How to calculate HashMap memory usage in Java?

Question

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.

Answer 1

I think that the question should be clarified because there is a difference between the size of the HashMap and the size of HashMap + the objects contained by the HashMap.

If you consider the size of the HashMap, in the example you provided, the HashMap stores one reference to the String "aby" and one reference to the List. So the multiple elements in the list do not matter. Only the reference to the list is stored in the value.

In a 32 bits JVM, in one Map entry, you have 4 bytes for the "aby" reference + 4 bytes for the List reference + 4 bytes for the "hashcode" int property of Map entry + 4 bytes for the "next" property of Map entry.

You also add the 4*(X-1) bytes references where the "X" is the number of empty buckets that the HashMap has created when you called the constructor new HashMap>() . According to http://docs.oracle.com/javase/6/docs/api/java/util/HashMap.html, it should be 16.

There are also loadFactor, modCount, threshold and size which are all primitive int type (16 more bytes) and header (8bytes).

So in the end, the size of your above HashMap would be 4 + 4 + 1 + (4*15) + 16 + 8 = 93 bytes

This is an approximation based on data that are owned by the HashMap. I think that maybe the interviewer was interested in seeing if you were aware of the way HashMap works (the fact for example that the default constructor create and array of 16 buckets for Map entry, the fact that the sizes of the objects stored in the HashMap do not affect the HashMap size since it only store the references).

HashMap are so widely used that under certain circumstances, it should be worth using the constructors with initial capacity and load factor.