Question

I have a Huffman Tree, and a character, and I want to return what that character’s encoding within the Huffman Tree should be.

I’ve implemented it using the breadth-first traversal method, and each time I’m checking the left and right tree, I’m checking if the tree’s data is equal to character I’m looking for. Each time I go right or left, though, I add 0 or 1 to the encoding so far. Eventually, when I find the character equal to the tree’s data, I return that tree’s encoding value.

Code:

    public static String findCharEncoding(BinaryTree<CharProfile> bTree, char character) {
        Queue<BinaryTree<CharProfile>> treeQueue = new LinkedList<BinaryTree<CharProfile>>();

        // Create a TreeWithEncoding object from the given arguments and add it to the queue
        treeQueue.add(bTree);

        while (!treeQueue.isEmpty()) {
            BinaryTree<CharProfile> t = treeQueue.remove();

->          if (t.getLeft().getData().getCharacter() == character) {
                return t.getLeft().getData().getEncoding();
            }
            if (t.getLeft() != null) {
                t.getLeft().getData().setEncoding(t.getLeft().getData().getEncoding() + "0");
                treeQueue.add(t.getLeft());
            }

            if (t.getRight().getData().getCharacter() == character) {
                return t.getRight().getData().getEncoding();
            }
            if (t.getRight() != null) {
                t.getRight().getData().setEncoding(t.getRight().getData().getEncoding() + "1");
                treeQueue.add(t.getRight());
            }
        }

        // If it gets to here, the while loop was unsuccessful in finding the encoding
        System.out.println("Unable to find.");
        return "-1";
    }

Which I’ve implemented as follows:

        for (int i = 0; i < charOccurrences.size(); i++) {
            char character = charOccurrences.get(i).getCharacter();

            charOccurrences.get(i).setEncoding(findCharEncoding(huffmanTree, character));
            System.out.println(charOccurrences.get(i).getEncoding());
        }

CharProfile is a custom class that holds the character value, the probability of the character and the encoding.

It keeps returning a NullPointerExceptionError at the line if (t.getLeft().getData().getCharacter() == character) {, which I’ve indicated with an arrow. I’ve tried and tried, but I can’t seem to figure out why, other than the fact that it’s t.getLeft().getData() that’s returning the error, not the whole thing, or t.getLeft(), but I still can’t figure out why.

Answer 1

If the left or right tree branch can actually be null–which, if this is Huffman, should probably only be happening at the leaf nodes–why do you call getData on them before you check? That would definitely cause a NullPointerExeption.

if (t.getLeft() != null) {
  // check nullity and THEN check match
  if (t.getLeft().getData().getCharacter() == character) {
    return t.getLeft().getData().getEncoding();
  }
  t.getLeft().getData().setEncoding(t.getLeft().getData().getEncoding() + "0");
  treeQueue.add(t.getLeft());
}

if (t.getRight() != null) {
  if (t.getRight().getData().getCharacter() == character) {
    return t.getRight().getData().getEncoding();
  }
  t.getRight().getData().setEncoding(t.getRight().getData().getEncoding() + "1");
  treeQueue.add(t.getRight());
}

p.s. You also might want to avoid returning “-1” if you would otherwise return a bitstring of 0s and 1s, and avoid setting the tree nodes’ encoding every time you findCharEncoding, but that’s just a wild guess. 🙂