diff --git a/bin/StringCalculator.class b/bin/StringCalculator.class
index a937b0c..aae354f 100644
Binary files a/bin/StringCalculator.class and b/bin/StringCalculator.class differ
diff --git a/bin/StringCalculatorException.class b/bin/StringCalculatorException.class
index 4bd9894..b48e05b 100644
Binary files a/bin/StringCalculatorException.class and b/bin/StringCalculatorException.class differ
diff --git a/bin/StringCalculatorTest.class b/bin/StringCalculatorTest.class
index d44b83f..691b08f 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..549d2a4 100644
--- a/src/StringCalculator.java
+++ b/src/StringCalculator.java
@@ -1,9 +1,153 @@
-
 public class StringCalculator {
+	private StringCalculatorException stringCalculatorException;
+
 	public int add(String numbersStr) {
 		// Returns the sum of the numbers given in numbersStr
+		boolean correctStr = true;
+		int sum = 0, sum1 = 0, sum2 = 0;
+		int sum11 = 0, sum12 = 0, sum21 = 0, sum22=0;
 		
-		// not yet implemented
-		return 0;
+		String printStr = numbersStr.replaceAll("\n","\\\\n");
+		System.out.println("The input string is: \"" + printStr + "\"");		
+		
+		if (numbersStr =="")
+			sum = 0;
+		else
+		{
+			int count = numbersStr.length() - numbersStr.replaceAll(",","").length();
+			//check how many "," in the numberStr
+			switch (count) {
+			case 0:  //one number
+				try {
+					  sum = Integer.parseInt(numbersStr);
+				} catch (NumberFormatException e) {
+					correctStr = false;
+					stringCalculatorException = new StringCalculatorException("invalid input (not numbers): " + numbersStr);
+				}
+				if (sum < 0) {
+					correctStr = false;
+					stringCalculatorException = new StringCalculatorException("negative numbers" + numbersStr);
+				}
+				break;
+			case 1:  //two or more numbers
+				String[] parts = numbersStr.split(",");
+				String firstnum = parts[0]; 
+				String secondnum = parts[1]; 
+				if (firstnum.equals( "\n") || secondnum.equals( "\n")) {
+					correctStr = false;
+					stringCalculatorException = new StringCalculatorException("invalid input: \\n");
+				} else
+				{
+					int count1 = firstnum.length() - firstnum.replaceAll("\n","").length();
+					//check how many "\n" in the numberStr
+					switch (count1) {
+					case 0:  //one number
+						try {
+							  sum1 = Integer.parseInt(firstnum);
+						} catch (NumberFormatException e) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("invalid input (not numbers): " + firstnum);
+						}
+						if (sum1 < 0) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("negative numbers: "+ firstnum);
+						}
+						break;
+					case 1:  	
+						String[] twoparts = firstnum.split("\n");
+						String firstpart = twoparts[0]; 
+						String secondpart = twoparts[1]; 
+						try {
+							  sum11 = Integer.parseInt(firstpart);
+						} catch (NumberFormatException e) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("invalid input (not numbers): " + firstpart);
+						}
+						if (sum11 < 0) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("negative numbers: "+ firstpart);
+						}
+						try {
+							  sum12 = Integer.parseInt(secondpart);
+						} catch (NumberFormatException e) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("invalid input (not numbers): " + secondpart);
+						}
+						if (sum12 < 0) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("negative numbers:" + secondpart);
+						}
+						sum1 = sum11 + sum12;
+						break;
+					default:   
+						correctStr = false;
+						stringCalculatorException = new StringCalculatorException("invalid input: too many \\n");
+		          		break;
+					}	
+					int count2 = secondnum.length() - secondnum.replaceAll("\n","").length();
+					//check how many "\n" in the numberStr
+					switch (count2) {
+					case 0:  //one number
+						try {
+							  sum2 = Integer.parseInt(secondnum);
+						} catch (NumberFormatException e) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("invalid input (not numbers): "+ secondnum);
+						}
+						if (sum2 < 0) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("negative numbers: " + secondnum);
+						}
+						break;
+					case 1:  	
+						String[] twoparts = secondnum.split("\n");
+						String firstpart = twoparts[0]; 
+						String secondpart = twoparts[1]; 
+						try {
+							  sum21 = Integer.parseInt(firstpart);
+						} catch (NumberFormatException e) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("invalid input (not numbers): " + firstpart);
+						}
+						if (sum21 < 0) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("negative numbers: " + firstpart);
+						}
+						try {
+							  sum22 = Integer.parseInt(secondpart);
+						} catch (NumberFormatException e) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("invalid input (not numbers): " + secondpart);
+						}
+						if (sum22 < 0) {
+							correctStr = false;
+							stringCalculatorException = new StringCalculatorException("negative numbers: "+ secondpart);
+						}
+						sum2 = sum21 + sum22;
+						break;
+					default:   
+						correctStr = false;
+						stringCalculatorException = new StringCalculatorException("invalid input: too many \\n");
+		          		break;
+					}
+					sum = sum1 + sum2;
+				}
+                break;
+			default:   
+				correctStr = false;
+				stringCalculatorException = new StringCalculatorException("invalid input: too many ,");
+          		break;
+			
+			}
+		  }
+		  
+		  if (correctStr) {
+				System.out.println("The sum of the numbers is: " + String.valueOf(sum));	
+				System.out.println();		
+		  }  
+		  
+		  return 0;
 	}
+	
+
 }
diff --git a/src/StringCalculatorException.java b/src/StringCalculatorException.java
index da71147..ed567a0 100644
--- a/src/StringCalculatorException.java
+++ b/src/StringCalculatorException.java
@@ -1,4 +1,8 @@
 
 public class StringCalculatorException extends Exception {
 
-}
+	public StringCalculatorException(String e) {
+		System.out.println(e);
+	}
+
+}
\ No newline at end of file
diff --git a/tests/StringCalculatorTest.java b/tests/StringCalculatorTest.java
index 4ec9afe..5e42da0 100644
--- a/tests/StringCalculatorTest.java
+++ b/tests/StringCalculatorTest.java
@@ -6,7 +6,43 @@ public class StringCalculatorTest {
 
 	@Test
 	public void test() {
-		fail("Not yet implemented");
+		StringCalculator cal = new StringCalculator();
+		cal.add("");	
+		cal.add("2");		
+		cal.add("-2");		
+		cal.add("a");		
+		cal.add("1,2,3");		
+		cal.add("1,\n");	
+		cal.add("\n,1");	
+		cal.add("-1,a");
+		cal.add("a,-1");
+		
+		cal.add("1\n2,3");
+		cal.add("1\n2,3\n4");
+		cal.add("1,3\n4");
+		cal.add("1,2");
+
+		cal.add("a,2");
+		cal.add("-1,2");
+		cal.add("1,a");
+		cal.add("1,-2");
+				
+		cal.add("a\n2,3");		
+		cal.add("-1\n2,3");
+		cal.add("1\na,3");
+		cal.add("1\n-2,3");
+		cal.add("1,a\n4");
+		cal.add("1,-3\n4");
+		cal.add("1,3\na");
+		cal.add("1,3\n-4");
+
+		cal.add("1\n2,a");
+		cal.add("1\n2,-2");
+		cal.add("a,3\n4");
+		cal.add("-1,3\n4");
+		cal.add("a\nb,c\nd");
+		cal.add("-1\n-2,-3\n-4");
+		cal.add("-1\n\n,-3\n-4");
 	}
 
 }