Demonstrating one off extension functions in Kotlin for improving readability

ArgumentBuilder.kt

public class ArgumentBuilder(private val runnerParameters: Map<String, String>) {

    public fun build(): List<String> {
        return arrayListOf<String>()
            .addIfSet("--Project", RunnerSettings.PROJECT_FILE)
            .addIfSet("--Output", RunnerSettings.OUTPUT)
            .addFlagIfSet("--Flag1", RunnerSettings.FLAG_1)
            .addFlagIfSet("--Flag2", RunnerSettings.FLAG_2)
            .addFlagIfSet("--Flag3", RunnerSettings.FLAG_3)
            .addMultipleIfSet("--InputFiles", RunnerSettings.INPUTS)
    }

    private fun MutableList<String>.addFlagIfSet(argument: String, setting: String): MutableList<String> {
        if (runnerParameters.doesNotContainKey(setting)) {
            return this;
        }
        add(argument)
        return this
    }

    private fun MutableList<String>.addIfSet(argument: String, setting: String): MutableList<String> {
        val value = runnerParameters.get(setting)
        if (value == null) {
            return this;
        }
        add(argument)
        add(value)
        return this;
    }

    private fun MutableList<String>.addMultipleIfSet(argument: String, setting: String): MutableList<String> {
        val value = runnerParameters.get(setting)
        if (value == null) {
            return this;
        }
        val values = value
            .splitOnLineBreaks()
            .filterEmptyLines()
        if (values.isEmpty()) {
            return this;
        }
        add(argument)
        addAll(values)
        return this;
    }
}

Revision 1 is before, Revision 2 is after the refactoring. Note there are two components, first using extension functions on a MutableList and second creating a fluent interface by returning the type being acted upon (MutuableList).

Focus on the readability of the build method. One could argue the fluent aspect is less readable within the extension functions, in that case just remove the fluent aspect and still have:

val arguments = ArrayList<String>()
arguments.addIfSet("--Project", RunnerSettings.PROJECT_FILE)
arguments.addIfSet("--Output", RunnerSettings.OUTPUT)
arguments.addFlagIfSet("--Flag1", RunnerSettings.FLAG_1)
arguments.addFlagIfSet("--Flag2", RunnerSettings.FLAG_2)
arguments.addFlagIfSet("--Flag3", RunnerSettings.FLAG_3)
arguments.addMultipleIfSet("--InputFiles", RunnerSettings.INPUTS)
return arguments