Compare and Merging two XML "STRINGS" in java

I want to compare and merge 2 XML formats in JAVA which is stored as strings. I want to merge the strings and give the output as a single XML format. I want to compare unique IDs in both XML strings and merge each record accordingly.For example if i have product ID, product name, product description in one XML and product ID, number of stocks in the other XML ,I want to compare product IDs in both XML and if they are same, then I want output in the format which has: product ID, product name, product description, number of stocks for each record.Please guide me with a sample code for this in Java. There shouldn't be any loop statement for this comparing and merging.