Add @FunctionalInterface annotation to interfaces where applicable (#7954)

Closes vaadin/framework8-issues#548

server/src/main/java/com/vaadin/data/BinderValidationStatusHandler.java

@@ -44,6 +44,7 @@ import com.vaadin.ui.AbstractComponent;
  *
  * @since 8.0
  */
+@FunctionalInterface
 public interface BinderValidationStatusHandler<BEAN>
         extends Consumer<BinderValidationStatus<BEAN>>, Serializable {
 

server/src/main/java/com/vaadin/data/ValidationStatusHandler.java

@@ -38,6 +38,7 @@ import com.vaadin.ui.AbstractComponent;
  * @since 8.0
  *
  */
+@FunctionalInterface
 public interface ValidationStatusHandler
         extends Consumer<ValidationStatus<?>>, Serializable {
 

server/src/main/java/com/vaadin/event/Action.java

@@ -90,6 +90,7 @@ public class Action implements Serializable {
      * Action.Handler interface.<br/>
      *
      */
+    @FunctionalInterface
     public interface Listener extends Serializable {
         public void handleAction(Object sender, Object target);
     }

server/src/main/java/com/vaadin/event/ContextClickEvent.java

@@ -46,6 +46,7 @@ public class ContextClickEvent extends ClickEvent {
     /**
      * Listener for {@link ContextClickEvent ContextClickEvents}.
      */
+    @FunctionalInterface
     public interface ContextClickListener extends Serializable {
 
         /**

server/src/main/java/com/vaadin/event/FieldEvents.java

@@ -109,6 +109,7 @@ public interface FieldEvents {
      * @see FocusEvent
      * @since 6.2
      */
+    @FunctionalInterface
     public interface FocusListener extends ConnectorEventListener {
 
         public static final Method focusMethod = ReflectTools
@@ -149,6 +150,7 @@ public interface FieldEvents {
      * @see BlurEvent
      * @since 6.2
      */
+    @FunctionalInterface
     public interface BlurListener extends ConnectorEventListener {
 
         public static final Method blurMethod = ReflectTools

server/src/main/java/com/vaadin/event/LayoutEvents.java

@@ -28,6 +28,7 @@ import com.vaadin.util.ReflectTools;
 
 public interface LayoutEvents {
 
+    @FunctionalInterface
     public interface LayoutClickListener extends ConnectorEventListener {
 
         public static final Method clickMethod = ReflectTools.findMethod(

server/src/main/java/com/vaadin/event/MouseEvents.java

@@ -192,6 +192,7 @@ public interface MouseEvents {
      * @author Vaadin Ltd.
      * @since 6.2
      */
+    @FunctionalInterface
     public interface ClickListener extends ConnectorEventListener {
 
         public static final Method clickMethod = ReflectTools
@@ -230,6 +231,7 @@ public interface MouseEvents {
      * @author Vaadin Ltd.
      * @since 6.2
      */
+    @FunctionalInterface
     public interface DoubleClickListener extends ConnectorEventListener {
 
         public static final Method doubleClickMethod = ReflectTools.findMethod(

server/src/main/java/com/vaadin/event/SortEvent.java

@@ -75,6 +75,7 @@ public class SortEvent extends Component.Event {
     /**
      * Listener for sort order change events.
      */
+    @FunctionalInterface
     public interface SortListener extends Serializable {
         /**
          * Called when the sort order has changed.

server/src/main/java/com/vaadin/event/UIEvents.java

@@ -39,6 +39,7 @@ public interface UIEvents {
      * @since 7.2
      * @author Vaadin Ltd
      */
+    @FunctionalInterface
     public interface PollListener extends Serializable {
         public static final Method POLL_METHOD = ReflectTools
                 .findMethod(PollListener.class, "poll", PollEvent.class);

server/src/main/java/com/vaadin/event/selection/MultiSelectionListener.java

@@ -28,6 +28,7 @@ import java.util.function.Consumer;
  * @param <T>
  *            the data type of the selection model
  */
+@FunctionalInterface
 public interface MultiSelectionListener<T>
         extends Consumer<MultiSelectionEvent<T>>, Serializable {
     @Override

