Scala Cookbook

1.10. Add Your Own Methods to the String Class

Problem

Rather than create a separate library of String utility methods, like a StringUtilities class, you want to add your own behavior(s) to the String class, so you can write code like this:

"HAL".increment

Instead of this:

StringUtilities.increment("HAL")

Solution

In Scala 2.10, you define an implicit class, and then define methods within that class to implement the behavior you want.

You can see this in the REPL. First, define your implicit class and method(s):

scala> implicit class StringImprovements(s: String) {
     |   def increment = s.map(c => (c + 1).toChar)
     | }
defined class StringImprovements

Then invoke your method on any String:

scala> val result = "HAL".increment
result: String = IBM

In real-world code, this is just slightly more complicated. According to SIP-13, Implicit Classes, “An implicit class must be defined in a scope where method definitions are allowed (not at the top level).” This means that your implicit class must be defined inside a class, object, or package object.

Put the implicit class in an object

One way to satisfy this condition is to put the implicit class inside an object. For instance, you can place the StringImprovements implicit class in an object such as a StringUtils object, as shown here:

package com.alvinalexander.utils

object StringUtils {
  implicit class StringImprovements(val s: String) {
    def increment = s.map(c => (c + 1).toChar)
  }
}

You can then use the increment method somewhere else in your code, after ...