How does the content of a Hashtable affect its size in memory?

If I have Hashtable A that has 5 million keys mapped to 5 million unique values, and I have Hashtable B that has 5 million keys mapped to 20 unique values, then approximately how much more memory would Hashtable A use compared to Hashtable B?

All of the keys and values are Strings that are approximately 20-50 characters in length.

My initial guess is that Hashtable A would take up roughly double the space as Hashtable B, but if you include the mappings then Hashtable B would use:

(5 million keys + 5 million mappings + 20 values) / (5 million keys + 5 million mappings + 5 million values) = .66

66.6% of the memory Hashtabl开发者_如何学JAVAe A uses. However I don't know if a mapping would use as much space as a key or value if the keys and values are Strings.

Comments?

I don't think this has to do much with the hash table, since the "values" of the hash table are merely references to what I assume are the existing values. The increase in total cost would be based primarily on the size of a value. After all, you could have every key mapped to null.

Also, depending on the size of your keys, this may or may not have an impact. For example, a mapping from 5 million heavy objects (like strings) to 5 million lighter objects like Integers would not be that different from mapping 5 million heavy objects to 20 different values of Integer.

If you're storing literal strings, then the JVM may intern them, in which case the 20 key version would use significantly less memory (just how much less I don't know how to calculate). But for a standard hash table implementation that isn't subject to such magic, they would both use the same amount of memory, since each "bucket" will store a value, regardless of if that value is also stored in other buckets.