Just a trick:) Not static inner classes give the opportunity elegantly implement Tree structure. 

Example (only immutable code is shown):

 interface INode
 { 
  INode getParentNode();
  Collection getChildrenNodes();
  Data getSomeNodeData();
 } 

 class Default''''''Node implements INode
 {
  // members, initialization and mutable code has been omited
  private class Inner''''''Node extends Default''''''Node 
  {
    public INode getParent''''''Node()
    {
      return Default''''''Node.this;// outer class
    }
  } 
     
  public INode getParent''''''Node()
  {
    return null;// or INode.ROOT if you use NULL object pattern...
  }
  public Collection getChildren''''''Nodes()
  {
    return childrenNodes;// returns the Collection of Inner''''''Nodes
  }
  public Data getSome''''''Node''''''Data()
  {
    return data;
  }
 }

-- KirillStepanosov