StringBuilder

(defn reverse-string [s]
  (.toString
    (.reverse
      (StringBuilder. s))))

In Clojure, as in Java, strings are immutable. It means that with every change we want to make to any string, we create a new string in memory.

String recreation can be very resource-intensive, especially when new strings are created in many steps. This is a common problem, so Java provides the StringBuilder class, which holds characters as a collection, allowing for modifications until we are ready to create the string by calling toString() method.

String builder has a built-in reverse() method, which we can use as shown above. The complete approach is to initialise a StringBuilder with the string we want to reverse. Then reverse it, and finally convert it back to string.

The shortcut

Is accessing the underlying Java Virtual Machine and Java classes necessary? Couldn't we just use the clojure.string/reverse function instead?

We could! This is the alternative way to reverse a string:

(defn reverse-string [s]
  (clojure.string/reverse s))

In fact, at some level, we could consider both approaches to be equivalent. Let's have a look at the implementation of the clojure.string/reverse function.

(defn ^String reverse
  "Returns s with its characters reversed."
  {:added "1.2"}
  [^CharSequence s]
  (.toString (.reverse (StringBuilder. s))))

While there is a little bit more going on in the syntax, at its core, it is the same as the code at the top of this approach.