Example
Given a binary tree, with this node structure:
class node { node left node right }One may implement a tree size procedure recursively:
function tree_size(node) { return 1 + tree_size(node.left) + tree_size(node.right) }Since the child nodes may not exist, one must modify the procedure by adding non-existence or null checks:
function tree_size(node) { set sum = 1 if node.left exists { sum = sum + tree_size(node.left) } if node.right exists { sum = sum + tree_size(node.right) } return sum }This however makes the procedure more complicated by mixing boundary checks with normal logic, and it becomes harder to read. Using the null object pattern, one can create a special version of the procedure but only for null nodes:
function tree_size(node) { return 1 + tree_size(node.left) + tree_size(node.right) } function tree_size(null_node) { return 0 }This separates normal logic from special case handling, and makes the code easier to understand.
Read more about this topic: Null Object Pattern
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