2010-07-20

Functions on Iterables :: Event Hooks

public interface Operator
{
   public void operate(T item);
}

public class Functional
{
   /**
    * Performs a callback on each element-visit, and each element removal
    */

    public static <T> Iterable<T> eventHook(final Iterable<T> iterable,
                                            final Operator<T> onVisit,
                                            final Operator<T> onRemove)
   {
      if (iterable == null)
         throw new NullPointerException();
      if (onVisit == null && onRemove == null)
         throw new NullPointerException("must specify either onVisit, onRemove or both");

      return new Iterable<T>()
      {
         @Override
         public Iterator<T> iterator()
         {
            final Iterator<T> iterator = iterable.iterator();

            return new Iterator<T>()
            {
               @Override
               public boolean hasNext()
               {
                  return iterator.hasNext();
               }

               @Override
               public T next()
               {
                  this.current = iterator.next();
                  if (onVisit != null)
                     onVisit.operate(this.current);
                  return this.current;
               }

               @Override
               public void remove()
               {
                  iterator.remove();
                  if (onRemove != null)
                     onRemove.operate(this.current);
               }

               //

               private T current;
            };
         }
      };
   }
}

Example:
Operator<String> onVisit= new Operator<String>()
{
   public void operate(String text)
   {
      System.out.println("visited: "+text);
   }
};

Operator<String> onRemove= new Operator<String>()
{
   public void operate(String text)
   {
      System.out.println("removed: "+text);
   }
};

Iterable<String> data = ...;

for(String item: eventHook(data, onVisit, onRemove))
{
   if(item.length() > String.valueOf(Math.PI))
      System.out.println("this text is longer than PI!");
}


Once you created an Iterable (say, a filtered view on a List) you can iterate over it as often as you want. No need to create a new Iterable.


With the support for closures in Java 7 a lot of the boilerplate code will disappear.

2 comments:

  1. The example is the one from the Transform post.

    ReplyDelete