Code cleanup (#12333)

- removed unused private methods
- removed unused private variables
- removed unnecessary initializations
- removed unnecessary substring(0)
- removed inner assignments
- renamed private methods that started with upper case
- renamed static final variables to use upper case
- converted to use non-deprecated options
- suppressed unavoidable warnings
- divided long Strings to multiple lines
- added missing types
- added missing JavaDoc parameters
- formatting
- updated comments & JavaDocs

17 files changed, 188 insertions, 121 deletions

diff --git a/server/src/main/java/com/vaadin/server/DragAndDropService.java b/server/src/main/java/com/vaadin/server/DragAndDropService.java
index 9d8ddf018e..903860c077 100644
--- a/server/src/main/java/com/vaadin/server/DragAndDropService.java
+++ b/server/src/main/java/com/vaadin/server/DragAndDropService.java
@@ -133,8 +133,7 @@ public class DragAndDropService implements VariableOwner, ClientConnector {
          * operation based on the info passed from the client widgets (drag
          * source for Transferable, drop target for DragDropDetails).
          */
-        Transferable transferable = constructTransferable(dropTarget,
-                variables);
+        Transferable transferable = constructTransferable(variables);
         TargetDetails dropData = constructDragDropDetails(dropTarget,
                 variables);
         DragAndDropEvent dropEvent = new DragAndDropEvent(transferable,
@@ -161,8 +160,7 @@ public class DragAndDropService implements VariableOwner, ClientConnector {
          * operation based on the info passed from the client widgets (drag
          * source for Transferable, current target for DragDropDetails).
          */
-        Transferable transferable = constructTransferable(dropTarget,
-                variables);
+        Transferable transferable = constructTransferable(variables);
         TargetDetails dragDropDetails = constructDragDropDetails(dropTarget,
                 variables);
 
@@ -207,8 +205,7 @@ public class DragAndDropService implements VariableOwner, ClientConnector {
     }
 
     @SuppressWarnings("unchecked")
-    private Transferable constructTransferable(DropTarget dropHandlerOwner,
-            Map<String, Object> variables) {
+    private Transferable constructTransferable(Map<String, Object> variables) {
         final Component sourceComponent = (Component) variables
                 .get("component");
 
diff --git a/server/src/main/java/com/vaadin/server/JsonCodec.java b/server/src/main/java/com/vaadin/server/JsonCodec.java
index 3d1d5af5e2..fffb560738 100644
--- a/server/src/main/java/com/vaadin/server/JsonCodec.java
+++ b/server/src/main/java/com/vaadin/server/JsonCodec.java
@@ -412,7 +412,7 @@ public class JsonCodec implements Serializable {
         // Arrays
         if (JsonConstants.VTYPE_ARRAY.equals(transportType)) {
 
-            return decodeObjectArray(targetType, (JsonArray) encodedJsonValue,
+            return decodeObjectArray((JsonArray) encodedJsonValue,
                     connectorTracker);
 
         } else if (JsonConstants.VTYPE_STRINGARRAY.equals(transportType)) {
@@ -625,13 +625,14 @@ public class JsonCodec implements Serializable {
         }
     }
 
+    @SuppressWarnings({ "rawtypes", "unchecked" })
     private static Object decodeEnum(Class<? extends Enum> cls,
             JsonString value) {
         return Enum.valueOf(cls, value.getString());
     }
 
-    private static Object[] decodeObjectArray(Type targetType,
-            JsonArray jsonArray, ConnectorTracker connectorTracker) {
+    private static Object[] decodeObjectArray(JsonArray jsonArray,
+            ConnectorTracker connectorTracker) {
         List<Object> list = decodeList(List.class, true, jsonArray,
                 connectorTracker);
         return list.toArray(new Object[list.size()]);
@@ -685,6 +686,7 @@ public class JsonCodec implements Serializable {
         }
     }
 
+    @SuppressWarnings("deprecation")
     public static EncodeResult encode(Object value, JsonValue diffState,
             Type valueType, ConnectorTracker connectorTracker) {
 
@@ -1000,6 +1002,7 @@ public class JsonCodec implements Serializable {
     /*
      * Encodes a connector map. Invisible connectors are skipped.
      */
+    @SuppressWarnings("deprecation")
     private static JsonObject encodeConnectorMap(Type valueType, Map<?, ?> map,
             ConnectorTracker connectorTracker) {
         JsonObject jsonMap = Json.createObject();
@@ -1040,6 +1043,7 @@ public class JsonCodec implements Serializable {
         return TYPE_TO_TRANSPORT_TYPE.get(getClassForType(valueType));
     }
 
+    @SuppressWarnings({ "rawtypes", "unchecked" })
     private static JsonValue serializeJson(Object value,
             ConnectorTracker connectorTracker) {
         JSONSerializer serializer = CUSTOM_SERIALIZERS.get(value.getClass());
diff --git a/server/src/main/java/com/vaadin/server/JsonPaintTarget.java b/server/src/main/java/com/vaadin/server/JsonPaintTarget.java
index 4a247981f4..8c498e0ea2 100644
--- a/server/src/main/java/com/vaadin/server/JsonPaintTarget.java
+++ b/server/src/main/java/com/vaadin/server/JsonPaintTarget.java
@@ -45,7 +45,7 @@ import com.vaadin.ui.CustomLayout;
  * @author Vaadin Ltd.
  * @since 5.0
  */
-@SuppressWarnings("serial")
+@SuppressWarnings({ "deprecation", "serial" })
 public class JsonPaintTarget implements PaintTarget {
 
     /* Document type declarations */
@@ -82,6 +82,7 @@ public class JsonPaintTarget implements PaintTarget {
      * Creates a new JsonPaintTarget.
      *
      * @param manager
+     *            communication manager
      * @param outWriter
      *            A character-output stream.
      * @param cachingRequired
@@ -124,6 +125,8 @@ public class JsonPaintTarget implements PaintTarget {
      *
      * @param tagName
      *            the name of the start tag.
+     * @param isChildNode
+     *            {@code true} if child node, {@code false} otherwise
      * @throws PaintException
      *             if the paint operation failed.
      *
@@ -181,9 +184,7 @@ public class JsonPaintTarget implements PaintTarget {
         if (!openJsonTags.isEmpty()) {
             final JsonTag parent = openJsonTags.pop();
 
