diff --git a/.gitignore b/.gitignore
index cd2946a..3aa82af 100644
--- a/.gitignore
+++ b/.gitignore
@@ -45,3 +45,25 @@ $RECYCLE.BIN/
 Network Trash Folder
 Temporary Items
 .apdisk
+
+*.class
+
+# Mobile Tools for Java (J2ME)
+.mtj.tmp/
+
+# Package Files #
+*.jar
+*.war
+*.ear
+
+# virtual machine crash logs, see http://www.java.com/en/download/help/error_hotspot.xml
+hs_err_pid*
+
+## IDE Specific
+.idea
+.settings
+.project
+.classpath
+
+## Excludes the generated files
+bin/
\ No newline at end of file
diff --git a/src/StringCalculator.java b/src/StringCalculator.java
index 487916b..ab48ed2 100644
--- a/src/StringCalculator.java
+++ b/src/StringCalculator.java
@@ -1,9 +1,66 @@
+import java.util.Arrays;
+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;
+class StringCalculator {
+
+	/**
+	 * add(String numbersStr)
+	 *
+	 * Adds the given numbers together.
+	 *
+	 * @param numbersStr - The numbers in a string
+	 * @return The sum of the numbers
+	 */
+	int add(String numbersStr) throws StringCalculatorException {
+
+		final int[] sum = {0};
+
+		// Check that the string is not null or empty
+		if (isStringNotNullAndNotEmpty(numbersStr)) {
+		    String preparedStr = prepareStringForCalculation(numbersStr); // Prepare the string for calculation
+
+			List<String> numbersList = Arrays.asList(preparedStr.split(",")); // Assign the expected numbers to an array
+            numbersList.forEach((currentItem) -> {
+                if(!currentItem.isEmpty()) {
+                    sum[0] = sum[0] + Integer.parseInt(currentItem);
+                }
+            });
+		}
+
+		return sum[0]; // Return the sum of the items
 	}
+
+    /**
+     * private boolean isStringNotNullAndNotEmpty
+     * Checks if the given String is null or empty. The null check is performed first to ensure no NPE is thrown.
+     *
+     * @param stringToBeChecked - The String that needs to be checked
+     * @return true/false depending on if the String was null/empty or not
+     */
+	private boolean isStringNotNullAndNotEmpty(String stringToBeChecked) {
+	    return (stringToBeChecked != null && !stringToBeChecked.isEmpty());
+    }
+
+    /**
+     * private String prepareStringForCalculation
+     * Prepares the user input for calculation.
+     *
+     * @param originalStr - The original String that needs to be prepared
+     * @return preparedStr - The prepared String
+     * @throws StringCalculatorException - Throw if the regex doesn't match
+     */
+    private String prepareStringForCalculation(String originalStr) throws StringCalculatorException{
+        String regex = "[0-9, /,]+"; //The regex that the check will be performed against
+
+        // Replace whitespace with comma
+        String preparedStr = originalStr.replaceAll("\\s", ",");
+
+        // Abort execution if string contains characters other than numbers [0-9] or comma [/,].
+        if (!preparedStr.matches(regex)) {
+            throw new StringCalculatorException();
+        }
+
+	    return preparedStr;
+    }
+
 }
diff --git a/src/StringCalculatorException.java b/src/StringCalculatorException.java
index da71147..cf231ec 100644
--- a/src/StringCalculatorException.java
+++ b/src/StringCalculatorException.java
@@ -1,4 +1,4 @@
 
-public class StringCalculatorException extends Exception {
+class StringCalculatorException extends Exception {
 
 }
diff --git a/tests/StringCalculatorTest.java b/tests/StringCalculatorTest.java
index 4ec9afe..87fa276 100644
--- a/tests/StringCalculatorTest.java
+++ b/tests/StringCalculatorTest.java
@@ -1,12 +1,101 @@
 import static org.junit.Assert.*;
 
+import org.junit.Before;
 import org.junit.Test;
 
 public class StringCalculatorTest {
 
+	private StringCalculator stringCalculator;
+
+	@Before
+	public void initializeStringCalculator() {
+		stringCalculator = new StringCalculator();
+	}
+
 	@Test
-	public void test() {
-		fail("Not yet implemented");
+	public void testAdd_2NumbersString() throws StringCalculatorException {
+		//Arrange
+		String twoNumberString = "42,42";
+		//Act
+		int returnedNumber = stringCalculator.add(twoNumberString);
+		//Assert
+		assertEquals("Returned number should be equal to the sum of the numbers in the string", 84, returnedNumber);
+	}
+
+	@Test
+	public void testAdd_2NumbersStringWithWhiteSpace() throws StringCalculatorException {
+		//Arrange
+		String twoNumberString = "42, 42";
+		//Act
+		int returnedNumber = stringCalculator.add(twoNumberString);
+		//Assert
+		assertEquals("Returned number should be equal to the sum of the numbers in the string", 84, returnedNumber);
+	}
+
+	@Test
+	public void testAdd_1NumberString() throws StringCalculatorException {
+		//Arrange
+		String singleNumberString = "42";
+		//Act
+		int returnedNumber = stringCalculator.add(singleNumberString);
+		//Assert
+		assertEquals("Returned number should be equal to given number.", Integer.parseInt(singleNumberString), returnedNumber);
+	}
+
+	@Test
+	public void testAdd_NullString() throws StringCalculatorException {
+		//Arrange
+		String nullString = null;
+		//Act
+		int returnedNumber = stringCalculator.add(nullString);
+		//Assert
+		assertEquals("Returned number should be 0", 0, returnedNumber);
+	}
+
+	@Test
+	public void testAdd_EmptyString() throws StringCalculatorException {
+		//Arrange
+		String emptyString = "";
+		//Act
+		int returnedNumber = stringCalculator.add(emptyString);
+		//Assert
+		assertEquals("Returned number should be 0", 0, returnedNumber);
+	}
+
+	@Test
+	public void testAdd_3NumbersStringWithWhitespace() throws StringCalculatorException {
+		//Arrange
+		String threeNumberString = "42,42, 42";
+		//Act
+		int returnedNumber = stringCalculator.add(threeNumberString);
+		//Assert
+		assertEquals("Returned number should be equal to the sum of the numbers in the string", 126, returnedNumber);
+	}
+
+	@Test
+	public void testAdd_stringWithNewLines() throws StringCalculatorException {
+		//Arrange
+		String threeNumberString = "42,42\n42";
+		//Act
+		int returnedNumber = stringCalculator.add(threeNumberString);
+		//Assert
+		assertEquals("Returned number should be equal to the sum of the numbers in the string", 126, returnedNumber);
+	}
+
+	@Test (expected = StringCalculatorException.class)
+	public void testAdd_NegativeNumber() throws StringCalculatorException {
+		//Arrange
+		String negativeNumberString = "-1";
+		//Act
+		int returnedNumber = stringCalculator.add(negativeNumberString);
+	}
+
+	@Test (expected = StringCalculatorException.class)
+	public void testAdd_InvalidInput() throws StringCalculatorException {
+		//Arrange
+		String invalidNumberString = "Hello there!";
+		//Act
+		int returnedNumber = stringCalculator.add(invalidNumberString);
 	}
 
 }