I recently ran into a case where I was violating the DRY principle by having to encode part of a string every time I added to it. After some digging I found the solution: Groovy Categories.

A bit of Groovy background

A couple key features of Groovy are that everything is an object and that operators are just syntactic sugar for calling methods on the objects. What’s cool about this is that with Groovy you can override the default behavior of these operators for certain classes. For example, 4 + 2 in Groovy really means 4.plus(2)

How to implement/override operators in Groovy

Groovy allows you to override a LOT of stuff, including final methods and operators. If I want to override a method on the String class, all I need to do is:

String.metaClass.plus {
    //It is a special term for the argument passed (if there is only 1)
    String stringToEncode = it
    //delegate is a special term for the caller of the method
    delegate << URLEncoder.encode(it, 'UTF-8')

String first = "my delegate "
String second = "twitter @eriwen"
//prints "my delegate twitter+%40eriwen"
println first + second

Notice that it (intentionally) only encodes the second String. Check out the list of operators and associated methods. This is great, but I need to restrict this to code blocks I so choose. That’s where Groovy Categories come in.

Overriding with Categories

Groovy Categories allow you to add functionality to any class at runtime. This means that you can add in methods to final classes (like java.lang.String). In this case I just want to override a method instead of adding one.

class MyUtils {
  //Takes 2 java.lang.Strings and returns a String - TYPES ARE OPTIONAL
  static String plus(String first, String second) {
    //Concatenates the Strings and returns
    first << URLEncoder.encode(second, encoding)

//my other class in another file...
import MyUtils

class MyOtherClass {
  String first = "first"
  String second = "to be encoded"
  use (MyUtils) {
    // This will now encode value
    list << first + second

Note that Groovy defaults to GStrings, so you can’t just use def actuallyMyString.

How would you make this even more Groovy? Enjoy!

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