-            String lastTag = "";
-
-            lastTag = mOpenTags.pop();
+            String lastTag = mOpenTags.pop();
             if (!tagName.equalsIgnoreCase(lastTag)) {
                 throw new PaintException("Invalid UIDL: wrong ending tag: '"
                         + tagName + "' expected: '" + lastTag + "'.");
diff --git a/server/src/main/java/com/vaadin/server/VaadinServlet.java b/server/src/main/java/com/vaadin/server/VaadinServlet.java
index 3ddbd956e9..f0ec0427bf 100644
--- a/server/src/main/java/com/vaadin/server/VaadinServlet.java
+++ b/server/src/main/java/com/vaadin/server/VaadinServlet.java
@@ -78,7 +78,7 @@ import elemental.json.Json;
 import elemental.json.JsonArray;
 import elemental.json.JsonObject;
 
-@SuppressWarnings("serial")
+@SuppressWarnings({ "deprecation", "serial" })
 public class VaadinServlet extends HttpServlet implements Constants {
 
     private class ScssCacheEntry implements Serializable {
@@ -197,9 +197,13 @@ public class VaadinServlet extends HttpServlet implements Constants {
 
     private VaadinServletService servletService;
 
-    // Mapped uri is for the jar file
-    static final Map<URI, Integer> openFileSystems = new HashMap<>();
-    private static final Object fileSystemLock = new Object();
+    /**
+     * Mapped uri is for the jar file.
+     * <p>
+     * FOR INTERNAL USE ONLY, may get renamed or removed.
+     */
+    static final Map<URI, Integer> OPEN_FILE_SYSTEMS = new HashMap<>();
+    private static final Object FILE_SYSTEM_LOCK = new Object();
 
     /**
      * Called by the servlet container to indicate to a servlet that the servlet
@@ -696,6 +700,7 @@ public class VaadinServlet extends HttpServlet implements Constants {
      * servers).
      *
      * @param servletContext
+     *            the {@link ServletContext} in which this servlet is running
      * @param path
      *            the resource path.
      * @return the resource path.
@@ -706,8 +711,7 @@ public class VaadinServlet extends HttpServlet implements Constants {
     @Deprecated
     protected static String getResourcePath(ServletContext servletContext,
             String path) {
-        String resultPath = null;
-        resultPath = servletContext.getRealPath(path);
+        String resultPath = servletContext.getRealPath(path);
         if (resultPath != null) {
             return resultPath;
         } else {
@@ -729,7 +733,8 @@ public class VaadinServlet extends HttpServlet implements Constants {
      * e.g. '(' and ')', so values should be safe in javascript too.
      *
      * @param themeName
-     * @return
+     *            name of the theme
+     * @return name of the theme without special characters
      *
      * @deprecated As of 7.0. Will likely change or be removed in a future
      *             version
@@ -782,7 +787,7 @@ public class VaadinServlet extends HttpServlet implements Constants {
     /**
      * Returns the default theme. Must never return null.
      *
-     * @return
+     * @return default theme name
      */
     public static String getDefaultTheme() {
         return DEFAULT_THEME_NAME;
@@ -825,7 +830,9 @@ public class VaadinServlet extends HttpServlet implements Constants {
      * @param response
      *            The response
      * @throws IOException
+     *             if an I/O exception occurs
      * @throws ServletException
+     *             if a servlet exception occurs
      *
      * @since 8.5
      */
@@ -971,6 +978,7 @@ public class VaadinServlet extends HttpServlet implements Constants {
      * @param resourceUrl
      *            The url to send
      * @throws IOException
+     *             if an I/O exception occurs
      */
     protected void writeStaticResourceResponse(HttpServletRequest request,
             HttpServletResponse response, URL resourceUrl) throws IOException {
@@ -1228,8 +1236,11 @@ public class VaadinServlet extends HttpServlet implements Constants {
      * outside the VAADIN directory if the method is overridden.
      *
      * @param request
+     *            current request
      * @param resourceUrl
-     * @return
+     *            URL of the resource to validate
+     * @return {@code true} if the resource is a valid VAADIN resource,
+     *         {@code false} otherwise
      *
      * @since 6.6.7
      *
@@ -1356,7 +1367,7 @@ public class VaadinServlet extends HttpServlet implements Constants {
 
     // Package protected for feature verification purpose
     FileSystem getFileSystem(URI resourceURI) throws IOException {
-        synchronized (fileSystemLock) {
+        synchronized (FILE_SYSTEM_LOCK) {
             URI fileURI = getFileURI(resourceURI);
             if (!fileURI.getScheme().equals("file")) {
                 throw new IOException("Can not read scheme '"
@@ -1364,7 +1375,7 @@ public class VaadinServlet extends HttpServlet implements Constants {
                         + " and will determine this as not a folder");
             }
 
-            Integer locks = openFileSystems.computeIfPresent(fileURI,
+            Integer locks = OPEN_FILE_SYSTEMS.computeIfPresent(fileURI,
                     (key, value) -> value + 1);
             if (locks != null) {
                 // Get filesystem is for the file to get the correct provider
@@ -1373,20 +1384,20 @@ public class VaadinServlet extends HttpServlet implements Constants {
             // Opened filesystem is for the file to get the correct provider
             FileSystem fileSystem = FileSystems.newFileSystem(resourceURI,
                     Collections.emptyMap());
-            openFileSystems.put(fileURI, 1);
+            OPEN_FILE_SYSTEMS.put(fileURI, 1);
             return fileSystem;
         }
     }
 
     // Package protected for feature verification purpose
     void closeFileSystem(URI resourceURI) {
-        synchronized (fileSystemLock) {
+        synchronized (FILE_SYSTEM_LOCK) {
             try {
                 URI fileURI = getFileURI(resourceURI);
-                Integer locks = openFileSystems.computeIfPresent(fileURI,
+                Integer locks = OPEN_FILE_SYSTEMS.computeIfPresent(fileURI,
                         (key, value) -> value - 1);
                 if (locks != null && locks == 0) {
-                    openFileSystems.remove(fileURI);
+                    OPEN_FILE_SYSTEMS.remove(fileURI);
                     // Get filesystem is for the file to get the correct
                     // provider
                     FileSystems.getFileSystem(resourceURI).close();
@@ -1459,7 +1470,8 @@ public class VaadinServlet extends HttpServlet implements Constants {
 
