在尝试添加第二个键之前，如何避免将我的哈希映射增加到 2 个键？

Question

I have a situation where I have tons of hashmaps (millions and millions) in my server, and 95% of them have literally only one key, ever.

The server runs out of memory; which probably may have something to do with the fact that initial HashMap default size is 16, so for every HashMap object I'm wasting a ton of memory.

Short of re-designing the server with a new data structure that flexibly stores the data as 1 element OR an entire hashmap (which I'd prefer avoiding), I'm trying to first optimize this by changing the initial size to 1:

Map<String, Type2> myMiniMap = new HashMap<>(1);

However, my concern is that due to default load factor of 0.75 in HashMaps, this would immediately get increased to 2 the moment I add the first key to the map (since 1*0.75 < 1 , which is how I understand hash sizing logic in Java)

Assuming my understanding above is correct (that by default, Java will create a space for 2 keys as soon as I add the first key to the hash), is there a way to prevent this from happening till I actually try to insert the second key ?

Eg, should I set loadFactor to zero or one?

Answer 1

If they're truly only ever going to be singletons, why not use Collections.singletonMap to create them? The downside of this is that the map that is created is immutable.

Alternatively, you could create your own class implementing Map that will store a key and value in class fields and then, if an attempt is made to add a second key value, it will switch to using a HashMap as its default backing store. It would be more tedious than difficult to accomplish.