Building a simple tree structure

Question

I'm trying to build this simple tree:

Multiverse
| 
|--- Universe
 |
 |--- Supercluster
 |
 |--- Galaxy
 |
 |--- Andromeda
 |
 |--- Milky way

I came up with this simple method which recursively walk a tree:

/**
 * recursively walk a tree structure
 */ 
public static void recursive(TreeViewItem treeNode){
 System.out.println(treeNode.getText());
 Iterator<TreeViewItem> itr = treeNode.getItems().iterator();
 while (itr.hasNext()) {
 TreeViewItem item = (TreeViewItem) itr.next();
 recursive(item);
 }
}

Is there a better way or more elegant way to do this?

Answer 1

You're visiting and printing the tree elements in a depth-first manner, so depthFirstPrint would be a much better name than "recursive" (which is always a terrible name for any recursive function).

You didn't need this cast:

TreeViewItem item = (TreeViewItem) itr.next();

because you correctly declared the iterator as Iterator<TreeViewItem> itr

As of Java 1.5, you can use a for-each loop, which is simple and elegant:

for (TreeViewItem item : treeNode.getItems()) {
 depthFirstPrint(item);
}

Answer 2

There are three kinds of recursive binary tree traversals: pre-order, in-order, and post-order. You've implemented pre-order traversal, so I suggest naming your function preOrderTraversal().

Your function is actually doing two things: traversing the tree and printing each node. It would be nice to separate them, so that you have a generic tree traversal algorithm, and a separate place to specify what to do with each node as it is encountered. With Java ≤ 7, this can be done using a visitor pattern. In Java 8, the visitor pattern can be much simpler, since the language allows functions to be passed around easily:

import java.util.function.Consumer;
...
public static void preOrderTraversal(TreeViewItem treeNode, Consumer<TreeViewItem> action) {
 action.accept(treeNode);
 for (TreeViewItem child : treeNode.getItems()) {
 preOrderTraversal(child, action);
 }
}
// Call that code with
preOrderTraversal(rootNode, (TreeViewItem node) -> System.out.println(node.getText()));

Your TreeViewItem class could have a public Iterator<TreeViewItem> iterator() method, so that TreeViewItem implements Iterable<TreeViewItem> to iterate of the children directly, without having to call .getItems(). Whether you want to make this change depends on whether there might be anything else in a TreeViewItem that might be worth iterating over, besides the child nodes.