     /**
      * @param request
-     * @return
+     *            the request that is to be evaluated
+     * @return request type
      *
      * @deprecated As of 7.0. This is no longer used and only provided for
      *             backwards compatibility. Each {@link RequestHandler} can
@@ -1574,6 +1586,7 @@ public class VaadinServlet extends HttpServlet implements Constants {
      *
      * @param request
      *            the HTTP request.
+     * @return current application URL
      * @throws MalformedURLException
      *             if the application is denied access to the persistent data
      *             store represented by the given URL.
@@ -1687,6 +1700,7 @@ public class VaadinServlet extends HttpServlet implements Constants {
      * characters" to keep the text somewhat readable.
      *
      * @param unsafe
+     *            the string that needs to be made safe
      * @return a safe string to be added inside an html tag
      *
      * @deprecated As of 7.0. Will likely change or be removed in a future
@@ -1714,10 +1728,9 @@ public class VaadinServlet extends HttpServlet implements Constants {
 
     private static boolean isSafe(char c) {
         return //
-        c > 47 && c < 58 || // alphanum
-                c > 64 && c < 91 || // A-Z
-                c > 96 && c < 123 // a-z
-        ;
+        (c > 47 && c < 58) || // alphanum
+                (c > 64 && c < 91) || // A-Z
+                (c > 96 && c < 123); // a-z
     }
 
     private static final Logger getLogger() {
diff --git a/server/src/main/java/com/vaadin/server/communication/ResourceWriter.java b/server/src/main/java/com/vaadin/server/communication/ResourceWriter.java
index 9ae5fd41fd..f9b687b567 100644
--- a/server/src/main/java/com/vaadin/server/communication/ResourceWriter.java
+++ b/server/src/main/java/com/vaadin/server/communication/ResourceWriter.java
@@ -26,7 +26,6 @@ import java.util.logging.Level;
 import java.util.logging.Logger;
 
 import com.vaadin.server.JsonPaintTarget;
-import com.vaadin.server.LegacyCommunicationManager;
 import com.vaadin.ui.CustomLayout;
 import com.vaadin.ui.UI;
 
@@ -53,10 +52,6 @@ public class ResourceWriter implements Serializable {
     public void write(UI ui, Writer writer, JsonPaintTarget target)
             throws IOException {
 
-        // TODO PUSH Refactor so that this is not needed
-        LegacyCommunicationManager manager = ui.getSession()
-                .getCommunicationManager();
-
         // Precache custom layouts
 
         // TODO We should only precache the layouts that are not
diff --git a/server/src/main/java/com/vaadin/server/communication/ServerRpcHandler.java b/server/src/main/java/com/vaadin/server/communication/ServerRpcHandler.java
index 2c9d15bdd3..460e458aba 100644
--- a/server/src/main/java/com/vaadin/server/communication/ServerRpcHandler.java
+++ b/server/src/main/java/com/vaadin/server/communication/ServerRpcHandler.java
@@ -31,7 +31,6 @@ import java.util.logging.Logger;
 import com.vaadin.server.ClientConnector;
 import com.vaadin.server.Constants;
 import com.vaadin.server.JsonCodec;
-import com.vaadin.server.LegacyCommunicationManager;
 import com.vaadin.server.LegacyCommunicationManager.InvalidUIDLSecurityKeyException;
 import com.vaadin.server.ServerRpcManager;
 import com.vaadin.server.ServerRpcManager.RpcInvocationException;
@@ -64,6 +63,7 @@ import elemental.json.impl.JsonUtil;
  * @author Vaadin Ltd
  * @since 7.1
  */
+@SuppressWarnings("deprecation")
 public class ServerRpcHandler implements Serializable {
 
     /**
@@ -212,6 +212,7 @@ public class ServerRpcHandler implements Serializable {
      * @param reader
      *            The {@link Reader} used to read the JSON.
      * @param request
+     *            The {@link VaadinRequest} to handle.
      * @throws IOException
      *             If reading the message fails.
      * @throws InvalidUIDLSecurityKeyException
@@ -332,8 +333,7 @@ public class ServerRpcHandler implements Serializable {
             Set<Connector> enabledConnectors = new HashSet<>();
 
             List<MethodInvocation> invocations = parseInvocations(
-                    ui.getConnectorTracker(), invocationsData,
-                    lastSyncIdSeenByClient);
+                    ui.getConnectorTracker(), invocationsData);
             for (MethodInvocation invocation : invocations) {
                 final ClientConnector connector = connectorTracker
                         .getConnector(invocation.getConnectorId());
@@ -489,14 +489,10 @@ public class ServerRpcHandler implements Serializable {
      * @param invocationsJson
      *            JSON containing all information needed to execute all
      *            requested RPC calls.
-     * @param lastSyncIdSeenByClient
-     *            the most recent sync id the client has seen at the time the
-     *            request was sent
      * @return list of MethodInvocation to perform
      */
     private List<MethodInvocation> parseInvocations(
-            ConnectorTracker connectorTracker, JsonArray invocationsJson,
-            int lastSyncIdSeenByClient) {
+            ConnectorTracker connectorTracker, JsonArray invocationsJson) {
         int invocationCount = invocationsJson.length();
         List<MethodInvocation> invocations = new ArrayList<>(invocationCount);
 
@@ -507,8 +503,7 @@ public class ServerRpcHandler implements Serializable {
             JsonArray invocationJson = invocationsJson.getArray(i);
 
             MethodInvocation invocation = parseInvocation(invocationJson,
-                    previousInvocation, connectorTracker,
-                    lastSyncIdSeenByClient);
+                    previousInvocation, connectorTracker);
             if (invocation != null) {
                 // Can be null if the invocation was a legacy invocation and it
                 // was merged with the previous one or if the invocation was
@@ -522,7 +517,7 @@ public class ServerRpcHandler implements Serializable {
 
     private MethodInvocation parseInvocation(JsonArray invocationJson,
             MethodInvocation previousInvocation,
-            ConnectorTracker connectorTracker, long lastSyncIdSeenByClient) {
+            ConnectorTracker connectorTracker) {
         String connectorId = invocationJson.getString(0);
         String interfaceName = invocationJson.getString(1);
         String methodName = invocationJson.getString(2);
@@ -536,7 +531,6 @@ public class ServerRpcHandler implements Serializable {
             }
 
             return parseLegacyChangeVariablesInvocation(connectorId,
-                    interfaceName, methodName,
                     (LegacyChangeVariablesInvocation) previousInvocation,
                     parametersJson, connectorTracker);
         } else {
@@ -547,7 +541,7 @@ public class ServerRpcHandler implements Serializable {
     }
 
     private LegacyChangeVariablesInvocation parseLegacyChangeVariablesInvocation(
-            String connectorId, String interfaceName, String methodName,
+            String connectorId,
             LegacyChangeVariablesInvocation previousInvocation,
             JsonArray parametersJson, ConnectorTracker connectorTracker) {
         if (parametersJson.length() != 2) {
@@ -643,10 +637,12 @@ public class ServerRpcHandler implements Serializable {
     }
 
