This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I want to update the properties file without changing the sequence of key,value pairs.
When I do it using Java.util.Proeprties.setProperty , the format of file is lost, although the data gets persisted.
You could extend the Properties with your own class that overrides the "load" and "store" method with versions that remember (load) and honor (store) the order of the original file. If you start with the code of the original methods, that shouldn't be too hard.