Functions on Iterables :: Filter

Functional programming has its strengths when iterating over data. If you want a view on a subset data, you can make a filter, and iterate over your data only seeing the elements that the filter accepted:

public interface Filter<T>
{
   public boolean accept(T value);
}

public class Functional
{
   /**
    * Filter items from the view of the returned Iterable<T>
    */

   public static <T> Iterable<T> filter(final Iterable<T> iterable, final Filter<T> filter)
   {
      if (iterable == null)
         throw new NullPointerException();
      if (filter == null)
         throw new NullPointerException();

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

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

               @Override
               public T next()
               {
                  this.ensureReady();
                  if (!this.ready)
                     throw new NoSuchElementException();

                  T result = this.current;
                  this.ready = false;
                  this.current = null;
                  return result;
               }

               @Override
               public void remove()
               {
                  iterator.remove();
               }

               //

               private boolean ready = false;
               private T       current;

               private void ensureReady()
               {
                  while (!this.ready && iterator.hasNext())
                  {
                     T item = iterator.next();
                     if (!filter.accept(item))
                        continue;

                     this.ready = true;
                     this.current = item;
                  }
               }
            };
         }
      };
   }
}

Example:

Filter<String> nonNull = new Filter<String>()
{
   public boolean accept(String value)
   {
      return value != null;
   }
};

Iterable<String> data = ...;

for(String item: filter(data, nonNull))
{
   System.out.println("item: "+item); // guaranteed not to be null
}