Theoretical limit for number of keys (objects) that can be stored in a HashMap?

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Is there a theoretical limit for the number of key entries that can be stored in a HashMap or does it purely depend on the heapmemory available ?

Looking at the documentation of that class, I would say that the theoretical limit is Integer.MAX_VALUE (231-1 = 2147483647) elements.

This is because to properly implement this class, the size() method is obliged to return an int representing the number of key/value pairs.

From the documentation of HashMap.size()

Returns: the number of key-value mappings in this map

Note: This question is very similar to How many data a list can hold at the maximum.

which data structure is the best to store a very large number of objects(say several hundred thousand objects)?

I would say it depends on what you need to store and what type of access you require. All built in collections are probably well optimized for large quantities.

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