Use of emptyIterator in java [duplicate]

Question

Can anyone let me know what is the real time use of an Empty Iterator in java? I'm curious to know why is it needed? things like,

 1.  public static <T> Iterator<T> emptyIterator()
 2.  public static <T> ListIterator<T> emptyListIterator()
 3.  public static final <T> Set<T> emptySet(), etc..

source: http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#emptyIterator()

Answer 1

You can use an empty iterator in cases where an API that you implement requires an iterator, but some of your code's logic can yield no items in the result. In that case, instead of returning a null, you return an empty iterator. You can also use an empty iterator to save some memory and for testing purposes.

Here is some example code that prevents returning null and saves some memory at the same time:

class LazyObjectInitialization {

   private Collection<String> items; 

   public final Iterator<String> items() {

      if(items == null || items.isEmpty()) {
         return Collections.emptyIterator(); 
      }

      return items.iterator();
   }

   public final add(String item) { 

      if(items == null) {
         items = new ArrayList<>();
      }

      items.add(item);
   }
}

In the above class, the field items is not initialized until an element is added. So to provide expected behavior in method items() we return an empty iterator. The benefit from this are as follow:

• Smaller memory consumption

  • The class allocates memory only when it is really needed.

• Smaller memory mutation

  • Until we add something to the object, we never create a new instance of the iterator.

• We never return null.