Hashmap vs Array performance

HashMap uses an array underneath so it can never be faster than using an array correctly.

Random.nextInt() is many times slower than what you are testing, even using array to test an array is going to bias your results.

The reason your array benchmark is so slow is due to the equals comparisons, not the array access itself.

HashTable is usually much slower than HashMap because it does much the same thing but is also synchronized.

A common problem with micro-benchmarks is the JIT which is very good at removing code which doesn't do anything. If you are not careful you will only be testing whether you have confused the JIT enough that it cannot workout your code doesn't do anything.

This is one of the reason you can write micro-benchmarks which out perform C++ systems. This is because Java is a simpler language and easier to reason about and thus detect code which does nothing useful. This can lead to tests which show that Java does "nothing useful" much faster than C++ ;)

arrays when the indexes are know are faster (HashMap uses an array of linked lists behind the scenes which adds a bit of overhead above the array accesses not to mention the hashing operations that need to be done)

and FYI HashMap<String,SomeObject> objects = HashMap<String,SomeObject>(); makes it so you won't have to cast

For the example shown, HashTable wins, I believe. The problem with the array approach is that it doesn't scale. I imagine you want to have more than two entries in the table, and the condition branch tree in doSomethingToObject will quickly get unwieldly and slow.