diff --git a/bin/StringCalculator.class b/bin/StringCalculator.class
index a937b0c..2879614 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..053abfb 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..1d4c076 100644
--- a/src/StringCalculator.java
+++ b/src/StringCalculator.java
@@ -1,9 +1,31 @@
 
 public class StringCalculator {
-	public int add(String numbersStr) {
-		// Returns the sum of the numbers given in numbersStr
+	public int add(String numbersStr) throws StringCalculatorException {
+		int total = 0;
+		int list = 0;	
 		
-		// not yet implemented
-		return 0;
+		if (numbersStr.length()==0) {
+			return 0;
+		}
+		else {
+	
+			while (list < numbersStr.length()) {				
+				char character  = numbersStr.charAt(list);
+				if (character !=',' && character !='\n') {
+					if (Character.isDigit(character) == true) {
+						total+= Character.getNumericValue(character);
+					} else {
+						throw new StringCalculatorException();
+					}
+				} else {
+					if (numbersStr.length() > (list + 1) &&
+						Character.isDigit(numbersStr.charAt(list + 1)) == false) {
+						throw new StringCalculatorException();
+					} 
+				}									
+				list++;
+			}
+			return total;
+		}
 	}
 }
diff --git a/tests/StringCalculatorTest.java b/tests/StringCalculatorTest.java
index 4ec9afe..e9b852c 100644
--- a/tests/StringCalculatorTest.java
+++ b/tests/StringCalculatorTest.java
@@ -3,10 +3,59 @@
 import org.junit.Test;
 
 public class StringCalculatorTest {
-
+	
+	@Test
+	public void testOneNumber() throws StringCalculatorException{
+		StringCalculator calculator = new StringCalculator();
+		// Assert
+		assertEquals(1,calculator.add("1"));	
+	}
+	
 	@Test
-	public void test() {
-		fail("Not yet implemented");
+	public void testTwoNumbers() throws StringCalculatorException{
+		StringCalculator calculator = new StringCalculator();
+		// Assert
+		assertEquals(3,calculator.add("1,2"));
+		assertEquals(6,calculator.add("4,2"));
+		assertEquals(6,calculator.add("4\n2"));
+
 	}
+	
+	@Test
+	public void testManyNumbers2() throws StringCalculatorException{
+		StringCalculator calculator = new StringCalculator();
+		// Assert
+		assertEquals(23,calculator.add("4,5,6,8"));	
 
-}
+	}
+	
+	@Test
+	public void testZero() throws StringCalculatorException{
+		StringCalculator calculator = new StringCalculator();
+		// Assert
+		assertEquals(0, calculator.add(""));
+	}
+	@Test
+	public void testNewLine() throws StringCalculatorException {
+		StringCalculator calculator = new StringCalculator();
+		
+		assertEquals(6, calculator.add("1\n2,3"));
+		assertEquals(22, calculator.add("4,5\n6,7"));
+		assertEquals(18, calculator.add("1,8\n9"));
+	}
+	
+	@Test (expected = StringCalculatorException.class)
+	public void testWrongValues() throws StringCalculatorException{
+		StringCalculator calculator = new StringCalculator();
+		calculator.add("1234");
+		calculator.add(",,,,,1");
+		calculator.add("1,");
+		calculator.add("t");
+	}
+	@Test (expected = StringCalculatorException.class)
+	public void testhNegativeNumbers() throws StringCalculatorException {
+		StringCalculator calculator = new StringCalculator();
+		
+		calculator.add("-3,-4");
+	}
+}
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