diff --git a/simple-algorithms/array-diff/src/main/java/com/josdem/kata/Differentiator.java b/simple-algorithms/array-diff/src/main/java/com/josdem/kata/Differentiator.java
index 49a6a45..ce7f113 100644
--- a/simple-algorithms/array-diff/src/main/java/com/josdem/kata/Differentiator.java
+++ b/simple-algorithms/array-diff/src/main/java/com/josdem/kata/Differentiator.java
@@ -5,6 +5,6 @@
 public class Differentiator {
 
     public List<Integer> diff(List<Integer> a, List<Integer> b) {
-        return a;
+        return a.stream().filter(number -> !b.contains(number)).toList();
     }
 }
diff --git a/simple-algorithms/array-diff/src/test/java/com/josdem/kata/DifferentiatorTest.java b/simple-algorithms/array-diff/src/test/java/com/josdem/kata/DifferentiatorTest.java
index 16fb18f..ca7945d 100644
--- a/simple-algorithms/array-diff/src/test/java/com/josdem/kata/DifferentiatorTest.java
+++ b/simple-algorithms/array-diff/src/test/java/com/josdem/kata/DifferentiatorTest.java
@@ -21,7 +21,7 @@ class DifferentiatorTest {
     @DisplayName("Get difference")
     void shouldGetDifferenceInArrays() {
         assertEquals(List.of(2), differentiator.diff(List.of(1, 2), List.of(1)));
-        assertEquals(List.of(2, 2), differentiator.diff(List.of(1, 2), List.of(1)));
+        assertEquals(List.of(2, 2), differentiator.diff(List.of(1, 2, 2), List.of(1)));
         assertEquals(List.of(1), differentiator.diff(List.of(1, 2, 2), List.of(2)));
         assertEquals(List.of(1, 2, 2), differentiator.diff(List.of(1, 2, 2), new ArrayList<>()));
         assertEquals(new ArrayList<>(), differentiator.diff(new ArrayList<>(), List.of(1, 2)));