To expand on David Richerby's answer, the term "hash function" is a little overloaded. Often, when we talk about a hash function we think of MD5, SHA-1, or something like Java's
.hashCode() method, which turns some input into a single number. However the domain of this number (i.e. is maximum value) is very unlikely to be the same size as the hashtable you're trying to store data in. (MD5 is 16 bytes, SHA-1 is 20 bytes, and
.hashCode() is an
int - 4 bytes).
So your question is about that next step - once we have a hash function that can map arbitrary inputs to numbers, how do we put them into a data structure of a particular size? With another function, also called a "hash function"!
A trivial example of such a function is modulo; you can easily map a number of arbitrary size to a specific index in an array with modulo. This is introduced in CLRS as "the division method":
In the division method for creating hash functions, we map a key $k$ into one of $m$ slots by taking the remainder of $k$ divided by $m$. That is, the hash function is
$ h(k) = k$ mod $m$.
When using the division method we usually avoid certain values of $m$. For example, $m$ should not be a power of 2, since if $m = 2^p$ then $h(k)$ is just the $p$ lowest-order bits of $k$.
~Introduction to Algorithms, §11.3.1 - CLRS
So modulo isn't a great hash function, since it restricts what sizes we can safely use for our underlying data structure. The next section introduces a slighly more complex "multiplication method", which also uses modulo but is advantageous because "the value of $m$ is not critical". It however works best with some prior knowledge of "characteristics of the data being hashed" - something we often don't know.
HashMap uses a modified version of the division method that does a pre-processing step to account for weak
.hashCode() implementations so that it can use power-of-two sized arrays. You can see exactly what's happening in the
.getEntry() method (comments are mine):
// hash() transforms key.hashCode() to protect against bad hash functions
int hash = (key == null) ? 0 : hash(key.hashCode());
// indexOf() converts the resulting hash to a value between 0 and table.length-1
for (Entry<K,V> e = table[indexFor(hash, table.length)];
Java 8 brought along a rewrite of
HashMap which is even faster, but a little harder to read. It uses the same general principle for index lookup, however.