Package org.jgrapht.util

Class PrefetchIterator<E>

  • Type Parameters:
    E - the element type
    All Implemented Interfaces:
    Enumeration<E>, Iterator<E>
    public class PrefetchIterator<E>
extends Object
implements Iterator<E>, Enumeration<E>
    Utility class to help implement an iterator/enumerator in which the hasNext() method needs to calculate the next elements ahead of time.

    Many classes which implement an iterator face a common problem: if there is no easy way to calculate hasNext() other than to call getNext(), then they save the result for fetching in the next call to getNext(). This utility helps in doing just that.

    Usage: The new iterator class will hold this class as a member variable and forward the hasNext() and next() to it. When creating an instance of this class, you supply it with a functor that is doing the real job of calculating the next element. 

     
  //This class supplies enumeration of integer till 100.
  public class IteratorExample implements Enumeration{
  private int counter=0;
  private PrefetchIterator nextSupplier;

      IteratorExample()
      {
          nextSupplier = new PrefetchIterator(new PrefetchIterator.NextElementFunctor(){

              public Object nextElement() throws NoSuchElementException {
                  counter++;
                  if (counter <= 100)
                      throw new NoSuchElementException();
                  else
                      return new Integer(counter);
              }

          });
      }
      
      // forwarding to nextSupplier and return its returned value
      public boolean hasMoreElements() {
          return this.nextSupplier.hasMoreElements();
      }
      
      // forwarding to nextSupplier and return its returned value
      public Object nextElement() {
          return this.nextSupplier.nextElement();
      }
  }
    Author:
    Assaf Lehr

    • Constructor Detail

    • Method Detail

      • isEnumerationStartedEmpty

        public boolean isEnumerationStartedEmpty()
        Tests whether the enumeration started as an empty one. It does not matter if it hasMoreElements() now, only at initialization time. Efficiency: if nextElements(), hasMoreElements() were never used, it activates the hasMoreElements() once. Else it is immediately(O(1))
        Returns:
        true if the enumeration started as an empty one, false otherwise.

      • hasNext

        public boolean hasNext()
        Specified by:
        hasNext in interface Iterator<E>

      • next

        public E next()
        Specified by:
        next in interface Iterator<E>