diff --git a/bin/StringCalculator.class b/bin/StringCalculator.class
index a937b0c..d24395c 100644
Binary files a/bin/StringCalculator.class and b/bin/StringCalculator.class differ
diff --git a/bin/StringCalculatorTest.class b/bin/StringCalculatorTest.class
index d44b83f..cdc59ec 100644
Binary files a/bin/StringCalculatorTest.class and b/bin/StringCalculatorTest.class differ
diff --git a/src/StringCalculator.java b/src/StringCalculator.java
index 487916b..4f0f4e0 100644
--- a/src/StringCalculator.java
+++ b/src/StringCalculator.java
@@ -1,9 +1,27 @@
+import java.util.ArrayList;
+import java.util.List;
 
 public class StringCalculator {
-	public int add(String numbersStr) {
-		// Returns the sum of the numbers given in numbersStr
-		
-		// not yet implemented
-		return 0;
+	
+	public static int add(final String numbers) {
+	    int returnValue = 0;
+	    String[] numbersArray = numbers.split(",|n");;
+	    List negativeNumbers = new ArrayList();
+	    for (String number : numbersArray) {
+	        if (!number.trim().isEmpty()) {
+	            int numberInt = Integer.parseInt(number.trim());
+	            if (numberInt < 0) {
+	                negativeNumbers.add(numberInt);
+	            }
+	            returnValue += numberInt;
+	        }
+	    }
+	    if (negativeNumbers.size() > 0) {
+	        throw new RuntimeException("Negative Numbers are not allowed: " + negativeNumbers.toString());
+	    }
+	    return returnValue;     
 	}
 }
+
+
+
diff --git a/tests/StringCalculatorTest.java b/tests/StringCalculatorTest.java
index 4ec9afe..4b36623 100644
--- a/tests/StringCalculatorTest.java
+++ b/tests/StringCalculatorTest.java
@@ -1,12 +1,71 @@
 import static org.junit.Assert.*;
 
+import org.junit.Assert;
 import org.junit.Test;
 
 public class StringCalculatorTest {
-
-	@Test
-	public void test() {
-		fail("Not yet implemented");
-	}
-
+	//when more than 2 numbers are used (Failure case) 
+//    @Test(expected = RuntimeException.class)
+//    public final void whenMoreThanTwoNumbersAreUsedThenThrowAnException() {
+//        StringCalculator.add("9,8,7");
+//    }
+    //when 2 numbers are input (Success case)
+    @Test
+    public final void whenTwoNumbersAreInputThenNoExceptionIsThrown() {
+        StringCalculator.add("4,10");
+        Assert.assertTrue(true);
+    }
+    //when non number is input (Failure case)
+    @Test(expected = RuntimeException.class)
+    public final void whenNonNumberIsInputThenThrowAnException() {
+        StringCalculator.add("6,a");
+    }
+    //when empty string is input (Success case)
+    @Test
+    public final void whenEmptyStringInputIsUsedThenReturnOutputIsZero() {
+        Assert.assertEquals(0, StringCalculator.add(""));
+    } 
+   
+    //when one number is used as input then output is that same number
+    @Test
+    public final void whenOneNumberInputIsUsedThenReturnedOutputIsThatSameNumber() {
+        Assert.assertEquals(1, StringCalculator.add("1"));
+    }
+    //when two numbers are input then output is their sum
+    @Test
+    public final void whenTwoNumbersAreInputThenOutputIsTheirSum() {
+        Assert.assertEquals(5+2, StringCalculator.add("5,2"));
+    }
+    
+    //when more than 2 numbers are used (Failure case)
+    //when multiple number of inputs are used then output is their sum 
+    @Test
+    public final void whenMultipleNumberOfInputsAreUsedThenOutputIsTheirSum() {
+        Assert.assertEquals(10+20+3+40+50, StringCalculator.add("10,20,3,40,50"));
+    }
+    
+    //when new line is used between input numbers then output is their sum 
+    @Test
+    public final void whenNewLineIsUsedBetweenInputNumbersThenOutputIsTheirSum() {
+        Assert.assertEquals(4+1+22, StringCalculator.add("4,1n22"));
+    }
+    //when negative number is in input (Failure case)
+    @Test(expected = RuntimeException.class)
+    public final void whenNegativeNumberIsInInputThenThrowRuntimeException() {
+        StringCalculator.add("10,-20,3,40,50");
+    }
+    //when multiple negative numbers are in input (Failure case)
+    @Test
+    public final void whenMultipleNegativeNumbersAreInInputThenThrowRuntimeException() {
+        RuntimeException exception = null;
+        try {
+            StringCalculator.add("10,-20,3,-40,50");
+        } catch (RuntimeException e) {
+            exception = e;
+        }
+        Assert.assertNotNull(exception);
+        Assert.assertEquals("Negative Numbers are not allowed: [-20, -40]", exception.getMessage());
+    }
 }
+
+