diff --git a/src/StringCalculator.java b/src/StringCalculator.java
index 487916b..71e41b2 100644
--- a/src/StringCalculator.java
+++ b/src/StringCalculator.java
@@ -1,9 +1,59 @@
 
 public class StringCalculator {
-	public int add(String numbersStr) {
-		// Returns the sum of the numbers given in numbersStr
+	
+	static final int[] validNumbers = {0, 1, 2};
+	
+	// Returns the sum of the numbers given in numbersStr
+	public int add(String numbersStr) throws StringCalculatorException {
+		int [] numbers;
+		int result;
 		
-		// not yet implemented
-		return 0;
+		if (numbersStr.length() == 0) {
+			return 0;
+		}
+		
+		numbers = StringArrayToInt(numbersStr.split(",|\\n"));
+		result = calculateNumbers(numbers);
+		
+		return result;
+	}
+	
+	private int[] StringArrayToInt(String[] strNumbers) throws StringCalculatorException {
+		int[] intNumbers = new int[strNumbers.length];
+		int tempInt;
+		for (int i = 0; i < strNumbers.length; i++) {
+			
+			// Try converting part of the string to a integer
+			try {
+				tempInt = Integer.parseInt(strNumbers[i]);
+			}
+			catch(Exception e) {
+				throw (new StringCalculatorException());
+			}
+			
+			validateNumber(tempInt);
+			intNumbers[i] = tempInt;
+			
+		}
+		
+		return intNumbers;
 	}
+	
+	private void validateNumber(int number) throws StringCalculatorException {
+		for (int i: validNumbers) {
+			if (i == number) {
+				return;
+			}
+		}
+		throw (new StringCalculatorException());
+	}
+	
+	private int calculateNumbers(int[] numbers) {
+		int sum = 0;
+		for (int x: numbers) {
+			sum += x;
+		}
+		return sum;
+	}
+
 }
diff --git a/tests/StringCalculatorTest.java b/tests/StringCalculatorTest.java
index 4ec9afe..46c06c9 100644
--- a/tests/StringCalculatorTest.java
+++ b/tests/StringCalculatorTest.java
@@ -4,9 +4,79 @@
 
 public class StringCalculatorTest {
 
+	StringCalculator calc = new StringCalculator();
+
+	@Test
+	public void StringCalculatorTest_EmptyString() throws StringCalculatorException  {
+		int result = calc.add("");
+		assertEquals("Empty string returns wrong value", 0, result);
+	}
+
+	@Test
+	public void StringCalculatorTest_NonEmptyString() throws StringCalculatorException  {
+		int result = calc.add("1,2");
+		assertTrue("Non Empty string returns wrong value", 0 != result);
+	}
+
+	@Test(expected=StringCalculatorException.class)
+	public void StringCalculatorTest_invalid_string() throws StringCalculatorException {
+		int result = calc.add("asdasd");
+	}
+
+	@Test(expected=StringCalculatorException.class)
+	public void StringCalculatorTest_invalid_numbers() throws StringCalculatorException  {
+		int result = calc.add("3,2");
+	}
+	
+	@Test
+	public void StringCalculatorTest_1_2_3() throws StringCalculatorException  {
+		int result = calc.add("1,2");
+		assertTrue("1 + 2 returns a wrong answer " + result, result == 3);
+	}
+	
 	@Test
-	public void test() {
-		fail("Not yet implemented");
+	public void StringCalculatorTest_1_2_2_5() throws StringCalculatorException  {
+		int result = calc.add("1,2,2");
+		assertTrue("1 + 2 + 2 returns a wrong answer " + result, result == 5);
+	}
+	
+	@Test
+	public void StringCalculatorTest_1_0_0_1() throws StringCalculatorException  {
+		int result = calc.add("1,0,0");
+		assertTrue("1 + 0 + 0 returns a wrong answer " + result, result == 1);
+	}
+	
+	@Test
+	public void StringCalculatorTest_0_0_0() throws StringCalculatorException  {
+		int result = calc.add("0,0");
+		assertTrue("0 + 0 returns a wrong answer " + result, result == 0);
+	}
+	
+	@Test
+	public void StringCalculatorTest_0() throws StringCalculatorException  {
+		int result = calc.add("0");
+		assertTrue("0 returns a wrong answer " + result, result == 0);
+	}
+	
+	@Test(expected=StringCalculatorException.class)
+	public void StringCalculatorTest_negative() throws StringCalculatorException  {
+		int result = calc.add("-1,0");
+	}
+	
+	@Test
+	public void StringCalculatorTest_1_1_1_1_1_5() throws StringCalculatorException  {
+		int result = calc.add("1,1,1,1,1");
+		assertTrue("1+1+1+1+1 returns a wrong answer " + result, result == 5);
+	}
+	
+	@Test
+	public void StringCalculatorTest_1_2_3_new_line() throws StringCalculatorException  {
+		int result = calc.add("1\n2");
+		assertTrue("1 + 2 with new line returns a wrong answer " + result, result == 3);
+	}
+	@Test(expected=StringCalculatorException.class)
+	public void StringCalculatorTest_invalid_input_new_line() throws StringCalculatorException  {
+		int result = calc.add("1,\n");
 	}
 
 }