Flattening a collection

Question

Say I have a Map>

I can get the values of the map easily enough, and iterate over it to produce a single

Answer 1

If you just want to iterate through values, you can avoid all these addAll methods.

All you have to do is write a class that encapsulates your Map, and that implements the Iterator :

public class ListMap implements Iterator
{
  private final Map> _map;
  private Iterator>> _it1 = null;
  private Iterator _it2 = null;

  public ListMap(Map> map)
  {
    _map = map;
    _it1 = map.entrySet().iterator(); 
    nextList();
  }

  public boolean hasNext()
  {
    return _it2!=null && _it2.hasNext();
  }

  public V next()
  {
    if(_it2!=null && _it2.hasNext())
    {
      return _it2.next();
    }
    else
    {
      throw new NoSuchElementException();
    }
    nextList();
  } 

  public void remove()
  {
    throw new NotImplementedException();
  }

  private void nextList()
  {
    while(_it1.hasNext() && !_it2.hasNext())
    {
      _it2 = _it1.next().value();
    }
  }
}