Question

From my understanding,

TreeMap :
1. Insert O( logN )
2. Delete O( logN )
3. Retrieve O( logN )
HashMap :
1. Insert O( 1 ) -> O( N )
2. Delete O( 1 ) -> O( N )
3. Retrieve O( 1 ) -> O( N )

I know that TreeMap using Red-Black tree as internal data structure. However, I’m not so sure about HashMap’s internal data structure.

1. Is HaspMap implemented using
   Hash-Table?
2. If the answer is “Yes”, then does it use array-base’s implementation
   or reference-base’s implementation?
3. And if it used array-base then is it sorted or unsorted?

I’m working on a small project using Java to demonstrate the runtime complexity of all operations( insert, delete, retrieve ) between HashMap and TreeMap, but I don’t really know how to link the theory formula with actual result from running a program. For example, from running a quick test :
1. Insert 10,000 elements
2. Delete 100 elements
3. Retrieve 100 elements

I got this info:

     HashMap
Create time : 6348015 nano seconds.
Remove time : 98458 nano seconds.
Retrieve Found time : 59762 nano seconds.
Retrieve Not Found time : 39097 nano seconds.
 --- end ---

     TreeMap
Create time : 20518163 nano seconds.
Remove time : 274221 nano seconds.
Retrieve Found time : 112072 nano seconds.
Retrieve Not Found time : 168442 nano seconds.
 --- end ---

I would like to know how can I find the link these times with the theoretical time complexity such as O( N ) or O( logN )?
And this result surprised me since I always think TreeMap would beat HashMap badly. Could anyone give me some brief explanation about these things? Thanks in advance.

Answer 1

If you want to show the complexity of some operation, you can’t just use one data point, you have to show how the time changes when N changes.

Also, often algorithms or data-structures with better theoretical complexity have worse running times for smaller data-sets.

Another thing to think about is to have representative values. For example if you insert values 1, 2, 3, … into an RB-tree, that’s the worst-case scenario (I think), because it has to rebalance quite often. Inserting random values will probably give different result.