Theoretical limit for number of keys (objects) that can be stored in a HashMap?
HashMap holds the values in an array, which can hold up to Integer.MAX_VALUE. But this does not count collisions. Each Entry has a next field, which is also an entry. This is how collisions (two or more objects with the same hashcode) are resolved. So I wouldn't say there is any limit (apart from the available memory)
Note that if you exceed Integer.MAX_VALUE, you'll get unexpected behaviour from some methods, like size(), but get() and put() will still work. And they will work, because the hashCode() of any object will return an int, hence by definition each object will fit in the map. And then each object will collide with an existing one.
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.