     /**
-     * Generates an error message when the client is trying to to something
+     * Generates an error message when the client is trying to do something
      * ('what') with a connector which is disabled or invisible.
      *
      * @since 7.1.8
+     * @param what
+     *            the ignored operation
      * @param connector
      *            the connector which is disabled (or invisible)
      * @return an error message
diff --git a/server/src/main/java/com/vaadin/server/communication/UidlRequestHandler.java b/server/src/main/java/com/vaadin/server/communication/UidlRequestHandler.java
index 43a3e0d664..b0e779ad5e 100644
--- a/server/src/main/java/com/vaadin/server/communication/UidlRequestHandler.java
+++ b/server/src/main/java/com/vaadin/server/communication/UidlRequestHandler.java
@@ -46,6 +46,7 @@ import elemental.json.JsonException;
  * @author Vaadin Ltd
  * @since 7.1
  */
+@SuppressWarnings("deprecation")
 public class UidlRequestHandler extends SynchronizedRequestHandler
         implements SessionExpiredHandler {
 
@@ -89,7 +90,7 @@ public class UidlRequestHandler extends SynchronizedRequestHandler
         try {
             rpcHandler.handleRpc(uI, request.getReader(), request);
 
-            writeUidl(request, response, uI, stringWriter);
+            writeUidl(response, uI, stringWriter);
         } catch (JsonException e) {
             getLogger().log(Level.SEVERE, "Error writing JSON to response", e);
             // Refresh on client side
@@ -117,8 +118,8 @@ public class UidlRequestHandler extends SynchronizedRequestHandler
         UIInitHandler.commitJsonResponse(request, response, json);
     }
 
-    private void writeUidl(VaadinRequest request, VaadinResponse response,
-            UI ui, Writer writer) throws IOException {
+    private void writeUidl(VaadinResponse response, UI ui, Writer writer)
+            throws IOException {
         openJsonMessage(writer, response);
 
         new UidlWriter().write(ui, writer, false);
diff --git a/server/src/main/java/com/vaadin/server/widgetsetutils/ClassPathExplorer.java b/server/src/main/java/com/vaadin/server/widgetsetutils/ClassPathExplorer.java
index 4d51d288c5..1aec57c1a4 100644
--- a/server/src/main/java/com/vaadin/server/widgetsetutils/ClassPathExplorer.java
+++ b/server/src/main/java/com/vaadin/server/widgetsetutils/ClassPathExplorer.java
@@ -52,8 +52,6 @@ import java.util.jar.Manifest;
  */
 public class ClassPathExplorer {
 
-    private static final String VAADIN_ADDON_VERSION_ATTRIBUTE = "Vaadin-Package-Version";
-
     /**
      * File filter that only accepts directories.
      */
diff --git a/server/src/main/java/com/vaadin/ui/AbstractDateField.java b/server/src/main/java/com/vaadin/ui/AbstractDateField.java
index d194363b94..c22aff114d 100644
--- a/server/src/main/java/com/vaadin/ui/AbstractDateField.java
+++ b/server/src/main/java/com/vaadin/ui/AbstractDateField.java
@@ -427,7 +427,7 @@ public abstract class AbstractDateField<T extends Temporal & TemporalAdjuster &
     }
 
     /**
-     * Parses string representaion of date range limit into date type
+     * Parses string representation of date range limit into date type.
      *
      * @param temporalStr
      *            the string representation
@@ -1005,7 +1005,7 @@ public abstract class AbstractDateField<T extends Temporal & TemporalAdjuster &
                         .orElse(null));
     }
 
-    private Iterable<R> getResolutionsHigherOrEqualTo(R resoution) {
+    private Iterable<R> getResolutionsHigherOrEqualTo(R resolution) {
         return getResolutions().skip(resolution.ordinal())
                 .collect(Collectors.toList());
     }
@@ -1141,10 +1141,11 @@ public abstract class AbstractDateField<T extends Temporal & TemporalAdjuster &
     }
 
     /**
-     * Control whether value change event is emitted when user input value 
-     * does not meet the integrated range validator.
+     * Control whether value change event is emitted when user input value does
+     * not meet the integrated range validator.
      *
-     * @param preventInvalidInput Set to false to disable the value change event.
+     * @param preventInvalidInput
+     *            Set to false to disable the value change event.
      *
      * @since 8.13
      */
@@ -1153,8 +1154,8 @@ public abstract class AbstractDateField<T extends Temporal & TemporalAdjuster &
     }
 
     /**
-     * Check whether value change is emitted when user input value does
-     * not meet integrated range validator. The default is false.
+     * Check whether value change is emitted when user input value does not meet
+     * integrated range validator. The default is false.
      *
      * @return a Boolean value
      *
diff --git a/server/src/main/java/com/vaadin/ui/Grid.java b/server/src/main/java/com/vaadin/ui/Grid.java
index 7ffd333430..137f947401 100644
--- a/server/src/main/java/com/vaadin/ui/Grid.java
+++ b/server/src/main/java/com/vaadin/ui/Grid.java
@@ -275,7 +275,7 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          *            <code>true</code> if event is a result of user
          *            interaction, <code>false</code> if from API call
          */
-        public ColumnReorderEvent(Grid source, boolean userOriginated) {
+        public ColumnReorderEvent(Grid<?> source, boolean userOriginated) {
             super(source);
             this.userOriginated = userOriginated;
         }
@@ -306,6 +306,8 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          *
          * @param source
          *            the grid where the event originated from
+         * @param column
+         *            the column that was resized
          * @param userOriginated
          *            <code>true</code> if event is a result of user
          *            interaction, <code>false</code> if from API call
@@ -356,6 +358,16 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          * Creates a new {@code ItemClick} event containing the given item and
          * Column originating from the given Grid.
          *
+         * @param source
+         *            the grid where the event originated from
+         * @param column
+         *            the column that contains the clicked cell
+         * @param item
+         *            the item that was clicked
+         * @param mouseEventDetails
+         *            mouse event details about the click
+         * @param rowIndex
+         *            the index of the row that contains the clicked cell
          */
         public ItemClick(Grid<T> source, Column<T, ?> column, T item,
                 MouseEventDetails mouseEventDetails, int rowIndex) {
@@ -390,6 +402,7 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          * @return the grid
          */
         @Override
+        @SuppressWarnings("unchecked")
         public Grid<T> getSource() {
             return (Grid<T>) super.getSource();
         }
@@ -501,6 +514,7 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
         }
 
