diff --git a/bin/StringCalculator.class b/bin/StringCalculator.class
index a937b0c..2401ef7 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..05f5a70 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..d3643ec 100644
--- a/src/StringCalculator.java
+++ b/src/StringCalculator.java
@@ -1,9 +1,56 @@
 
 public class StringCalculator {
+	
+	
 	public int add(String numbersStr) {
-		// Returns the sum of the numbers given in numbersStr
 		
-		// not yet implemented
-		return 0;
+		boolean lengthCheck = checkLength(numbersStr);
+		
+		int sum = 0;
+		if (lengthCheck == true) {
+			boolean inputCheck = checkInput(numbersStr);
+			if (inputCheck == true) {
+				String[] strArray = numbersStr.split(",|\\\n");
+				int[] intArray = new int[strArray.length];
+				for(int i = 0; i < strArray.length; i++) {
+				    intArray[i] = Integer.parseInt(strArray[i]);
+				}
+				
+				for (int j = 0; j < strArray.length; j++ ) {
+					sum = sum + intArray[j];
+				}
+			}
+		} 
+		
+		return sum;
+	}
+	
+	public boolean checkLength (String numbersStr) {
+		int stringLength = numbersStr.length();
+		
+		if (stringLength <= 0) {
+			return false;
+		} else {
+			String[] strArray = numbersStr.split(",|\\\n");
+			int[] intArray = new int[strArray.length];
+			for(int i = 0; i < strArray.length; i++) {
+				if (i > 1) {
+					return false;
+				}
+			    intArray[i] = Integer.parseInt(strArray[i]);
+			}
+		}
+		return true;
+	}
+	
+	public boolean checkInput (String numbersStr) {
+		String[] strArray = numbersStr.split(",|\\\n");
+		
+		if (numbersStr.matches("^[0-9,\n]+$")) {
+			return true;
+		} 
+		
+		return false;
 	}
+	
 }
diff --git a/tests/StringCalculatorTest.java b/tests/StringCalculatorTest.java
index 4ec9afe..df9b511 100644
--- a/tests/StringCalculatorTest.java
+++ b/tests/StringCalculatorTest.java
@@ -4,9 +4,94 @@
 
 public class StringCalculatorTest {
 
+	//comment
 	@Test
-	public void test() {
-		fail("Not yet implemented");
+	public void testStringEmpty() {
+		
+		StringCalculator calculator = new StringCalculator();
+		String numbersStr = "";
+		
+		assertEquals(false , calculator.checkLength(numbersStr));
+	}
+	
+	@Test
+	public void testStringTooLong() {
+		
+		StringCalculator calculator = new StringCalculator();
+		String numbersStr = "1,2,3";
+		
+		assertEquals(false , calculator.checkLength(numbersStr));
+	}
+	
+	@Test
+	public void testStringLength() {
+		
+		StringCalculator calculator = new StringCalculator();
+		String numbersStr = "11,22";
+		
+		assertEquals(true , calculator.checkLength(numbersStr));
+	}
+	
+	@Test
+	public void testSumComma() {
+		
+		StringCalculator calculator = new StringCalculator();
+		String numbersStr = "1,2";
+		
+		assertEquals(3 , calculator.add(numbersStr));
+	}
+	
+	@Test
+	public void testSumNewLine() {
+		
+		StringCalculator calculator = new StringCalculator();
+		String numbersStr = "1\n2";
+		
+		assertEquals(3 , calculator.add(numbersStr));
 	}
 
+	@Test
+	public void testLetters() {
+		
+		StringCalculator calculator = new StringCalculator();
+		String numbersStr = "abc";
+		
+		assertEquals(false , calculator.checkInput(numbersStr));
+	}
+	
+	@Test
+	public void testLetters2() {
+		
+		StringCalculator calculator = new StringCalculator();
+		String numbersStr = "n12";
+		
+		assertEquals(false , calculator.checkInput(numbersStr));
+	}
+	
+	@Test
+	public void testSumEmpty() {
+		
+		StringCalculator calculator = new StringCalculator();
+		String numbersStr = "";
+		
+		assertEquals(0 , calculator.add(numbersStr));
+	}
+	
+	@Test
+	public void testSum() {
+		
+		StringCalculator calculator = new StringCalculator();
+		String numbersStr = "11,22";
+		
+		assertEquals(33 , calculator.add(numbersStr));
+	}
+	
+	@Test
+	public void testOneNumber() {
+		
+		StringCalculator calculator = new StringCalculator();
+		String numbersStr = "10";
+		
+		assertEquals(10 , calculator.add(numbersStr));
+	}
 }