Re: Efficiently sequencing a stream of objects

Piotr Kobzda <>
Thu, 22 Mar 2007 03:22:32 +0100
Chris wrote:

Generics, unfortunately, are worthless for performance improvements.
They provide only compile-time type checking. At runtime, all the casts
are still there.

Not all cast are there. Compare your version with "genericified" one
(attached below).


public class MergingIterator<E extends Comparable<? super E>> implements
Iterator<E> {
   private PriorityQueue<Carrier> queue = new PriorityQueue<Carrier>();

   public void add(Iterator<? extends E> it) {
     if (!it.hasNext())
       return; // do nothing, do not add

     Carrier carrier = new Carrier(); = it; =;

   public boolean hasNext() {
     return !queue.isEmpty();

   public E next() {
      Carrier carrier = queue.poll();
      E next =;

      // if there's another object to be had from this stream, insert it.
      // if not, the queue just gets smaller
      if ( { =;

      return next;

   public void remove() {
     throw new UnsupportedOperationException();

   private class Carrier implements Comparable<Carrier> {
     Iterator<? extends E> it;
     E next;

     public int compareTo(Carrier other) {
       return next.compareTo(;

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