diff --git a/app/src/main/java/gui/SudokuType.java b/app/src/main/java/gui/SudokuType.java
index c8f3d8e..c7646a7 100644
--- a/app/src/main/java/gui/SudokuType.java
+++ b/app/src/main/java/gui/SudokuType.java
@@ -41,6 +41,7 @@ public enum SudokuType {
 		return this.paramCount;
 	}
 
+	@FunctionalInterface
 	private static interface SudokuMaker {
 		MultiDoku makeSudoku(List<Constraint> constraints, int... params);
 	}