Set whenever we want to get the uniqe option from the dropdown.
Or whenever we want to handle multiple tabs such as getwindowhandles.
HashMaps data from 2 columns from tables. 
If you want to get a data from Json file, properties file. 
HashMap is for pair of data. You can get the data from JSON or properties files. POJO Class. Key and value for.
What is different between list and set. 
Set does not accept duplicates. List takes duplicates.
And index number different.