diff --git a/src/main/kotlin/com/beust/kobalt/api/Project.kt b/src/main/kotlin/com/beust/kobalt/api/Project.kt
index 258e090f..e49f047d 100644
--- a/src/main/kotlin/com/beust/kobalt/api/Project.kt
+++ b/src/main/kotlin/com/beust/kobalt/api/Project.kt
@@ -44,7 +44,7 @@ open public class Project(
         return sources
     }
 
-    var sourceDirectories : ArrayList<String> = arrayListOf()
+    var sourceDirectories : HashSet<String> = hashSetOf()
         get() = if (field.isEmpty()) projectInfo.defaultSourceDirectories else field
         set(value) {
             field = value
@@ -57,7 +57,7 @@ open public class Project(
         return sources
     }
 
-    var sourceDirectoriesTest : ArrayList<String> = arrayListOf()
+    var sourceDirectoriesTest : HashSet<String> = hashSetOf()
         get() = if (field.isEmpty()) projectInfo.defaultTestDirectories
                 else field
         set(value) {
@@ -94,7 +94,7 @@ open public class Project(
     val projectName: String get() = name
 }
 
-public class Sources(val project: Project, val sources: ArrayList<String>) {
+public class Sources(val project: Project, val sources: HashSet<String>) {
     @Directive
     public fun path(vararg paths: String) {
         sources.addAll(paths)
diff --git a/src/main/kotlin/com/beust/kobalt/internal/BuildGenerator.kt b/src/main/kotlin/com/beust/kobalt/internal/BuildGenerator.kt
index 190a583b..df47c36d 100644
--- a/src/main/kotlin/com/beust/kobalt/internal/BuildGenerator.kt
+++ b/src/main/kotlin/com/beust/kobalt/internal/BuildGenerator.kt
@@ -12,8 +12,8 @@ import java.util.*
  * Abstract base class for the build generators that use build-template.mustache.
  */
 abstract class BuildGenerator : IInitContributor {
-    abstract val defaultSourceDirectories : ArrayList<String>
-    abstract val defaultTestDirectories : ArrayList<String>
+    abstract val defaultSourceDirectories : HashSet<String>
+    abstract val defaultTestDirectories : HashSet<String>
     abstract val directive : String
     abstract val name : String
     abstract val fileMatch : (String) -> Boolean
diff --git a/src/main/kotlin/com/beust/kobalt/internal/ProjectInfo.kt b/src/main/kotlin/com/beust/kobalt/internal/ProjectInfo.kt
index 2c8ee288..81653a92 100644
--- a/src/main/kotlin/com/beust/kobalt/internal/ProjectInfo.kt
+++ b/src/main/kotlin/com/beust/kobalt/internal/ProjectInfo.kt
@@ -6,6 +6,6 @@ import java.util.*
  * Data that is useful for projects to have but should not be specified in the DSL.
  */
 interface IProjectInfo {
-    val defaultSourceDirectories: ArrayList<String>
-    val defaultTestDirectories: ArrayList<String>
+    val defaultSourceDirectories: HashSet<String>
+    val defaultTestDirectories: HashSet<String>
 }
diff --git a/src/main/kotlin/com/beust/kobalt/plugin/java/JavaPlugin.kt b/src/main/kotlin/com/beust/kobalt/plugin/java/JavaPlugin.kt
index 11307667..6cfd288f 100644
--- a/src/main/kotlin/com/beust/kobalt/plugin/java/JavaPlugin.kt
+++ b/src/main/kotlin/com/beust/kobalt/plugin/java/JavaPlugin.kt
@@ -89,7 +89,7 @@ public class JavaPlugin @Inject constructor(
         return result
     }
 
-    private fun findSourceFiles(dir: String, sourceDirectories: List<String>): List<String> {
+    private fun findSourceFiles(dir: String, sourceDirectories: Collection<String>): List<String> {
         val projectDir = File(dir)
         return files.findRecursively(projectDir,
                 sourceDirectories.map { File(it) }) { it: String -> it.endsWith(".java") }
diff --git a/src/main/kotlin/com/beust/kobalt/plugin/java/JavaProjectInfo.kt b/src/main/kotlin/com/beust/kobalt/plugin/java/JavaProjectInfo.kt
index 9b5513ec..f0ae3ff3 100644
--- a/src/main/kotlin/com/beust/kobalt/plugin/java/JavaProjectInfo.kt
+++ b/src/main/kotlin/com/beust/kobalt/plugin/java/JavaProjectInfo.kt
@@ -5,6 +5,6 @@ import com.google.inject.Singleton
 
 @Singleton
 class JavaProjectInfo : IProjectInfo {
-    override val defaultSourceDirectories = arrayListOf("src/main/java", "src/main/resources")
-    override val defaultTestDirectories = arrayListOf("src/test/java", "src/test/resources")
+    override val defaultSourceDirectories = hashSetOf("src/main/java", "src/main/resources")
+    override val defaultTestDirectories = hashSetOf("src/test/java", "src/test/resources")
 }
diff --git a/src/main/kotlin/com/beust/kobalt/plugin/kotlin/KotlinProjectInfo.kt b/src/main/kotlin/com/beust/kobalt/plugin/kotlin/KotlinProjectInfo.kt
index 77a07678..30f9c153 100644
--- a/src/main/kotlin/com/beust/kobalt/plugin/kotlin/KotlinProjectInfo.kt
+++ b/src/main/kotlin/com/beust/kobalt/plugin/kotlin/KotlinProjectInfo.kt
@@ -5,7 +5,7 @@ import com.google.inject.Singleton
 
 @Singleton
 class KotlinProjectInfo : IProjectInfo {
-    override val defaultSourceDirectories = arrayListOf("src/main/kotlin", "src/main/resources")
-    override val defaultTestDirectories = arrayListOf("src/test/kotlin", "src/test/resources")
+    override val defaultSourceDirectories = hashSetOf("src/main/kotlin", "src/main/resources")
+    override val defaultTestDirectories = hashSetOf("src/test/kotlin", "src/test/resources")
 }