[REEF-781] Fix NPE on receiving filename query without arguments

This PR addresses the followings issues in HttpServerReefEventHandler:
* Fix NullPointerException on `filename` query without arguments
* Use typed list to avoid dynamic type-casting
* Remove mixed use of both `getWriter()` and `getOutputStream()'

JIRA:
  [REEF-781](https://issues.apache.org/jira/browse/REEF-781)

Pull Request:
  Closes apache#520

lang/java/reef-webserver/src/main/java/org/apache/reef/webserver/HttpServerReefEventHandler.java

@@ -29,6 +29,7 @@
 import org.apache.reef.util.logging.LoggingScopeFactory;
 import org.apache.reef.util.logging.LoggingScopeImpl;
 import org.apache.reef.wake.EventHandler;
+
 import javax.inject.Inject;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServletResponse;
@@ -169,20 +170,22 @@ public void onHttpRequest(
       writeLines(response, result, "Current Stages...");
       break;
     case "logfile":
-      final List names = parsedHttpRequest.getQueryMap().get("filename");
+      final List<String> names = parsedHttpRequest.getQueryMap().get("filename");
+      final PrintWriter writer = response.getWriter();
       if (names == null || names.size() == 0) {
-        response.getWriter().println(String.format("File name is not provided"));
-      }
-
-      final String fileName = (String)names.get(0);
-      if (!fileName.equals(driverStdoutFile) && !fileName.equals(driverStderrFile)) {
-        response.getWriter().println(String.format("Unsupported file names: [%s] ", fileName));
-      }
-      try {
-        final byte[] outputBody = readFile((String) names.get(0)).getBytes(StandardCharsets.UTF_8);
-        response.getOutputStream().write(outputBody);
-      } catch(final IOException e) {
-        response.getWriter().println(String.format("Cannot find the log file: [%s].", fileName));
+        writer.println("File name is not provided");
+      } else {
+        final String fileName = names.get(0);
+        if (!fileName.equals(driverStdoutFile) && !fileName.equals(driverStderrFile)) {
+          writer.println(String.format("Unsupported file names: [%s] ", fileName));
+        } else {
+          try {
+            final byte[] outputBody = readFile(names.get(0)).getBytes(StandardCharsets.UTF_8);
+            writer.print(outputBody);
+          } catch (final IOException e) {
+            writer.println(String.format("Cannot find the log file: [%s].", fileName));
+          }
+        }
       }
       break;
     // TODO[JIRA REEF-798] Use this provider in the HTTP
@@ -280,7 +283,7 @@ private void writeEvaluatorInfoWebOutput(
    * Get all evaluator ids and send it back to response as JSON.
    */
   private void writeEvaluatorsJsonOutput(final HttpServletResponse response) throws IOException {
-    LOG.log(Level.INFO, "HttpServerReefEventHandler getEvaluators is called");
+    LOG.log(Level.INFO, "HttpServerReefEventHandler writeEvaluatorsJsonOutput is called");
     try {
       final EvaluatorListSerializer serializer =
           Tang.Factory.getTang().newInjector().getInstance(EvaluatorListSerializer.class);
@@ -302,7 +305,7 @@ private void writeEvaluatorsJsonOutput(final HttpServletResponse response) throw
    */
   private void writeEvaluatorsWebOutput(final HttpServletResponse response) throws IOException {
 
-    LOG.log(Level.INFO, "HttpServerReefEventHandler getEvaluators is called");
+    LOG.log(Level.INFO, "HttpServerReefEventHandler writeEvaluatorsWebOutput is called");
 
     final PrintWriter writer = response.getWriter();
 
@@ -329,6 +332,9 @@ private void writeEvaluatorsWebOutput(final HttpServletResponse response) throws
    * Write Driver Info as JSON string to Response.
    */
   private void writeDriverJsonInformation(final HttpServletResponse response) throws IOException {
+
+    LOG.log(Level.INFO, "HttpServerReefEventHandler writeDriverJsonInformation invoked.");
+
     try {
       final DriverInfoSerializer serializer =
           Tang.Factory.getTang().newInjector().getInstance(DriverInfoSerializer.class);
@@ -355,7 +361,7 @@ private void writeResponse(final HttpServletResponse response, final String data
    */
   private void writeDriverWebInformation(final HttpServletResponse response) throws IOException {
 
-    LOG.log(Level.INFO, "HttpServerReefEventHandler writeDriverInformation invoked.");
+    LOG.log(Level.INFO, "HttpServerReefEventHandler writeDriverWebInformation invoked.");
 
     final PrintWriter writer = response.getWriter();