Data computation in Java is ususally realized with the help of SQL, but for a complex data process, SQL seems so difficult and complex. Sometimes, the datasource is not a database, it could be TXT or Excel. In my java project, the data source is from dynamic Excel Sheet and the data is being updated by now and then. One Excel Sheet is Customer Information, the other is Purchase Order. Based on the information given, how could I figure out the the group of Customers who have purchased the same product ? Your ideas will be welcomed.