The Filter Pattern - Selective Iterators

The following code is from: http://www.erik-rasmussen.com/blog/2008/01/18/the-filter-pattern-java-conditional-abstraction-with-iterables/ 


import java.util.Iterator;

import java.util.NoSuchElementException;


public abstract class Filter<T> {

  public abstract boolean passes(T object);


  public Iterator<T> filter(Iterator<T> iterator) {

    return new FilterIterator(iterator);

  }


  public Iterable<T> filter(Iterable<T> iterable) {

    return new Iterable<T>() {

      public Iterator<T> iterator() {

        return filter(iterable.iterator());

      }

    };

  }


  private class FilterIterator implements Iterator<T> {

    private Iterator<T> iterator;

    private T next;


    private FilterIterator(Iterator<T> iterator) {

      this.iterator = iterator;

      toNext();

    }


    public boolean hasNext() {

      return next != null;

    }


    public T next() {

      if (next == null)

        throw new NoSuchElementException();

      T returnValue = next;

      toNext();

      return returnValue;

    }


    public void remove() {

      throw new UnsupportedOperationException();

    }


    private void toNext() {

      next = null;

      while (iterator.hasNext()) {

        T item = iterator.next();

        if (item != null && passes(item)) {

          next = item;

          break;

        }

      }

    }

  }

}