diff --git a/pyproject.toml b/pyproject.toml
index 770538e..9436458 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "ontouml-py"
-version = "0.1.0"
+version = "0.1.1"
 description = "OntoUML Python Library"
 license = "Apache-2.0"
 authors = ["Pedro Paulo F. Barcelos <p.p.favatobarcelos@utwente.nl>"]
diff --git a/tests/abstract_classes/test_relation.py b/tests/abstract_classes/test_relation.py
new file mode 100644
index 0000000..bb873af
--- /dev/null
+++ b/tests/abstract_classes/test_relation.py
@@ -0,0 +1,102 @@
+import pytest
+
+from ontouml_py.classes.abstract_classes.relation import Relation
+from ontouml_py.classes.concrete_classes.binaryrelation import BinaryRelation
+from ontouml_py.classes.concrete_classes.naryrelation import NaryRelation
+from ontouml_py.classes.enumerations.relationstereotype import RelationStereotype
+
+
+def test_relation_initialization_with_valid_stereotype() -> None:
+    """Test the initialization of a Relation subclass with a valid stereotype."""
+
+    class BinaryRelation(Relation):
+        def __init__(self, **data):
+            super().__init__(**data)
+
+    relation = BinaryRelation(stereotype=RelationStereotype.MATERIAL)
+    assert (
+        relation.stereotype == RelationStereotype.MATERIAL
+    ), "Relation should be initialized with the specified stereotype"
+
+
+def test_relation_initialization_with_invalid_stereotype() -> None:
+    """Test the initialization of a Relation subclass with an invalid stereotype."""
+
+    class BinaryRelation(Relation):
+        def __init__(self, **data):
+            super().__init__(**data)
+
+    with pytest.raises(ValueError):
+        BinaryRelation(stereotype="invalid_stereotype")
+
+
+def test_relation_subclass_validation() -> None:
+    """Test the validation of allowed subclasses for Relation."""
+
+    class InvalidRelation(Relation):
+        def __init__(self, **data):
+            super().__init__(**data)
+
+    with pytest.raises(ValueError):
+        InvalidRelation()
+
+
+def test_binary_relation_as_valid_subclass() -> None:
+    """Test that BinaryRelation is a valid subclass of Relation."""
+    binary_relation = BinaryRelation()
+    assert isinstance(binary_relation, Relation), "BinaryRelation should be a valid subclass of Relation"
+
+
+def test_nary_relation_as_valid_subclass() -> None:
+    """Test that NaryRelation is a valid subclass of Relation."""
+    nary_relation = NaryRelation()
+    assert isinstance(nary_relation, Relation), "NaryRelation should be a valid subclass of Relation"
+
+
+def test_relation_default_values() -> None:
+    """Test the default values of attributes in the Relation class."""
+
+    class BinaryRelation(Relation):
+        def __init__(self, **data):
+            super().__init__(**data)
+
+    relation = BinaryRelation()
+    assert relation.stereotype is None, "Default value of stereotype should be None"
+
+
+def test_relation_attribute_assignment() -> None:
+    """Test attribute assignment in the Relation class."""
+
+    class BinaryRelation(Relation):
+        def __init__(self, **data):
+            super().__init__(**data)
+
+    relation = BinaryRelation()
+    relation.stereotype = RelationStereotype.MATERIAL
+    assert relation.stereotype == RelationStereotype.MATERIAL, "Stereotype should be assignable in Relation class"
+
+
+def test_relation_initialization_with_each_stereotype() -> None:
+    """Test the initialization of a Relation subclass with each stereotype in RelationStereotype."""
+
+    class BinaryRelation(Relation):
+        def __init__(self, **data):
+            super().__init__(**data)
+
+    for stereotype in RelationStereotype:
+        relation = BinaryRelation(stereotype=stereotype)
+        assert relation.stereotype == stereotype, f"Relation should be initialized with the stereotype {stereotype}"
+
+
+def test_relation_initialization_with_incorrect_type_for_stereotype() -> None:
+    """Test the initialization of a Relation subclass with incorrect type for stereotype."""
+
+    class BinaryRelation(Relation):
+        def __init__(self, **data):
+            super().__init__(**data)
+
+    with pytest.raises(ValueError):
+        BinaryRelation(stereotype=123)  # Using an integer instead of a RelationStereotype member
+
+    with pytest.raises(ValueError):
+        BinaryRelation(stereotype="non_existent_stereotype")  # Using a non-existent stereotype string
diff --git a/tests/utils/test_nonemptyset.py b/tests/utils/test_nonemptyset.py
index bc58583..30dfeaa 100644
--- a/tests/utils/test_nonemptyset.py
+++ b/tests/utils/test_nonemptyset.py
@@ -3,61 +3,61 @@
 from ontouml_py.classes.utils.nonemptyset import NonEmptySet
 
