Java中的二进制树静态方法

Question

I have these instance methods in my Java implementation of Binary Tree and Search Binary Tree: getSize() , getHeight() , getDepth() , getPreOrder() , getInOrder() , getPostOrder() and getLevelOrder() . These methods use the root of the tree in others recursive methods that have a parameter Node . Which is more appropiate to use from the point of view of the OOP:

1. Using these recursive methods as static methods, because they use an object ( Node ) that doesn't belong to the actual class, and they don't use any class attributes,
2. They can be instance methods because they can use in a subtree of this tree and they don't use any static attributes,
3. or they may be in other static class like UtilsTree() ?

Answer 1

From an OOP perspective I believe approach number 2 is the way to go. (Statics are in general often frowned upon in OOP.)

As I understand it, the method uses this as root, and then traverses the rest of the tree without calling any instance methods? This isn't too bad considering that the other nodes are of the same class, which means that the code is already shared among all objects. (The method can access private members of other instances and so forth.)

That being said, I think getSize , getHeight , getDepth , getPreOrder , getInOrder , getPostOrder and getLevelOrder can all be implemented as proper recursive instance methods. (Correct me if I'm wrong.)

A fourth option which isn't too bad would be to use a visitor pattern and have a NodeVisitor interface.

Answer 2

Some of those methods definitely should be non-static members since they relate directly to a specific instance.

tree.getSize() or tree.getDepth() is much easier to read and understancd than BinaryTree.getDepth(tree) .

However one could argue that the methods getPreOrder() , getInOrder() , getPostOrder() can be static or even in their own class. You can think of that as a StrategyPattern for how to walk the tree.

TraversalStrategy.PREORDER.walk(tree);

might be a good idea instead of adding more methods. That way, if you ever need to add different ways to walk to the tree you don't have to keep adding methods to BinaryTree (Following the Open-Closed Principle)

Answer 3

A class represents a pattern for a set of Object s. A class can have static members and methods, that is, class -level properties and abilities. An instance of a class is an Object , which has its particular, non- static members and methods. In your case, we are talking about a bunch of getters. The key question is:

whose are those methods?

and the answer is: they are methods, that is, abilities of your Tree Object . If you have several Tree s, the other Tree s have nothing to do with the size and stuff of a given Tree . As a result, all the mentioned methods should be public , instance-level methods unless you want to use them internally, in which case the affected methods should be protected , non- static methods.