added negative test case

.idea/vcs.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>
\ No newline at end of file

src/main/java/com/nisum/ZigZagConversionAlgo.java

@@ -13,6 +13,7 @@ public class ZigZagConversionAlgo {
     }
 
     public static String convertString(String str, int rows){
+        if (rows < 1) return "Row integer out of bounds.";
         resultString = "";
         listOfSubstrings = populateArrayList(rows);
         distributeLetters(str,rows);

src/test/java/com/nisum/ZigZagConversionAlgoTest.java

@@ -16,14 +16,15 @@ public class ZigZagConversionAlgoTest {
         Assert.assertEquals("result 3 incorrect", res3, "A");
     }
 
-//    @Test
-//    public void handlesOutOfBounds(){
-//        String res1 = ZigZagConversionAlgo.convertString("PAYPALISHIRING", -1);
-//
-//        Assert.assertFalse(res1.getClass() == "String");
-//        Assert.assertTrue;
-//
-//    }
+
+    @Test
+    public void handlesOutOfBounds(){
+        String res1 = ZigZagConversionAlgo.convertString("PAYPALISHIRING", -1);
+
+        Assert.assertFalse(res1 == "PAYPALISHIRING");
+        Assert.assertTrue(res1 == "Row integer out of bounds.");
+
+    }
 
 }
 /*