User parameterized with ignoring case, fix bug.

Author: designatednerd

AndroidListenerExamples/src/androidTest/java/com/designatednerd/androidlistenerexamples/test/ParameterTestExample.java

@@ -79,4 +79,10 @@ public void findCommonLetters() {
         String foundCommonLetters = mLetterFinder.commonLetters(mFirstString, mSecondString);
         assertEquals(mExpectedCommonLetters, foundCommonLetters);
     }
+
+    @Test
+    public void findCommonLettersIgnoringCase() {
+        String foundCommonLetters = mLetterFinder.commonLettersCaseInsensitive(mFirstString, mSecondString);
+        assertEquals(mExpectedCommonLetters, foundCommonLetters);
+    }
 }

AndroidListenerExamples/src/main/java/com/designatednerd/androidlistenerexamples/model/CommonLetterFinder.java

@@ -31,6 +31,10 @@ public String commonLetters(String aFirstString, String aSecondString) {
     }
 
     public String commonLettersCaseInsensitive(String aFirstString, String aSecondString) {
-        return commonLetters(aFirstString.toLowerCase(), aSecondString.toLowerCase());
+        if (aFirstString != null && aSecondString != null) {
+            return commonLetters(aFirstString.toLowerCase(), aSecondString.toLowerCase());
+        } else {
+            return null;
+        }
     }
 }