Why a hashtable? Why not just a non-hashed associative array?

I’ve been learning about using a hashtable to efficiently check for items in a list without looping through the whole thing, but there’s one thing that I don’t get:

Is hash calculated before/after compression?

I had a question regarding compression and calculation of checksum/hash of data.

Which hashing algorithm is best for uniqueness and speed?

Which hashing algorithm is best for uniqueness and speed? Example (good) uses include hash dictionaries.

Which hashing algorithm is best for uniqueness and speed?

Which hashing algorithm is best for uniqueness and speed? Example (good) uses include hash dictionaries.

Does multiple Bloom filters make sense?

In order to decrease the number of false positives, I came up with a couple of possible solutions.

Track a series of choices with a single number

I want to track a series of scored questions in a single number (the total score). Is that possible? and how?

How early does a hashable type need to become immutable?

Suppose I have a class that looks vaguely like this:

How early does a hashable type need to become immutable?

Suppose I have a class that looks vaguely like this:

How early does a hashable type need to become immutable?

Suppose I have a class that looks vaguely like this:

How early does a hashable type need to become immutable?

Suppose I have a class that looks vaguely like this: