I’ve got an existing instance of HashMap simply called sale (it is Map>) I use it to log customers and items history.

Is there a way to create a new instance of HashMap, that effectively reverses this usage? i.e will show each item purchased as a unique key and the corresponding value as a String set of the customers that have purchased that product. I suspect there is a simple process using keySet() in some way to iterate over the sales map but I just can’t see how to do this. Any help would be much appreciated.