server/src/main/java/com/vaadin/navigator/ViewDisplay.java

@@ -27,6 +27,7 @@ import java.io.Serializable;
  * @author Vaadin Ltd
  * @since 7.0
  */
+@FunctionalInterface
 public interface ViewDisplay extends Serializable {
     /**
      * Remove previously shown view and show the newly selected view in its

server/src/main/java/com/vaadin/server/ClientConnector.java

@@ -55,6 +55,7 @@ public interface ClientConnector extends Connector {
      * Interface for listening {@link AttachEvent connector attach events}.
      *
      */
+    @FunctionalInterface
     public static interface AttachListener extends ConnectorEventListener {
         public static final Method attachMethod = ReflectTools
                 .findMethod(AttachListener.class, "attach", AttachEvent.class);
@@ -83,6 +84,7 @@ public interface ClientConnector extends Connector {
      * Interface for listening {@link DetachEvent connector detach events}.
      *
      */
+    @FunctionalInterface
     public static interface DetachListener extends ConnectorEventListener {
         public static final Method detachMethod = ReflectTools
                 .findMethod(DetachListener.class, "detach", DetachEvent.class);

server/src/main/java/com/vaadin/server/ErrorHandler.java

@@ -20,6 +20,7 @@ import java.io.Serializable;
 /**
  * Interface for listening to errors in the application.
  */
+@FunctionalInterface
 public interface ErrorHandler extends Serializable {
 
     /**

server/src/main/java/com/vaadin/server/Page.java

@@ -49,6 +49,7 @@ public class Page implements Serializable {
      *
      * @see UI#addListener(BrowserWindowResizeListener)
      */
+    @FunctionalInterface
     public interface BrowserWindowResizeListener extends Serializable {
         /**
          * Invoked when the browser window containing a UI has been resized.
@@ -251,6 +252,7 @@ public class Page implements Serializable {
      *
      * @see Page#addUriFragmentChangedListener(UriFragmentChangedListener)
      */
+    @FunctionalInterface
     public interface UriFragmentChangedListener extends Serializable {
         /**
          * Event handler method invoked when the URI fragment of the page
@@ -326,6 +328,7 @@ public class Page implements Serializable {
         }
     }
 
+    @FunctionalInterface
     private static interface InjectedStyle extends Serializable {
         public void paint(int id, PaintTarget target) throws PaintException;
     }

server/src/main/java/com/vaadin/server/RequestHandler.java

@@ -26,6 +26,7 @@ import com.vaadin.ui.UI;
  * added on a {@link VaadinService service} level, common for all users, or on a
  * {@link VaadinSession session} level for only a single user.
  */
+@FunctionalInterface
 public interface RequestHandler extends Serializable {
 
     /**

server/src/main/java/com/vaadin/server/ServiceDestroyListener.java

@@ -28,6 +28,7 @@ import java.io.Serializable;
  * @since 7.2
  * @author Vaadin Ltd
  */
+@FunctionalInterface
 public interface ServiceDestroyListener extends Serializable {
     /**
      * Invoked when a service is destroyed

server/src/main/java/com/vaadin/server/SessionDestroyListener.java

@@ -27,6 +27,7 @@ import java.io.Serializable;
  * @author Vaadin Ltd
  * @since 7.0.0
  */
+@FunctionalInterface
 public interface SessionDestroyListener extends Serializable {
     /**
      * Called when a Vaadin service session is no longer used.

server/src/main/java/com/vaadin/server/SessionInitListener.java

@@ -32,6 +32,7 @@ import java.io.Serializable;
  * @author Vaadin Ltd
  * @since 7.0.0
  */
+@FunctionalInterface
 public interface SessionInitListener extends Serializable {
     /**
      * Invoked when a new Vaadin service session is initialized for that

server/src/main/java/com/vaadin/server/StreamResource.java

@@ -153,6 +153,7 @@ public class StreamResource implements ConnectorResource {
      * @author Vaadin Ltd.
      * @since 3.0
      */
+    @FunctionalInterface
     public interface StreamSource extends Serializable {
 
