In an object-oriented language like Java, there are a large amount of types for every situation. Clojure takes another approach. It has a small amount of types and many different functions for them. However, being pragmatic, Clojure realizes that polymorphism is flexible and useful for some situations. Let’s take a look at a few ways that Clojure can flex its polymorphic muscles.

If we wanted to have a function that would behave differently based on the kind of input we had, we could use a case like statement. This example uses a function called cond that behaves differently depending on whether the argument is a keyword, string, or number, and returns the caterpillar’s questions to Alice:

The class input is compared, and if it is a string it will return "String - Who are you?"

If it is a keyword, it will return "Keyword - Who are you?"

If it is a number (class of Long), it will return "Number - who are you?"

When called with a keyword, returns the clause that matched the keyword class.

When called with a string, returns the clause that matched the string class.

When called with a number, returns the clause that matched the number class.

When called with a boolean, returns nil because there is no matching cond clause.

When a new language comes into being, it faces the library problem. That is, to be useful in everyday situations, a language needs to do all the things that current dominant languages do. These current dominant languages have a full array of libraries that support things like parsing JSON and logging.

Clojure solved this new language library problem by running on the JVM and having interoperability with Java classes. When you use Clojure, you can use Java classes and Java libraries. Clojure builds on the strength of the production-hardened and tested JVM and existing Java libraries. In fact, many of the popular Clojure libraries in use today utilize Java libraries as fundamental building blocks. We are going to cover the most common areas that you will encounter: how to import Java libraries/classes, how to create new instances of them, and how to interact with their methods.