 
-def test_nonemptyset_initialization():
+def test_nonemptyset_initialization() -> None:
     """Test initialization of NonEmptySet with non-empty set."""
     non_empty_set = NonEmptySet({1, 2, 3})
     assert len(non_empty_set) == 3, "NonEmptySet should initialize with the correct number of elements"
 
 
-def test_nonemptyset_initialization_empty_set():
+def test_nonemptyset_initialization_empty_set() -> None:
     """Test initialization of NonEmptySet with an empty set raises ValueError."""
     with pytest.raises(ValueError, match="Initial set cannot be empty."):
         NonEmptySet(set())
 
 
-def test_nonemptyset_add_element():
+def test_nonemptyset_add_element() -> None:
     """Test adding an element to the NonEmptySet."""
     non_empty_set = NonEmptySet({1})
     non_empty_set.add(2)
     assert 2 in non_empty_set, "Element should be added to NonEmptySet"
 
 
-def test_nonemptyset_remove_element():
+def test_nonemptyset_remove_element() -> None:
     """Test removing an element from the NonEmptySet."""
     non_empty_set = NonEmptySet({1, 2})
     non_empty_set.remove(1)
     assert 1 not in non_empty_set, "Element should be removed from NonEmptySet"
 
 
-def test_nonemptyset_remove_last_element():
+def test_nonemptyset_remove_last_element() -> None:
     """Test removing the last element from the NonEmptySet raises ValueError."""
     non_empty_set = NonEmptySet({1})
     with pytest.raises(ValueError, match="Cannot remove the last element from the set."):
         non_empty_set.remove(1)
 
 
-def test_nonemptyset_update():
+def test_nonemptyset_update() -> None:
     """Test updating NonEmptySet with another set."""
     non_empty_set = NonEmptySet({1})
     non_empty_set.update({2, 3})
     assert non_empty_set == NonEmptySet({1, 2, 3}), "NonEmptySet should be updated correctly"
 
 
-def test_nonemptyset_discard_existing_element():
+def test_nonemptyset_discard_existing_element() -> None:
     """Test discarding an existing element from NonEmptySet."""
     non_empty_set = NonEmptySet({1, 2})
     non_empty_set.discard(1)
     assert 1 not in non_empty_set, "Existing element should be discarded from NonEmptySet"
 
 
-def test_nonemptyset_discard_last_element():
+def test_nonemptyset_discard_last_element() -> None:
     """Test discarding the last element from NonEmptySet raises ValueError."""
     non_empty_set = NonEmptySet({1})
     with pytest.raises(ValueError, match="Cannot discard the last element from the set."):
         non_empty_set.discard(1)
 
 
-def test_nonemptyset_pop_element():
+def test_nonemptyset_pop_element() -> None:
     """Test popping an element from NonEmptySet."""
     non_empty_set = NonEmptySet({1, 2})
     popped = non_empty_set.pop()
@@ -65,40 +65,40 @@ def test_nonemptyset_pop_element():
     assert len(non_empty_set) == 1, "NonEmptySet should have one less element after pop"
 
 
-def test_nonemptyset_pop_last_element():
+def test_nonemptyset_pop_last_element() -> None:
     """Test popping the last element from NonEmptySet raises ValueError."""
     non_empty_set = NonEmptySet({1})
     with pytest.raises(ValueError, match="Cannot pop the last element from the set."):
         non_empty_set.pop()
 
 
-def test_nonemptyset_clear():
+def test_nonemptyset_clear() -> None:
     """Test clearing NonEmptySet raises ValueError."""
     non_empty_set = NonEmptySet({1, 2, 3})
     with pytest.raises(ValueError, match="Cannot clear a NonEmptySet."):
         non_empty_set.clear()
 
 
-def test_nonemptyset_convert_set():
+def test_nonemptyset_convert_set() -> None:
     """Test converting a regular set to NonEmptySet."""
     regular_set = {1, 2, 3}
     non_empty_set = NonEmptySet.convert_set(regular_set)
     assert non_empty_set == NonEmptySet({1, 2, 3}), "NonEmptySet should be correctly created from a regular set"
 
 
-def test_nonemptyset_convert_empty_set():
+def test_nonemptyset_convert_empty_set() -> None:
     """Test converting an empty set to NonEmptySet raises ValueError."""
     with pytest.raises(ValueError, match="Cannot create a NonEmptySet from an empty set"):
         NonEmptySet.convert_set(set())
 
 
-def test_nonemptyset_str_representation():
+def test_nonemptyset_str_representation() -> None:
     """Test the string representation of NonEmptySet."""
     non_empty_set = NonEmptySet({1, 2, 3})
     assert str(non_empty_set) == "{1, 2, 3}", "String representation of NonEmptySet should be correct"
 
 
-def test_nonemptyset_repr_representation():
+def test_nonemptyset_repr_representation() -> None:
     """Test the official string representation of NonEmptySet."""
     non_empty_set = NonEmptySet({1, 2})
     assert (