         /**

server/src/main/java/com/vaadin/server/SystemMessagesProvider.java

@@ -27,6 +27,7 @@ import com.vaadin.ui.UI;
  * @author Vaadin Ltd
  * @since 7.0.0
  */
+@FunctionalInterface
 public interface SystemMessagesProvider extends Serializable {
     /**
      * Gets the system messages to use in the given context. The

server/src/main/java/com/vaadin/server/VaadinServiceInitListener.java

@@ -36,6 +36,7 @@ import java.util.ServiceLoader;
  * @since
  * @author Vaadin Ltd
  */
+@FunctionalInterface
 public interface VaadinServiceInitListener extends EventListener, Serializable {
     /**
      * Run when a {@link VaadinService} instance is initialized.

server/src/main/java/com/vaadin/server/ViewportGenerator.java

@@ -26,6 +26,7 @@ import java.io.Serializable;
  *
  * @author Vaadin Ltd
  */
+@FunctionalInterface
 public interface ViewportGenerator extends Serializable {
     /**
      * Generates a viewport tag based on a request.

server/src/main/java/com/vaadin/server/data/DataProviderListener.java

@@ -24,6 +24,7 @@ import java.io.Serializable;
  * @author Vaadin Ltd
  * @since 8.0
  */
+@FunctionalInterface
 public interface DataProviderListener extends Serializable {
 
     /**

server/src/main/java/com/vaadin/ui/AbstractSplitPanel.java

@@ -503,6 +503,7 @@ public abstract class AbstractSplitPanel extends AbstractComponentContainer {
      * @see SplitterClickEvent
      * @since 6.2
      */
+    @FunctionalInterface
     public interface SplitterClickListener extends ConnectorEventListener {
 
         public static final Method clickMethod = ReflectTools.findMethod(
@@ -533,6 +534,7 @@ public abstract class AbstractSplitPanel extends AbstractComponentContainer {
      *
      * @since 7.5.0
      */
+    @FunctionalInterface
     public interface SplitPositionChangeListener
             extends ConnectorEventListener {
 

server/src/main/java/com/vaadin/ui/Button.java

@@ -288,6 +288,7 @@ public class Button extends AbstractFocusable
      * @author Vaadin Ltd.
      * @since 3.0
      */
+    @FunctionalInterface
     public interface ClickListener extends Serializable {
 
         public static final Method BUTTON_CLICK_METHOD = ReflectTools
@@ -302,7 +303,6 @@ public class Button extends AbstractFocusable
          *            An event containing information about the click.
          */
         public void buttonClick(ClickEvent event);
-
     }
 
     /**

server/src/main/java/com/vaadin/ui/HasComponents.java

@@ -92,6 +92,7 @@ public interface HasComponents extends Component, Iterable<Component> {
     /**
      * Component attach listener interface.
      */
+    @FunctionalInterface
     public interface ComponentAttachListener extends Serializable {
 
         public static final Method attachMethod = ReflectTools.findMethod(
@@ -110,6 +111,7 @@ public interface HasComponents extends Component, Iterable<Component> {
     /**
      * Component detach listener interface.
      */
+    @FunctionalInterface
     public interface ComponentDetachListener extends Serializable {
 
         public static final Method detachMethod = ReflectTools.findMethod(

server/src/main/java/com/vaadin/ui/JavaScriptFunction.java

@@ -34,6 +34,7 @@ import elemental.json.JsonArray;
  * @author Vaadin Ltd
  * @since 7.0.0
  */
+@FunctionalInterface
 public interface JavaScriptFunction extends Serializable {
     /**
      * Invoked whenever the corresponding JavaScript function is called in the

server/src/main/java/com/vaadin/ui/LoginForm.java

@@ -99,6 +99,7 @@ public class LoginForm extends AbstractSingleComponentContainer {
     /**
      * Listener triggered when a login occurs in a {@link LoginForm}.
      */
+    @FunctionalInterface
     public interface LoginListener extends Serializable {
         /**
          * Event method invoked when the login button is pressed in a login

server/src/main/java/com/vaadin/ui/MenuBar.java

@@ -434,6 +434,7 @@ public class MenuBar extends AbstractComponent
      * clicks on the containing {@link com.vaadin.ui.MenuBar.MenuItem}. The
      * selected item is given as an argument.
      */
+    @FunctionalInterface
     public interface Command extends Serializable {
         public void menuSelected(MenuBar.MenuItem selectedItem);
     }

server/src/main/java/com/vaadin/ui/PopupView.java

@@ -399,6 +399,7 @@ public class PopupView extends AbstractComponent implements HasComponents {
      * visibility of the popup changes.
      *
      */
+    @FunctionalInterface
     public interface PopupVisibilityListener extends Serializable {
         /**
          * Pass to {@link PopupView#PopupVisibilityEvent} to start listening for

server/src/main/java/com/vaadin/ui/TabSheet.java

@@ -809,6 +809,7 @@ public class TabSheet extends AbstractComponentContainer
      *
      * @since 3.0
      */
+    @FunctionalInterface
     public interface SelectedTabChangeListener extends Serializable {
 
