Class structure: How should a binary tree and it's node class be implemented in Java in terms of class files?

Question

In implementing a binary tree in Java, should the node class be a separate class file independent of the BinaryTreeclass, or should it be a default class in the same class file as the BinaryTree class?

---

First Example: Node is in separate class file

BinaryTree.java

public class BinaryTree {
    ...
}

BinaryTreeNode.java

public class BinaryTreeNode {
    ...
}

---

Second Example: Node class is default class in same class file

BinaryTree.java

public class BinaryTree {
    ....
}
class BinaryTreeNode {
    ...
}

I almost never see the use case for putting more than one class inside of the same class file, but this might be the first time I see it being useful. Does this make sense, or would this be considered sloppy code?

Answer 1

The problem with using default (package-protected) classes is that you won't be able to use that class outside of the package. For something like this, you typically want to.

There's a third option that I think is preferable. Use a nested class:

public class BinaryTree {
    // ...

    public static class Node {
        // ...
    }
}

Whether you make this a 'static' nested class or not depends on whether you want to implicitly associate each instance with a single parent BinaryTree or want to be able to create Nodes that can exist in 0-to-many trees.

In terms of compilation and usage, the static nested class is just like having a public class called BinaryTreeNode but with the name BinaryTree.Node.

There are a few advantages to this. One is that you keep the highly coupled code together in a single file. One really nice thing is that The BinaryTree can use private methods and values of the Node and vice-versa So you can keep things really well-encapsulated.