         @Override
+        @SuppressWarnings("unchecked")
         public Grid<T> getComponent() {
             return (Grid<T>) super.getComponent();
         }
@@ -849,18 +863,18 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
     public static class Column<T, V> extends AbstractExtension {
 
         /**
-         * behavior when parsing nested properties which may contain
-         * <code>null</code> values in the property chain
+         * Behavior when parsing nested properties which may contain
+         * <code>null</code> values in the property chain.
          */
         public enum NestedNullBehavior {
             /**
-             * throw a NullPointerException if there is a nested
-             * <code>null</code> value
+             * Throw a NullPointerException if there is a nested
+             * <code>null</code> value.
              */
             THROW,
             /**
-             * silently ignore any exceptions caused by nested <code>null</code>
-             * values
+             * Silently ignore any exceptions caused by nested <code>null</code>
+             * values.
              */
             ALLOW_NULLS
         }
@@ -1578,6 +1592,7 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          *            the expand ratio of this column. {@code 0} to not have it
          *            expand at all. A negative number to clear the expand
          *            value.
+         * @return this column
          * @throws IllegalStateException
          *             if the column is no longer attached to any grid
          * @see #setWidth(double)
@@ -1607,6 +1622,7 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          * <p>
          * Equal to calling {@link #setExpandRatio(int) setExpandRatio(-1)}
          *
+         * @return this column
          * @throws IllegalStateException
          *             if the column is no longer attached to any grid
          */
@@ -1790,6 +1806,7 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          *
          * @param pixels
          *            the maximum width
+         * @return this column
          * @throws IllegalStateException
          *             if the column is no longer attached to any grid
          * @see #setExpandRatio(int)
@@ -1826,6 +1843,7 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          * @param resizable
          *            {@code true} if this column should be resizable,
          *            {@code false} otherwise
+         * @return this column
          * @throws IllegalStateException
          *             if the column is no longer attached to any grid
          */
@@ -1961,8 +1979,8 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          *            {@code true} if column is editable; {@code false} if not
          * @return this column
          * @throws IllegalStateException
-         *            if editable is true and column has no editor binding or
-         *            component defined
+         *             if editable is true and column has no editor binding or
+         *             component defined
          *
          * @see #setEditorComponent(HasValue, Setter)
          * @see #setEditorBinding(Binding)
@@ -2045,6 +2063,9 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          * <strong>Note:</strong> The same component cannot be used for multiple
          * columns.
          *
+         * @param <C>
+         *            a class that extends both {@link HasValue} and
+         *            {@link Component}
          * @param editorComponent
          *            the editor component
          * @param setter
@@ -2081,6 +2102,11 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          * <strong>Note:</strong> The same component cannot be used for multiple
          * columns.
          *
+         * @param <F>
+         *            a value type
+         * @param <C>
+         *            a class that extends both {@link HasValue} (for type
+         *            {@code <F>}) and {@link Component}
          * @param editorComponent
          *            the editor component
          * @return this column
@@ -2113,8 +2139,8 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
          * all currently available row data to be recreated and sent to the
          * client.
          *
-         * Note: Setting a new renderer will reset presentation provider if
-         * it exists.
+         * Note: Setting a new renderer will reset presentation provider if it
+         * exists.
          *
          * @param renderer
          *            the new renderer
@@ -2660,6 +2686,8 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
      * @see Grid#Grid()
      * @see Grid#Grid(Class)
      *
+     * @param <BEAN>
+     *            the grid bean type
      * @param propertySet
      *            the property set implementation to use, not <code>null</code>.
      * @return a new grid using the provided property set, not <code>null</code>
@@ -2875,6 +2903,8 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
      *
      * @param valueProvider
      *            the value provider
+     * @param <V>
+     *            the column value type
      *
      * @return the new column
      */
@@ -3061,8 +3091,8 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
     /**
      * Removes the given column from this {@link Grid}.
      *
-     * Note: If you have Editor with binding in this Grid to this property, you need to remove that
-     * using removeBinding method provided by Binder.
+     * Note: If you have Editor with binding in this Grid to this property, you
+     * need to remove that using removeBinding method provided by Binder.
      *
      * @param column
      *            the column to remove
@@ -4014,6 +4044,7 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
      *
      * @see Column#setId(String)
      */
+    @SuppressWarnings("unchecked")
     public void setColumns(String... columnIds) {
         // Must extract to an explicitly typed variable because otherwise javac
         // cannot determine which overload of setColumnOrder to use
@@ -4068,6 +4099,7 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
      * @param columns
      *            the columns in the order they should be
      */
+    @SuppressWarnings("unchecked")
     public void setColumnOrder(Column<T, ?>... columns) {
         setColumnOrder(Stream.of(columns));
     }
@@ -4133,6 +4165,11 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
         return new GridSingleSelect<>(this);
     }
 
