A simple example of using the CompositePattern:
  // Copyright(C) 2000 Yoshinori Oota All rights reserved.
  
  public class CompositeSample
  { 
    static public void main(String args[])
    {
      /* [???]
      aComposite --aLeaf   
        --aLeaf   
        --Composite --aLeaf 
          --aLeaf 
      */
      Component aComponent1 = new Composite();
      Component aComponent2 = new Leaf();
      Component aComponent3 = new Leaf();
      aComponent1.AddComponent(aComponent2);
      aComponent1.AddComponent(aComponent3);
      
      Component aComponent4 = new Composite();
      Component aComponent5 = new Leaf();
      Component aComponent6 = new Leaf();
      aComponent4.AddComponent(aComponent5);
      aComponent4.AddComponent(aComponent6);
      aComponent1.AddComponent(aComponent4);
      
      // Opearation [???]
      aComponent1.Operation(); 
    } 
  }
  
  abstract class Component
  { 
    abstract public void Operation();
    abstract public void AddComponent(Component aComponent); 
  }
  
  class Leaf extends Component
  { 
    public Leaf() { }
    
    public void Operation()
    { 
      System.out.println("**I'm Leaf"); 
    }
    
    public void AddComponent(Component aComponent) { } 
  }
  
  class Composite extends Component
  { 
    private Vector _components; //children
    
    public Composite()
    { 
      _components = new Vector(); 
    }
    
    public void Operation()
    { 
      System.out.println("I'm Composite");
      Enumeration componentList = _components.elements();
      while(componentList.hasMoreElements())
      { 
        // forall Component in _components
        // aComponent.Operation();
        
        Component aComponent = (Component)componentList.nextElement();
        aComponent.Operation(); 
      } 
    } 
    
    public void AddComponent(Component aComponent)
    { 
      _components.addElement(aComponent); 
    } 
  } 
------
Graphically:

http://www.javaworld.com/javaworld/jw-09-2002/images/jw-0913-designpatternsf1.jpg

Worth noting: This is not the GOF implementation, which puts the collection methods on the component. Heated discussion on CompositeConsideredHarmful.