使用哈希映射而不是数组会产生空间开销吗？

Question

I was trying to implement a trie and read in an example implementation that it would be more space efficient to use a small array of size 26 to store the children because then you wouldn't have to waste space with a HashMap (the code was in Java, if that makes a difference)

But wouldn't a map be more space efficient since you don't necessarily need to store all 26 values? Or is a HashMap object that contains Character objects as keys just more space because a simple int[] type does not use extra space in the background that the implementation for these more complex objects would use?

Just wanting to check if maybe this person was mistaken or if there's some overhead involved in using object types like HashMap that I should be aware of.

Answer 1

A hashmap stores keys and values, so if you were to implement a trie using a hashmap, you would be storing not only the values, but also the keys. If you use an array, then the key is actually the index of the value in the array, so you do not have to store it anywhere.

Besides that, hashmaps are less space efficient than arrays because they always have a load factor which is smaller than one, which is the same as saying that they keep more entries allocated than necessary due to the way they work. I am not expanding on this, because it is not related to your issue, but if you are curious, search for hashmap load factor .

Answer 2

This is a classic programming compromise. In this case, using a HashMap will take more space, but that may be a price you're willing to pay for improved performance. Or it might not. Depends on your problem.