It is possible to avoid duplication of immutable objects... Say if in a object tree, you have the same immutable object twice, we could consider it as a single object. That would save streaming size and possibly CPU as the serialization time would be probably higher than looking up in a HashMap. A question that I have though: Dates are not immutable... Is there any date object known to be immutable? Michael Nascimento, the one who originally requested this issue is driving a JSR about dates.. I guess he would be able to give some ideas on this subject also. Clebert View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4034296#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...