced are gone. You wont be able to run the code below since it needs some other classes, but in my main method where all those printlns are know that the return comes back as -, null, null, null, null, null, null with - being the thing on the stack that is SUPPOSED to link to the rest of them, yet for some reason they all come back null? any suggestions? heres the code:
import java.util.StringTokenizer;
public class ConverterTree
     String stringToConvert;
     static QueueLi returnQueue = new QueueLi();
     public ConverterTree (String toConvert)
          stringToConvert = toConvert;
     public static boolean isOperator(String op)
          char c = op.charAt(0);
          return (c == '*'|| c =='/' || c == '+' || c =='-');
     public static boolean isLParen (String op )
          char c = op.charAt(0);
          return c== '(';
     public static boolean isRParen (String op)
          char c = op.charAt(0);
          return c == ')';
     public static boolean isSpace(String op)
          char c = op.charAt(0);
          return c == ' ';
     public static boolean isOperand (String op)
          return (!( isOperator(op) || isLParen(op) || isRParen(op) || isSpace(op)));
     public Node toTree (String toConvert)
          String delims = new String("+-/* ");
          StringTokenizer tokenizer = new StringTokenizer(stringToConvert, delims, true);
          String token = new String();
          StackLi stack = new StackLi();
          while (tokenizer.hasMoreTokens())
               token = tokenizer.nextToken();
               System.out.println("token: " + token);
               if (isOperand(token))
                    ListNode node1 = new ListNode(token);
                    System.out.println("This operand is being pushed: " + token);
                    stack.push(token);
               else if (isOperator(token))
                    Node op2 = new Node(stack.topAndPop());
                    Node op1 = new Node(stack.topAndPop());
                    Node opTree = new Node(token, op1, op2);
                    System.out.println("node and left and right: " + opTree.element + opTree.left + opTree.right);
                    stack.push(opTree);
                    System.out.println("node and left and right: " + opTree.element + opTree.left + opTree.right);
          Node returnNode = new Node(stack.topAndPop());
          return returnNode;
     public static void main (String [] args)
          String toConvert1 = "2 5 + 6 8 + -";
          ConverterTree treeTest = new ConverterTree(toConvert1);
          Node testNode = treeTest.toTree(toConvert1);
          Node right = new Node(testNode.right);
          Node left = new Node(testNode.left);
          System.out.println(testNode);
          System.out.println(left);
          System.out.println(right);
          System.out.println(left.left);
          System.out.println(left.right);
          System.out.println(right.left);
          System.out.println(right.right);
}