Re: Hashtable enumeration

[Bryce McKinlay]

This is not a bug in classpath's Hashtable implementation. The behaviour 
here is undefined, and it is very dangerous to add elements to a 
Hashtable while an enumeration is in progress. For example if a rehash 
occurs, the table will be reordered and elements which have already been 
enumerated could be returned again.

However, I can see the value in adjusting our implementation if it means 
that we can run this common benchmark. As I understand it, 213_javac is 
expecting that if a call to hasMoreElements() returns true, then the 
next nextElement() will return an object even if the structure of the 
table has changed between these calls. This patch changes our 
implementation to make that guarantee. Its a different behaviour, but no 
more correct than before. Patrick, can you try this in your tree against 
the 213_javac. If it solves the problem you are seeing, I'll check it in.

regards

Bryce.


Cierniak, Michal wrote:

> Patrick,
> Gansha has submitted all the necessary fixes to Classpath.  I don't remember
> if the email you mentioned is sufficient by itself, but certainly the
> combined fixes submitted by Gansha to the mailing list should fix all the
> SPEC JVM98 213_javac problems -- we can run 213_javac without any problems.
>
> Those fixes have not been checked into the Classpath CVS tree yet.
>
> Michal
>
>
> > -----Original Message-----
> > From: Patrick Doyle [mailto:address@hidden
> > Sent: Tuesday, August 28, 2001 9:44 AM
> > To: address@hidden
> > Subject: Hashtable enumeration
> >
> >
> >
> > I have run into a problem with Hashtable enumeration, when 
> > running javac.
> > When an element is added to the Hashtable in a certain way, the
> > enumeration fails with a NoSuchElementException, while it 
> > succeeds under
> > the Sun libraries.  This is an issue which has arisen before, 
> > apparently:
> >
> >  http://www.mail-archive.com/address@hidden/msg03259.html
> >
> > Has this been resolved?  Is the latest CVS version of Hashtable fixed?
> >
> > --
> > Patrick Doyle
> > address@hidden

2001-09-06  Bryce McKinlay  <address@hidden>

        * java/util/Hashtable.java (Enumerator): Ensure that if 
        hasMoreElements() returns true, nextElement() will always return
        something even if the table has been modified.

Index: Hashtable.java
===================================================================
RCS file: /cvs/gcc/gcc/libjava/java/util/Hashtable.java,v
retrieving revision 1.12
diff -u -r1.12 Hashtable.java
--- Hashtable.java      2001/03/24 08:05:29     1.12
+++ Hashtable.java      2001/09/06 08:32:35
@@ -833,44 +833,57 @@
     static final int VALUES = 1;
     
     int type;
-    // The total number of elements returned by nextElement(). Used to 
-    // determine if there are more elements remaining.
-    int count;
     // current index in the physical hash table.
     int idx;
-    // the last Entry returned.
+    // the last Entry returned by nextEntry().
     Entry last;
+    // Entry which will be returned by the next nextElement() call.
+    Entry next;
     
     Enumerator(int type)
     {
       this.type = type;
-      this.count = 0;
       this.idx = buckets.length;
     }
+    
+    private Entry nextEntry()
+    {
+      Entry e = null;
 
+      if (last != null)
+        e = last.next;
+
+      while (e == null && idx > 0)
+       {
+         e = buckets[--idx];
+       }
+      last = e;
+      return e;
+    }
+
     public boolean hasMoreElements()
     {
-      return count < Hashtable.this.size;    
+      if (next != null)
+        return true;
+      next = nextEntry();
+      return (next != null);
     }
 
     public Object nextElement()
     {
-      if (count >= size)
-        throw new NoSuchElementException();
-      count++;
       Entry e = null;
-      if (last != null)
-        e = last.next;
-
-      while (e == null)
+      if (next != null)
         {
-         e = buckets[--idx];
+          e = next;
+         next = null;
        }
-
-      last = e;
+      else
+        e = nextEntry();
+      if (e == null)
+        throw new NoSuchElementException("Hashtable Enumerator");
       if (type == VALUES)
         return e.value;
       return e.key;
     }
-  }  
+  }
 }