diff --git a/src/__pycache__/interfaces.cpython-310.pyc b/src/__pycache__/interfaces.cpython-310.pyc
index 9c2c27ce8dbc3a9cbbc432aef7c2022428ab19de..d7953f25e6f8b9e6f58dfc3cdb55b061b176f472 100644
Binary files a/src/__pycache__/interfaces.cpython-310.pyc and b/src/__pycache__/interfaces.cpython-310.pyc differ
diff --git a/src/__pycache__/stringcalculator.cpython-310.pyc b/src/__pycache__/stringcalculator.cpython-310.pyc
new file mode 100644
index 0000000000000000000000000000000000000000..938708f3482e2cedde5ca28d3caf0cfceea2ad7f
Binary files /dev/null and b/src/__pycache__/stringcalculator.cpython-310.pyc differ
diff --git a/src/interfaces.py b/src/interfaces.py
index bc8c8c3ef35efbce40861f4feeedc7c44c0a04dd..7a3cc8e1962344df1a13d7895841f695451edb06 100644
--- a/src/interfaces.py
+++ b/src/interfaces.py
@@ -3,4 +3,9 @@ from abc import ABC, abstractmethod
 class IRomanNumber(ABC):
     @abstractmethod
     def convert(self, n: int) -> str:
+        pass
+
+class IStringCalculator(ABC):
+    @abstractmethod
+    def add(self, numbers: str) -> int:
         pass
\ No newline at end of file
diff --git a/src/stringcalculator.py b/src/stringcalculator.py
index e69de29bb2d1d6434b8b29ae775ad8c2e48c5391..41f666fdc4d0e39d619a00d678088cdbfca15376 100644
--- a/src/stringcalculator.py
+++ b/src/stringcalculator.py
@@ -0,0 +1,7 @@
+from src.interfaces import IStringCalculator
+
+class StringCalculator(IStringCalculator):
+    def add(self, numbers: str) -> int:
+        if not numbers:
+            return 0
+        return sum(map(int, numbers.split(',')))
\ No newline at end of file
diff --git a/tests/__pycache__/test_romannumerals.cpython-310.pyc b/tests/__pycache__/test_romannumerals.cpython-310.pyc
index 2ef85d1aa0aaa5d708fc349e4ed24856b6f600f4..1809b2b966feaa1479662587eca1207db19195ad 100644
Binary files a/tests/__pycache__/test_romannumerals.cpython-310.pyc and b/tests/__pycache__/test_romannumerals.cpython-310.pyc differ
diff --git a/tests/__pycache__/test_stringcalculator.cpython-310.pyc b/tests/__pycache__/test_stringcalculator.cpython-310.pyc
new file mode 100644
index 0000000000000000000000000000000000000000..31a224455e3a18f535acc76eafba13b427a27f89
Binary files /dev/null and b/tests/__pycache__/test_stringcalculator.cpython-310.pyc differ
diff --git a/tests/test_romannumerals.py b/tests/test_romannumerals.py
index abc5266bfc1e67c1c6007e637d7a5648b218e114..35e2af35f754d89a2da55ab9537741a29fd40960 100644
--- a/tests/test_romannumerals.py
+++ b/tests/test_romannumerals.py
@@ -97,4 +97,5 @@ class TestRomanNumber(unittest.TestCase):
 if __name__ == "__main__":
     unittest.main()
 
-#python -m unittest discover -s tests
\ No newline at end of file
+# python -m unittest tests.test_romannumerals
+# python -m unittest discover -s tests
\ No newline at end of file
diff --git a/tests/test_stringcalculator.py b/tests/test_stringcalculator.py
index b8e4935f3a5abe52c006b13965c8f9abf2433d46..f7c4c1221c4d42297357b1fccd3569a34afc3a21 100644
--- a/tests/test_stringcalculator.py
+++ b/tests/test_stringcalculator.py
@@ -2,4 +2,27 @@
 # Bei Eingabe keiner Zahl soll eine 0 ausgegeben werden
 # Bei Eingabe einer 1 soll 1 ausgegeben werden
 # Bei Eingabe einer 3,4 soll 7 ausgegeben werden
-# Bei Eingabe einer 10,20 soll 30 ausgegeben werden
\ No newline at end of file
+# Bei Eingabe einer 10,20 soll 30 ausgegeben werden
+
+import unittest
+from src.interfaces import IStringCalculator
+from src.stringcalculator import StringCalculator
+
+class TestStringCalculator(unittest.TestCase):
+    def setUp(self):
+        self.calculator: IStringCalculator = StringCalculator()  # Zugriff über das Interface
+
+    def test_add_empty_string(self):
+        self.assertEqual(self.calculator.add(""), 0)
+
+    def test_add_single_number(self):
+        self.assertEqual(self.calculator.add("1"), 1)
+
+    def test_add_two_numbers(self):
+        self.assertEqual(self.calculator.add("10,20"), 30)
+
+if __name__ == "__main__":
+    unittest.main()
+
+# python -m unittest tests.test_stringcalculator
+# python -m unittest discover -s tests
\ No newline at end of file