How to calculate HashMap memory usage in Java?

I was asked in an interview to calculate the memory usage for HashMap and how much estimated memory it will consume if you have 2 million items in it.

For example:

Map <String,List<String>> mp=new HashMap <String,List<String>>();

The mapping is like this.

key   value
----- ---------------------------
abc   ['hello','how']
abz   ['hello','how','are','you']

How would I estimate the memory usage of this HashMap Object in Java?