[JENKINS-15465] tweaking subList implementation and iterator.

I can't really think of how null ends up in the resulting list, but adding a check in Iterator to detect that situation.

Also, improved the efficiency of the subList implementation.
(cherry picked from commit 807dc717)

core/src/main/java/hudson/model/RunMap.java

@@ -36,6 +36,7 @@ import java.util.Collections;
 import java.util.Comparator;
 import java.util.Iterator;
 import java.util.Map;
+import java.util.NoSuchElementException;
 import java.util.SortedMap;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -104,6 +105,8 @@ public final class RunMap<R extends Run<?,R>> extends AbstractLazyLoadRunMap<R>
                 R last = next;
                 if (last!=null)
                     next = last.getPreviousBuild();
+                else
+                    throw new NoSuchElementException();
                 return last;
             }
 

core/src/main/java/hudson/util/RunList.java

@@ -44,6 +44,7 @@ import java.util.Comparator;
 import java.util.GregorianCalendar;
 import java.util.Iterator;
 import java.util.List;
+import java.util.NoSuchElementException;
 
 /**
  * {@link List} of {@link Run}s, sorted in the descending date order.
@@ -128,6 +129,25 @@ public class RunList<R extends Run> extends AbstractList<R> {
         return Iterators.get(iterator(),index);
     }
 
+    /**
+     * {@link AbstractList#subList(int, int)} isn't very efficient on our {@link Iterable} based implementation.
+     * In fact the range check alone would require us to iterate all the elements,
+     * so we'd be better off just copying into ArrayList.
+     */
+    @Override
+    public List<R> subList(int fromIndex, int toIndex) {
+        List<R> r = new ArrayList<R>();
+        Iterator<R> itr = iterator();
+        Iterators.skip(itr,fromIndex);
+        for (int i=toIndex-fromIndex; i>0; i--) {
+            if (itr.hasNext())
+                r.add(itr.next());
+            else
+                throw new NoSuchElementException();
+        }
+        return r;
+    }
+
     @Override
     public int indexOf(Object o) {
         int index=0;