Generic tree implementation in Java

Question

Is anyone aware of a generic tree (nodes may have multiple children) implementation for Java? It should come from a well trusted source and must be fully tested.

It just doesn't seem right implementing it myself. Almost reminds me of my university years when we were supposed to write all our collections ourselves.

EDIT: Found this project on java.net, might be worth looking into.

Answer 1

Here it comes:

abstract class TreeNode implements Iterable<TreeNode> {

  private Set<TreeNode> children;

  public TreeNode() {
    children = new HashSet<TreeNode>();
  }

  public boolean addChild(TreeNode n) {
    return children.add(n);
  }

  public boolean removeChild(TreeNode n) {
    return children.remove(n);
  }

  public Iterator<TreeNode> iterator() {
    return children.iterator();
  }
}

I am well trusted, but haven't tested the implementation.

Answer 2

There isn't a Tree class in the Collections libraries. However, there is one in the Swing Frameworks. DefaultTreeModel

I have used this in the past and it works well. It does pull in additional classes into your application though which may or may not be desirable.

You can also simulate a Tree using another collection and storing collections in it. Eg. List of Lists.

Answer 3

I use an XML DOM (XML describes a tree structure) and specifically the Open Source XOM (http://www.xom.nu). This is lightweight, nodes can be subclassed if required and very highly used and tested. It may be larger than you require but it has the advantage that any tree navigation methods (ancestors, siblings, etc.) are fully managed through XPath. You can also serialize the tree and transform it by tested XML methods. There is also a strong user community

Answer 4

I found a library that has a generic graph implementation: http://amino-cbbs.sourceforge.net/. Might be an overkill to import the whole library only because of the graph, but still...

Here's the direct link to the graph java doc.

Answer 5

I found an implementation of a Generic Tree (with tests) here:

http://vivin.net/2010/01/30/generic-n-ary-tree-in-java/

I think this is what you are looking for.

Answer 6

Ah, I was going to post a shameless plug to my solution and saw that someone already posted a link to it. Yeah, I had the same issue and I basically ended up writing my own Generic Tree. I've got tests for the tree node and the tree itself.

I implemented the node as an object having a data field and a list of nodes (which are the children of that node).

http://vivin.net/2010/01/30/generic-n-ary-tree-in-java/

Answer 7

I found an absolutely fantastic library http://jung.sourceforge.net, see the javadoc http://jung.sourceforge.net/doc/api/index.html . It is much more than just a graph implementation. With it you can visualize and layout graphs; plus, it has a bunch of standard graph algorithms you can use out of the box. Go, check it out! Although I ended up implementing my own basic graph (I didn't know of JUNG before), I use this library for visualization. It looks very neat!

Answer 8

It's rather hard to do a true generic tree implementation in Java that really separated the tree operations and properties from the underlying implementations, i.e. swap in a RedBlackTreeNode and override a couple of method to get a RedBlackTree implementation while retaining all the generic operations that a BinaryTree interface contains.

Also, an ideal abstraction would be able to swap out the low-level tree representation, e.g. an implicit binary tree structure stored in an array for a Heap or a Node-base interface with left and right child pointers, or multiple child pointers, or augmenting any of the above with parent pointers, or threading the leaf nodes, etc, etc, etc.

I did try and solve this myself, but ended up with quite a complicated interface that still enforces type safety. Here's the skeleton of the idea that sets up a abstract BinaryTree class with a non-trivial operation (Euler Tour) that will work even if the underlying node class or tree class is changed. It could probable be improved by introducing the idea of cursors for navigation and positions within the tree structure:

public interface Tree<E, P extends Tree.Entry<E, P>> extends Collection<E>
{
   public P getRoot();
   public Collection<P> children(P v);
   public E getValue(P v);

   public static interface Entry<T, Q extends Entry<T, Q>> { }
}

public interface BinaryTree<E, P extends BinaryTree.Entry<E, P>> extends Tree<E, P>
{
   public P leftChild(P v);
   public P rightChild(P v);

   public static interface Entry<T, Q extends Entry<T, Q>> extends Tree.Entry<T, Q>
   {
      public Q getLeft();
      public Q getRight();
   }
}

public interface TreeTraversalVisitor<E, P extends BinaryTree.Entry<E, P>, R> 
{
   public R visitLeft( BinaryTree<E, P> tree, P v, R result );
   public R visitCenter( BinaryTree<E, P> tree, P v, R result );
   public R visitRight( BinaryTree<E, P> tree, P v, R result );
}

public abstract class AbstractBinaryTree<E, P extends BinaryTree.Entry<E, P>> extends AbstractCollection<E> implements BinaryTree<E, P>
{
   public Collection<P> children( P v )
   {
      Collection<P> c = new ArrayList<P>( 2 );

      if ( hasLeft( v ))
         c.add( v.getLeft());

      if ( hasRight( v ))
         c.add( v.getRight());

      return c;
   }

   /**
    * Performs an Euler Tour of the binary tree
    */
   public static <R, E, P extends BinaryTree.Entry<E, P>> 
   R eulerTour( BinaryTree<E, P> tree, P v, TreeTraversalVisitor<E, P, R> visitor, R result )
   {
      if ( v == null )
         return result;

      result = visitor.visitLeft( tree, v, result );

      if ( tree.hasLeft( v ))
         result = eulerTour( tree, tree.leftChild( v ), visitor, result );

      result = visitor.visitCenter( tree, v, result );

      if ( tree.hasRight( v ))
         result = eulerTour( tree, tree.rightChild( v ), visitor, result );

      result = visitor.visitRight( tree, v, result );

      return result;
   }    
}