Unordered Collections

Maps

A map is a collection that maps keys to values. In Clojure, there are two different types—hashed and sorted.

Hash map

A hash map is the most used map in Clojure. It requires keys that correctly support hashCode and equals.

When should we use it?

They’re useful for:

• Representing entities

• When the order isn’t important, and we’re going to perform random access using keys

• When we don’t want to have duplicated keys

• Associating keys to values

• Removing key-value pairs based on the key

• Fast count

• Checking if a key exists in the map

How to instantiate a map

We normally instantiate a map using literals. The keys are keywords and need to be unique values.