Question

Is following code the safe way to remove a element in the Hashtable?

   Enumeration keys = siCache.keys();  //siCache is Hashtable

    while(keys.hasMoreElements())
    {
        String k = (String) keys.nextElement();
        Object v = siCache.get(k);

       if(condition) siCache.remove(k);

    }

Answer 1

Removing an element from a Hashtable while enumerating the keys is potentially risky. Here’s what the javadoc says:

“Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Hashtable’s keys and elements methods are not fail-fast.”

The implication is clear: arbitrary, non-deterministic behaviour is possible if you do that.

Solutions:

• If you are using J2SE, use keySet(). Or better still, don’t Use Hashtable.
• If you are using J2ME, build a list of keys to be removed, and remove them later … or pray hard :-).