diff --git a/bin/StringCalculator.class b/bin/StringCalculator.class
index a937b0c..081e8fe 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..e574231 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..3615705 100644
--- a/src/StringCalculator.java
+++ b/src/StringCalculator.java
@@ -1,9 +1,48 @@
+import java.util.Arrays;
+import java.util.List;
+import java.util.Scanner;
 
 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;
+		int size = numOfnums(numbersStr);
+		String corStr = replaceNewlines(numbersStr);
+		checkInput(corStr);
+		Scanner s = new Scanner(corStr).useDelimiter(",");
+		//ArrayList<Integer> arr = new ArrayList<Integer>();
+
+		int j = 0;
+		int x = 0;
+		if (size==0) return 0;
+		else {
+			while (size > j) {
+				x += s.nextInt();
+				j++;
+			}
+			return x;
+		}
+
+	}
+	
+	public int numOfnums(String numberStr) {
+		if (numberStr.length()==0) return 0;
+		String commaStr = replaceNewlines(numberStr);
+		List<String> splittedStr = Arrays.asList(commaStr.split(","));
+		int size = splittedStr.size();
+		return size;
+	}
+	
+	public String replaceNewlines(String str) {
+		String commaStr = str.replace("\n", ",");
+		return commaStr;
+	}
+	
+	public void checkInput(String str) throws StringCalculatorException{
+		String input = replaceNewlines(str);
+		if (input.length()>0 && !input.matches(".*\\d+.*")) throw new StringCalculatorException();
+		if (input.length()>1 && !input.matches("\\d.*\\d")) throw new StringCalculatorException();
+		if (input.matches(".*[a-z].*")) throw new StringCalculatorException();
+		if (input.contains("-")) throw new StringCalculatorException();
+		if (input.contains(" ")) throw new StringCalculatorException();
 	}
 }
diff --git a/tests/StringCalculatorTest.java b/tests/StringCalculatorTest.java
index 4ec9afe..e7c57b2 100644
--- a/tests/StringCalculatorTest.java
+++ b/tests/StringCalculatorTest.java
@@ -1,12 +1,85 @@
 import static org.junit.Assert.*;
 
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
 import org.junit.Test;
 
 public class StringCalculatorTest {
 
 	@Test
-	public void test() {
-		fail("Not yet implemented");
+	public void testNumOfnums() {
+		StringCalculator strC = new StringCalculator();
+		
+		String testStr = "";
+		int testSize = strC.numOfnums(testStr);
+		assertEquals(testSize, 0);
+		
+		String testStr2 = "1,2";
+		int testSize2 = strC.numOfnums(testStr2);
+		assertEquals(testSize2, 2);
+	}
+	
+	@Test
+	public void testReplaceNL() {
+		StringCalculator strC = new StringCalculator();
+		
+		String testStr = "1\n2";
+		String testStr2 = "1,2";
+		String testStr3 = "1";
+		
+		String newStr = strC.replaceNewlines(testStr);
+		String newStr2 = strC.replaceNewlines(testStr2);
+		String newStr3 = strC.replaceNewlines(testStr3);
+		
+		assertEquals(newStr, "1,2");
+		assertEquals(newStr2, "1,2");
+		assertEquals(newStr3, "1");
+	}
+	
+	@Test
+	public void testAdd() throws StringCalculatorException {
+		StringCalculator strC = new StringCalculator();
+		
+		String testStr1 = "1,2";
+		int result1 = strC.add(testStr1);
+		assertEquals(result1, 3);
+		
+		String testStr2 = "2\n2";
+		int result2 = strC.add(testStr2);
+		assertEquals(result2, 4);
+		
+		String testStr3 = "1";
+		int result3 = strC.add(testStr3);
+		assertEquals(result3, 1);
+		
+		String testStr4 = "";
+		int result4 = strC.add(testStr4);
+		assertEquals(result4, 0);
+	}
+	
+	@Test(expected = StringCalculatorException.class)
+	public void testCheckInput() throws StringCalculatorException {
+		StringCalculator strC = new StringCalculator();
+		
+		String testStr1 = "-1,2";
+		strC.checkInput(testStr1);
+		
+		String testStr2 = "///";
+		strC.checkInput(testStr2);
+		
+		String testStr3 = "1\na";
+		strC.checkInput(testStr3);
+		
+		String testStr4 = "abc";
+		strC.checkInput(testStr4);
+		
+		String testStr5 = "1, 2";
+		strC.checkInput(testStr5);
+		
+		String testStr6 = "1,,";
+		strC.checkInput(testStr6);
 	}
 
 }