diff --git a/bin/StringCalculator.class b/bin/StringCalculator.class
index a937b0c..bf98942 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..942de30 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..4623b01 100644
--- a/src/StringCalculator.java
+++ b/src/StringCalculator.java
@@ -1,9 +1,34 @@
 
 public class StringCalculator {
-	public int add(String numbersStr) {
-		// Returns the sum of the numbers given in numbersStr
+	public int add(String numbersStr) throws StringCalculatorException {
 		
-		// not yet implemented
-		return 0;
+		if (numbersStr.length() == 0) {
+			return 0;
+		}
+		
+		int sum = 0;
+		int index = 0;
+				
+		while (index < numbersStr.length()) {
+			
+			char character  = numbersStr.charAt(index);
+				
+			if (character != ',' && character != '\n') {
+				if (Character.isDigit(character) == false) {
+					throw new StringCalculatorException();
+				} else {
+					sum+= Character.getNumericValue(character);
+				}
+			} else {
+				if ((index + 1) < numbersStr.length() &&
+					Character.isDigit(numbersStr.charAt(index + 1)) == false) {
+					throw new StringCalculatorException();
+				} 
+			}
+								
+			index++;
+		}
+		
+		return sum;
 	}
 }
diff --git a/tests/StringCalculatorTest.java b/tests/StringCalculatorTest.java
index 4ec9afe..568adc4 100644
--- a/tests/StringCalculatorTest.java
+++ b/tests/StringCalculatorTest.java
@@ -5,8 +5,55 @@
 public class StringCalculatorTest {
 
 	@Test
-	public void test() {
-		fail("Not yet implemented");
+	public void testWith0And2And2NumbersSeparatedWithComma() throws StringCalculatorException {
+		StringCalculator calculator = new StringCalculator();
+		
+		assertEquals(0, calculator.add(""));
+		assertEquals(1, calculator.add("1"));
+		assertEquals(3, calculator.add("1,2"));
 	}
+	
+	@Test
+	public void testWithUnknownAmountOfNumbersSeparatedWithComma() throws StringCalculatorException {
+		StringCalculator calculator = new StringCalculator();
+		
+		assertEquals(12, calculator.add("4,7,1"));
+		assertEquals(17, calculator.add("4,7,1,5"));
+		assertEquals(23, calculator.add("1,2,3,8,9"));
+		
+	}
+		
+	@Test
+	public void testWithNewLinesBetweenNumbers() throws StringCalculatorException {
+		StringCalculator calculator = new StringCalculator();
+		
+		assertEquals(14, calculator.add("3\n5,6"));
+		assertEquals(21, calculator.add("3,5\n6,7"));
+		assertEquals(24, calculator.add("3,6,7\n8"));
+	}
+	
+	@Test (expected = StringCalculatorException.class)
+	public void testWithNegativeNumbers() throws StringCalculatorException {
+		StringCalculator calculator = new StringCalculator();
+		
+		//Negative numbers
+		calculator.add("-1,-5");
+	}
+
+	@Test (expected = StringCalculatorException.class)
+	public void testWithInvalidInput1() throws StringCalculatorException {
+		StringCalculator calculator = new StringCalculator();
+	
+		//Invalid input
+		calculator.add("1,\n");
 
+	}
+	
+	@Test (expected = StringCalculatorException.class)
+	public void testWithInvalidInput2() throws StringCalculatorException {
+		StringCalculator calculator = new StringCalculator();
+	
+		//Invalid input
+		calculator.add(",,,,,1");
+	}
 }