NullPointerException for BinaryTree created through ObjectInputStream

Question

I am building a binary tree from data (not nodes) saved in a binary file using the ObjectInputStream. However, it seems when the control is returned to the driver program, the tree Object created as a result is null and I see the NullPointerException. The exception occurs when the first method getHeight() in testTree is called. And when I try to iterate through the newly built tree object, it returns blank. I don't understand why. Any suggestions?

Here is the relevant code that I tried to keep brief:

public static void main(String [] args)
{
....
BST<Integer> T = new BST<Integer>();
T.buildTree("BST.dat");
testTree(T);
...
}

public class BST<T extends Comparable<? super T>>
         extends CBT<T> 
         implements STInterface<T> 
{
 private BN<T> root; 

 public BST()
 {
  super();
 }
//other methods here.

 }
 public class CBT<T extends Comparable<? super T>> extends BT<T> implements CTInterface<T>

{ 
   public ComparableBinaryTree()
    {
     super();
    }
}

public class BT<T> implements BTInterface<T>
{
  private BN<T> root;
  public int getHeight()
{
  return root.getHeight();
}

 ...

 public void buildTree(String fileName)
 {
  try
 {
  ObjectInputStream IS = new ObjectInputStream(
    new FileInputStream(fileName));
  double numberOfNodes = (double)IS.readInt();
  BN<T> root = new BN<>();
  RecReadTree(IS, numberOfNodes, root);
  IS.close();
 }
 catch (IOException e)
 {
  System.out.println("Reading problem");
 }
}
 ....
 }

  private void RecReadTree(ObjectInputStream IS, double numberOfNodes, BN<T> node)
  {
if (node != null)
  {
  try
  {
      if (numberOfNodes == 1)
      {
        @SuppressWarnings("unchecked")
        T data = (T) IS.readObject();
        node.setData(data);
        return;
      }

      double left = Math.ceil((numberOfNodes - 1)/2.0); //ceiling
      double right = Math.floor((numberOfNodes - 1)/2.0); //floor

      BN<T> leftChild = new BN<>();
      node.setLeftChild(leftChild);
      RecReadTree(IS, left, leftChild);
      @SuppressWarnings("unchecked")
      T data = (T) IS.readUnshared();
      node.setData(data);
      if (right > 0)
      {
        //new node
        BN<T> rightChild = new BN<>();
        node.setRightChild(rightChild);
        RecReadTree(IS, right, rightChild);
      }
  }
  catch (Exception e)
  {
    System.out.println("Rec Reading Problem" + e);
  }
 }
 }
}
}   

public class BN<T> 
  {
    public int getHeight()
  {
    return getHeight(this); 
  } // end getHeight

private int getHeight(BN<T> node)
 {
  int height = 0;

  if (node != null)
      height = 1 + Math.max(getHeight(node.getLeftChild()),
                             getHeight(node.getRightChild()));                      
  return height;
 }
}

EDIT: I have added the not null condition when building the binary tree. It still causes an error. It seems to be something to do with the scope and inheritance because I can access the getHeight() method no problem from inside RecReadTree, if I needed to.

possible duplicate of [What is a Null Pointer Exception, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it) — GhostCat, Apr 11 '15 at 18:12
`RecReadTree` used to have `if (node != null)` condition. It still caused a nullPointerException. — Aleks, Apr 11 '15 at 18:16

score 0 · Answer 1 · answered Apr 11 '15 at 18:39

The way I see this is you're trying to access a parent function from a child class but a value that the parent function uses inside it could be null due to uninitialization. There are many possibilities for this but this is just one of the obvious ones when it cones to inheritance. Cheers!

NullPointerException for BinaryTree created through ObjectInputStream

1 Answers1