+    /**
+     * Returns the {@link Editor} for this grid.
+     *
+     * @return the editor, not null
+     */
     public Editor<T> getEditor() {
         return editor;
     }
@@ -4223,6 +4260,7 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
      *
      * @see #getSelectionModel()
      * @see GridSelectionModel
+     * @return the items in the current selection, not null
      */
     public Set<T> getSelectedItems() {
         return getSelectionModel().getSelectedItems();
@@ -4234,6 +4272,8 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
      *
      * @see #getSelectionModel()
      * @see GridSelectionModel
+     * @param item
+     *            the item to select, not null
      */
     public void select(T item) {
         getSelectionModel().select(item);
@@ -4245,6 +4285,8 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
      *
      * @see #getSelectionModel()
      * @see GridSelectionModel
+     * @param item
+     *            the item to deselect, not null
      */
     public void deselect(T item) {
         getSelectionModel().deselect(item);
@@ -4849,10 +4891,10 @@ public class Grid<T> extends AbstractListing<T> implements HasComponents,
      */
     protected void writeData(Element body, DesignContext designContext) {
         getDataProvider().fetch(new Query<>())
-                .forEach(item -> writeRow(body, item, designContext));
+                .forEach(item -> writeRow(body, item));
     }
 
-    private void writeRow(Element container, T item, DesignContext context) {
+    private void writeRow(Element container, T item) {
         Element tableRow = container.appendElement("tr");
         tableRow.attr("item", serializeDeclarativeRepresentation(item));
         if (getSelectionModel().isSelected(item)) {
diff --git a/server/src/main/java/com/vaadin/ui/GridLayout.java b/server/src/main/java/com/vaadin/ui/GridLayout.java
index 42886fd650..a65a3717c0 100644
--- a/server/src/main/java/com/vaadin/ui/GridLayout.java
+++ b/server/src/main/java/com/vaadin/ui/GridLayout.java
@@ -641,6 +641,7 @@ public class GridLayout extends AbstractLayout
          * Constructs an <code>OverlapsException</code>.
          *
          * @param existingArea
+         *            the existing area that needs overlapping
          */
         public OverlapsException(Area existingArea) {
             this.existingArea = existingArea;
@@ -698,6 +699,7 @@ public class GridLayout extends AbstractLayout
          * detail message.
          *
          * @param areaOutOfBounds
+         *            the area that exceeds the bounds of the grid
          */
         public OutOfBoundsException(Area areaOutOfBounds) {
             super(String.format("%s, layout dimension: %sx%s", areaOutOfBounds,
@@ -837,6 +839,7 @@ public class GridLayout extends AbstractLayout
      * by GridLayout.
      *
      * @param cursorX
+     *            current cursor x-position
      */
     public void setCursorX(int cursorX) {
         this.cursorX = cursorX;
@@ -1058,7 +1061,9 @@ public class GridLayout extends AbstractLayout
      * @see #setWidth(float, Unit)
      *
      * @param columnIndex
+     *            The column index, starting from 0 for the leftmost row.
      * @param ratio
+     *            the expand ratio
      */
     public void setColumnExpandRatio(int columnIndex, float ratio) {
         columnExpandRatio.put(columnIndex, ratio);
@@ -1072,6 +1077,7 @@ public class GridLayout extends AbstractLayout
      * @see #setColumnExpandRatio(int, float)
      *
      * @param columnIndex
+     *            The column index, starting from 0 for the leftmost row.
      * @return the expand ratio, 0.0f by default
      */
     public float getColumnExpandRatio(int columnIndex) {
@@ -1103,6 +1109,7 @@ public class GridLayout extends AbstractLayout
      * @param rowIndex
      *            The row index, starting from 0 for the topmost row.
      * @param ratio
+     *            the expand ratio
      */
     public void setRowExpandRatio(int rowIndex, float ratio) {
         rowExpandRatio.put(rowIndex, ratio);
@@ -1195,6 +1202,7 @@ public class GridLayout extends AbstractLayout
      * MarginInfo )
      */
     @Override
+    @SuppressWarnings("deprecation")
     public void setMargin(MarginInfo marginInfo) {
         getState().marginsBitmask = marginInfo.getBitMask();
     }
@@ -1205,6 +1213,7 @@ public class GridLayout extends AbstractLayout
      * @see com.vaadin.ui.Layout.MarginHandler#getMargin()
      */
     @Override
+    @SuppressWarnings("deprecation")
     public MarginInfo getMargin() {
         return new MarginInfo(getState(false).marginsBitmask);
     }
@@ -1430,7 +1439,7 @@ public class GridLayout extends AbstractLayout
         }
 
         if (components.isEmpty()) {
-            writeEmptyColsAndRows(design, designContext);
+            writeEmptyColsAndRows(design);
             return;
         }
 
@@ -1566,10 +1575,8 @@ public class GridLayout extends AbstractLayout
      * components in the {@link GridLayout}
      *
      * @param design
-     * @param designContext
      */
-    private void writeEmptyColsAndRows(Element design,
-            DesignContext designContext) {
+    private void writeEmptyColsAndRows(Element design) {
         int rowCount = getState(false).rows;
         int colCount = getState(false).columns;
 
diff --git a/server/src/main/java/com/vaadin/ui/Panel.java b/server/src/main/java/com/vaadin/ui/Panel.java
index 750652653f..6c3df6e291 100644
--- a/server/src/main/java/com/vaadin/ui/Panel.java
+++ b/server/src/main/java/com/vaadin/ui/Panel.java
@@ -334,7 +334,7 @@ public class Panel extends AbstractSingleComponentContainer
     public void writeDesign(Element design, DesignContext designContext) {
         super.writeDesign(design, designContext);
         // handle tabindex
-        Panel def = designContext.getDefaultInstance(this);
+        designContext.getDefaultInstance(this);
     }
 
 }
diff --git a/server/src/main/java/com/vaadin/ui/TabSheet.java b/server/src/main/java/com/vaadin/ui/TabSheet.java
index 04c700e64e..64719e23a9 100644
--- a/server/src/main/java/com/vaadin/ui/TabSheet.java
+++ b/server/src/main/java/com/vaadin/ui/TabSheet.java
@@ -140,7 +140,7 @@ public class TabSheet extends AbstractComponentContainer
         registerRpc(new FocusAndBlurServerRpcDecorator(this, this::fireEvent));
 
         // expand horizontally by default
-        setWidth(100, UNITS_PERCENTAGE);
+        setWidth(100, Unit.PERCENTAGE);
         setCloseHandler(TabSheet::removeComponent);
     }
 
