Generic tree implementation in Java

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/

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.

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!

When in need of a tree I typically use following interface, and implement it accordingly.

  /**
   * Generic node interface
   * 
   * @param <T> type of contained data
   * @param <N> self-referential type boundary that captures the implementing type
   */
  interface Node<T, N extends Node<T, N>>
  {

    public T getObject();

    public boolean addChild(N node);

    public List<N> getChildren();

  }

An implementation could be

  class StringNode implements Node<String, StringNode>
  {

    private final String value;

    public StringNode(String value)
    {
      this.value = value;
    }

    @Override
    public String getObject()
    {
      return value;
    }

    @Override
    public boolean addChild(StringNode node)
    {
      // add child
      return false;
    }

    @Override
    public List<StringNode> getChildren()
    {
      // return children
      return Collections.emptyList();
    }

  }

The advantage here is the flexibility gained by implementing algorithms against the interface. A rather simple example could be

  public <T, N extends Node<T, ? extends N>> N performAlgorithm(N node)
  {
    if (!node.getChildren().isEmpty())
      return node.getChildren().get(0);

    return node;
  }

The method can be used with the inteface type or concrete implementations

StringNode sn = new StringNode("0");
Node<String, StringNode> node = sn;

// perform computations on the interface type
Node<String, StringNode> result = performAlgorithm(node);

// or use a concrete implementation
StringNode result2 = performAlgorithm(sn);

If you need an enterprise-level node tree, you could look into Java Content Repository (JCR). But it is far from the simple in-memory node tree solutions suggested here and more of a multi-user XML database with SQL and XPath.