NPE no longer thrown when requesting data for a timestamp not in the list

NoSuchElementException is thrown instead.
Closes #1

src/main/java/edu/unl/cse/soft160/json_connections/connector/OpenWeatherConnector.java

@@ -12,6 +12,7 @@ import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Date;
 import java.util.List;
+import java.util.NoSuchElementException;
 import java.util.Set;
 
 /**
@@ -372,10 +373,10 @@ public class OpenWeatherConnector {
 	 *
 	 * @param listName  the key to access the list
 	 * @param timestamp the timestamp of the list entry
-	 * @return the object of interest, or null if the list or the timestamp are not
-	 *         present
+	 * @return the object of interest, or null if the list is not present
+	 * @throws NoSuchElementException if the timestamp is not present in the list
 	 */
-	private JSONObject getListEntry(String listName, Date timestamp) {
+	private JSONObject getListEntry(String listName, Date timestamp) throws NoSuchElementException {
 		JSONObject listEntry = null;
 		if (data.containsKey(listName)) {
 			JSONArray list = (JSONArray) data.get(listName);
@@ -384,6 +385,9 @@ public class OpenWeatherConnector {
 					listEntry = (JSONObject) entry;
 				}
 			}
+			if (listEntry == null) {
+				throw new NoSuchElementException("The requested data is not available for " + timestamp + ".");
+			}
 		}
 		return listEntry;
 	}