@@ -450,8 +450,7 @@ public class TabSheet extends AbstractComponentContainer
 
     @Override
     public void moveComponentsFrom(ComponentContainer source) {
-        for (final Iterator<Component> i = source.getComponentIterator(); i
-                .hasNext();) {
+        for (final Iterator<Component> i = source.iterator(); i.hasNext();) {
             final Component c = i.next();
             String caption = null;
             Resource icon = null;
@@ -644,8 +643,7 @@ public class TabSheet extends AbstractComponentContainer
      */
     private boolean updateSelection() {
         Component originalSelection = selected;
-        for (final Iterator<Component> i = getComponentIterator(); i
-                .hasNext();) {
+        for (final Iterator<Component> i = iterator(); i.hasNext();) {
             final Component component = i.next();
 
             Tab tab = tabs.get(component);
@@ -851,7 +849,8 @@ public class TabSheet extends AbstractComponentContainer
      * @since 3.0
      */
     @FunctionalInterface
-    public interface SelectedTabChangeListener extends SerializableEventListener {
+    public interface SelectedTabChangeListener
+            extends SerializableEventListener {
 
         /**
          * Selected (shown) tab in tab sheet has has been changed.
@@ -1015,11 +1014,15 @@ public class TabSheet extends AbstractComponentContainer
 
         /**
          * Gets the caption for the tab.
+         *
+         * @return the caption
          */
         public String getCaption();
 
         /**
          * Gets the icon for the tab.
+         *
+         * @return tue icon resource
          */
         public Resource getIcon();
 
@@ -1036,6 +1039,8 @@ public class TabSheet extends AbstractComponentContainer
          *
          * @param icon
          *            the icon to set
+         * @param iconAltText
+         *            the alt text
          */
         public void setIcon(Resource icon, String iconAltText);
 
@@ -1043,6 +1048,8 @@ public class TabSheet extends AbstractComponentContainer
          * Gets the icon alt text for the tab.
          *
          * @since 7.2
+         *
+         * @return the alt text
          */
         public String getIconAlternateText();
 
@@ -1114,11 +1121,15 @@ public class TabSheet extends AbstractComponentContainer
          * TODO currently not sent to the client
          *
          * @see AbstractComponent#setComponentError(ErrorMessage)
+         *
+         * @return the error message
          */
         public ErrorMessage getComponentError();
 
         /**
          * Get the component related to the Tab.
+         *
+         * @return the component
          */
         public Component getComponent();
 
@@ -1407,6 +1418,7 @@ public class TabSheet extends AbstractComponentContainer
      * The default CloseHandler for TabSheet will only remove the tab.
      *
      * @param handler
+     *            the close handler that should be used
      */
     public void setCloseHandler(CloseHandler handler) {
         closeHandler = handler;
@@ -1434,7 +1446,8 @@ public class TabSheet extends AbstractComponentContainer
      *
      * @param tab
      *            The tab
-     * @return
+     * @return the index of the given tab, or -1 if there is no such tab in this
+     *         tabsheet
      */
     public int getTabPosition(Tab tab) {
         return components.indexOf(tab.getComponent());
@@ -1659,7 +1672,7 @@ public class TabSheet extends AbstractComponentContainer
     public void writeDesign(Element design, DesignContext designContext) {
         super.writeDesign(design, designContext);
         TabSheet def = designContext.getDefaultInstance(this);
-        Attributes attr = design.attributes();
+        design.attributes();
 
         // write tabs
         if (!designContext.shouldWriteChildren(this, def)) {
diff --git a/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerPopup.java b/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerPopup.java
index 409c930e1b..29925126cb 100644
--- a/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerPopup.java
+++ b/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerPopup.java
@@ -15,7 +15,6 @@
  */
 package com.vaadin.ui.components.colorpicker;
 
-import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
@@ -28,19 +27,19 @@ import com.vaadin.data.HasValue;
 import com.vaadin.shared.Registration;
 import com.vaadin.shared.ui.MarginInfo;
 import com.vaadin.shared.ui.colorpicker.Color;
-import com.vaadin.ui.TabSheet;
-import com.vaadin.ui.Window;
-import com.vaadin.ui.VerticalLayout;
-import com.vaadin.ui.HorizontalLayout;
-import com.vaadin.ui.Component;
-import com.vaadin.ui.Slider;
-import com.vaadin.ui.Layout;
+import com.vaadin.ui.AbstractColorPicker.Coordinates2Color;
 import com.vaadin.ui.Alignment;
 import com.vaadin.ui.Button;
-import com.vaadin.ui.HasComponents;
-import com.vaadin.ui.AbstractColorPicker.Coordinates2Color;
 import com.vaadin.ui.Button.ClickEvent;
+import com.vaadin.ui.Component;
+import com.vaadin.ui.HasComponents;
+import com.vaadin.ui.HorizontalLayout;
+import com.vaadin.ui.Layout;
+import com.vaadin.ui.Slider;
 import com.vaadin.ui.Slider.ValueOutOfBoundsException;
+import com.vaadin.ui.TabSheet;
+import com.vaadin.ui.VerticalLayout;
+import com.vaadin.ui.Window;
 
 /**
  * A component that represents color selection popup within a color picker.
@@ -206,11 +205,6 @@ public class ColorPickerPopup extends Window implements HasValue<Color> {
         history.setWidth("97%");
         history.setHeight("22px");
 
-        // Create the default colors
-        List<Color> defaultColors = new ArrayList<>();
-        defaultColors.add(Color.BLACK);
-        defaultColors.add(Color.WHITE);
-
         // Create the history
         VerticalLayout innerContainer = new VerticalLayout();
         innerContainer.setWidth("100%");
@@ -503,6 +497,7 @@ public class ColorPickerPopup extends Window implements HasValue<Color> {
     }
 
     @Override
+    @SuppressWarnings("deprecation")
     public Registration addValueChangeListener(
             ValueChangeListener<Color> listener) {
         Objects.requireNonNull(listener, "listener cannot be null");
diff --git a/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerPreview.java b/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerPreview.java
index 8782a5aa39..d70cf60313 100644
--- a/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerPreview.java
+++ b/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerPreview.java
@@ -49,8 +49,6 @@ public class ColorPickerPreview extends CssLayout implements HasValue<Color> {
     /** The field. */
     private final TextField field;
 
-    /** The old value. */
-    private String oldValue;
     private Registration valueChangeListenerRegistration = null;
 
     private boolean readOnly;
@@ -96,8 +94,6 @@ public class ColorPickerPreview extends CssLayout implements HasValue<Color> {
         field.setValue(colorCSS);
         field.setComponentError(null);
 
