RE: nullable pointers

A tree would be defined like so :

abstract class TreeNode {}
abstract class EmptyTreeNode extends TreeNode {}
abstract class FilledTreeNode extends TreeNode {
TreeNode first;
TreeNode second;
}

If you combine this with runtime polymorphism you can do something like this (won't work in Java, but I'm using java code anyway)

void processTreeNode(EmptyTreeNode et) {}
void processTreeNode(FilledTreeNode ft) {
// you might want to do something useful here
processTreeNode(ft.first);
processTreeNode(ft.second);
// or here
}

then simply call processTreeNode(tree), and the compiler would figure out whether to call the first or the second function.

I would love for java to support this though.