Re: Using a lot of Maps

From:

Lew <lew@lewscanon.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 22 Nov 2010 13:54:43 -0800 (PST)

Message-ID:

<29d86468-b877-431f-a9be-864eb6c63845@l32g2000yqc.googlegroups.com>

markspace wrote:

@Override
public int hashCode() {
  if( hash == 0 ) {
    for( Object o : hierarchy ) {
      hash = hash * 53 + o.hashCode();
    }
  }
  return hash;
}

There's a mistake above: I shouldn't use hash to accumulate the hash
code. Another thread could see a non-zero value that will be changed
later. Gotta use a temporary variable there.

   @Override
   public int hashCode() {
     if( hash == 0 ) {
       int x;
       for( Object o : hierarchy ) {
         x = x * 53 + o.hashCode();
       }
       hash = x;
     }
     return hash;
   }

If your goal is thread safety, you haven't achieved it with this.

Suppose Thread A comes along and sees '0' as the hash, calculates a
non-zero value and uses it. Some time later, Thread B comes along and
sees the object. It might see all the values in hierarchy that Thread
A saw, and still see a 0 value. Or, it might see different values in
the 'hierarchy' and see the calculated, non-zero value for 'hash' from
the old values. Or it might see some of the same values in
'hierarchy' but not others that have changed, and it might or might
not see 0 in 'hash'. Absent some synchronization, all bets are off.

--
Lew