-        oldValue = colorCSS;
-
         // Re-register listener
         valueChangeListenerRegistration = field
                 .addValueChangeListener(this::valueChange);
@@ -121,6 +117,7 @@ public class ColorPickerPreview extends CssLayout implements HasValue<Color> {
     }
 
     @Override
+    @SuppressWarnings("deprecation")
     public Registration addValueChangeListener(
             ValueChangeListener<Color> listener) {
         Objects.requireNonNull(listener, "listener cannot be null");
@@ -141,7 +138,6 @@ public class ColorPickerPreview extends CssLayout implements HasValue<Color> {
              */
             color = ColorUtil.stringToColor(value);
 
-            oldValue = value;
             fireEvent(new ValueChangeEvent<>(this, oldColor,
                     event.isUserOriginated()));
         } catch (NumberFormatException e) {
diff --git a/server/src/main/java/com/vaadin/ui/declarative/DesignContext.java b/server/src/main/java/com/vaadin/ui/declarative/DesignContext.java
index b8f355d4a2..29efe4f7b4 100644
--- a/server/src/main/java/com/vaadin/ui/declarative/DesignContext.java
+++ b/server/src/main/java/com/vaadin/ui/declarative/DesignContext.java
@@ -354,10 +354,14 @@ public class DesignContext implements Serializable {
      * Returns the default instance for the given class. The instance must not
      * be modified by the caller.
      *
+     * @param <T>
+     *            a component class
      * @param component
+     *            the component that determines the class
      * @return the default instance for the given class. The return value must
      *         not be modified by the caller
      */
+    @SuppressWarnings("unchecked")
     public <T> T getDefaultInstance(Component component) {
         // If the root is a @DesignRoot component, it can't use itself as a
         // reference or the written design will be empty
@@ -385,6 +389,9 @@ public class DesignContext implements Serializable {
     /**
      * Reads and stores the mappings from prefixes to package names from meta
      * tags located under <head> in the html document.
+     *
+     * @param doc
+     *            the document
      */
     protected void readPackageMappings(Document doc) {
         Element head = doc.head();
@@ -848,7 +855,8 @@ public class DesignContext implements Serializable {
             String value) {
         Map<String, String> map = customAttributes.get(component);
         if (map == null) {
-            customAttributes.put(component, map = new HashMap<>());
+            map = new HashMap<>();
+            customAttributes.put(component, map);
         }
         map.put(attribute, value);
     }
diff --git a/server/src/test/java/com/vaadin/server/VaadinServletTest.java b/server/src/test/java/com/vaadin/server/VaadinServletTest.java
index 006872b212..2c318e0897 100644
--- a/server/src/test/java/com/vaadin/server/VaadinServletTest.java
+++ b/server/src/test/java/com/vaadin/server/VaadinServletTest.java
@@ -233,7 +233,7 @@ public class VaadinServletTest {
     public void isAllowedVAADINResource_jarWarFileScheme_detectsAsStaticResources()
             throws IOException, URISyntaxException, ServletException {
         assertTrue("Can not run concurrently with other test",
-                VaadinServlet.openFileSystems.isEmpty());
+                VaadinServlet.OPEN_FILE_SYSTEMS.isEmpty());
 
         VaadinServlet servlet = new VaadinServlet();
         servlet.init(new MockServletConfig());
@@ -276,7 +276,7 @@ public class VaadinServletTest {
     public void isAllowedVAADINResource_jarInAJar_detectsAsStaticResources()
             throws IOException, URISyntaxException, ServletException {
         assertTrue("Can not run concurrently with other test",
-                VaadinServlet.openFileSystems.isEmpty());
+                VaadinServlet.OPEN_FILE_SYSTEMS.isEmpty());
 
         VaadinServlet servlet = new VaadinServlet();
         servlet.init(new MockServletConfig());
@@ -325,7 +325,7 @@ public class VaadinServletTest {
     public void openingJarFileSystemForDifferentFilesInSameJar_existingFileSystemIsUsed()
             throws IOException, URISyntaxException, ServletException {
         assertTrue("Can not run concurrently with other test",
-                VaadinServlet.openFileSystems.isEmpty());
+                VaadinServlet.OPEN_FILE_SYSTEMS.isEmpty());
 
         VaadinServlet servlet = new VaadinServlet();
         servlet.init(new MockServletConfig());
@@ -348,15 +348,15 @@ public class VaadinServletTest {
             servlet.getFileSystem(fileResourceURL.toURI());
 
             assertEquals("Same file should be marked for both resources",
-                    (Integer) 2, VaadinServlet.openFileSystems.entrySet()
+                    (Integer) 2, VaadinServlet.OPEN_FILE_SYSTEMS.entrySet()
                             .iterator().next().getValue());
             servlet.closeFileSystem(folderResourceURL.toURI());
             assertEquals("Closing resource should be removed from jar uri",
-                    (Integer) 1, VaadinServlet.openFileSystems.entrySet()
+                    (Integer) 1, VaadinServlet.OPEN_FILE_SYSTEMS.entrySet()
                             .iterator().next().getValue());
             servlet.closeFileSystem(fileResourceURL.toURI());
             assertTrue("Closing last resource should clear marking",
-                    VaadinServlet.openFileSystems.isEmpty());
+                    VaadinServlet.OPEN_FILE_SYSTEMS.isEmpty());
 
             try {
                 FileSystems.getFileSystem(folderResourceURL.toURI());
@@ -375,7 +375,7 @@ public class VaadinServletTest {
             throws IOException, InterruptedException, ExecutionException,
             URISyntaxException, ServletException {
         assertTrue("Can not run concurrently with other test",
-                VaadinServlet.openFileSystems.isEmpty());
+                VaadinServlet.OPEN_FILE_SYSTEMS.isEmpty());
 
         VaadinServlet servlet = new VaadinServlet();
         servlet.init(new MockServletConfig());
@@ -547,7 +547,7 @@ public class VaadinServletTest {
     private void ensureFileSystemsCleared(URL fileResourceURL)
             throws URISyntaxException {
         assertFalse("URI should have been cleared",
-                VaadinServlet.openFileSystems
+                VaadinServlet.OPEN_FILE_SYSTEMS
                         .containsKey(fileResourceURL.toURI()));
         try {
             FileSystems.getFileSystem(fileResourceURL.toURI());