         /**
@@ -1288,6 +1289,7 @@ public class TabSheet extends AbstractComponentContainer
      * @since 6.2.0
      *
      */
+    @FunctionalInterface
     public interface CloseHandler extends Serializable {
 
         /**

server/src/main/java/com/vaadin/ui/UI.java

@@ -1847,6 +1847,7 @@ public abstract class UI extends AbstractSingleComponentContainer
      * 
      * @see Window.WindowOrderChangeEvent
      */
+    @FunctionalInterface
     public interface WindowOrderUpdateListener extends ConnectorEventListener {
 
         public static final Method windowOrderUpdateMethod = ReflectTools

server/src/main/java/com/vaadin/ui/Upload.java

@@ -201,6 +201,7 @@ public class Upload extends AbstractComponent
      * @author Vaadin Ltd.
      * @since 3.0
      */
+    @FunctionalInterface
     public interface Receiver extends Serializable {
 
         /**
@@ -214,7 +215,6 @@ public class Upload extends AbstractComponent
          * @return Stream to which the uploaded file should be written.
          */
         public OutputStream receiveUpload(String filename, String mimeType);
-
     }
 
     /* Upload events */
@@ -544,6 +544,7 @@ public class Upload extends AbstractComponent
      * @author Vaadin Ltd.
      * @since 5.0
      */
+    @FunctionalInterface
     public interface StartedListener extends Serializable {
 
         /**
@@ -561,6 +562,7 @@ public class Upload extends AbstractComponent
      * @author Vaadin Ltd.
      * @since 3.0
      */
+    @FunctionalInterface
     public interface FinishedListener extends Serializable {
 
         /**
@@ -578,6 +580,7 @@ public class Upload extends AbstractComponent
      * @author Vaadin Ltd.
      * @since 3.0
      */
+    @FunctionalInterface
     public interface FailedListener extends Serializable {
 
         /**
@@ -595,6 +598,7 @@ public class Upload extends AbstractComponent
      * @author Vaadin Ltd.
      * @since 3.0
      */
+    @FunctionalInterface
     public interface SucceededListener extends Serializable {
 
         /**
@@ -611,6 +615,7 @@ public class Upload extends AbstractComponent
      *
      * @since 7.2
      */
+    @FunctionalInterface
     public interface ChangeListener extends Serializable {
 
         Method FILENAME_CHANGED = ReflectTools.findMethod(ChangeListener.class,
@@ -954,6 +959,7 @@ public class Upload extends AbstractComponent
     /**
      * ProgressListener receives events to track progress of upload.
      */
+    @FunctionalInterface
     public interface ProgressListener extends Serializable {
         /**
          * Updates progress to listener

server/src/main/java/com/vaadin/ui/Window.java

@@ -435,6 +435,7 @@ public class Window extends Panel
      * 
      * @see UI.WindowOrderUpdateListener
      */
+    @FunctionalInterface
     public interface WindowOrderChangeListener extends ConnectorEventListener {
 
         public static final Method windowOrderChangeMethod = ReflectTools
@@ -495,6 +496,7 @@ public class Window extends Panel
      * fires the CloseListener.
      * </p>
      */
+    @FunctionalInterface
     public interface CloseListener extends Serializable {
         /**
          * Called when the user closes a window. Use
@@ -596,6 +598,7 @@ public class Window extends Panel
      * will be called whenever the window is maximized (
      * {@link WindowMode#MAXIMIZED}) or restored ({@link WindowMode#NORMAL} ).
      */
+    @FunctionalInterface
     public interface WindowModeChangeListener extends Serializable {
 
         public static final Method windowModeChangeMethod = ReflectTools
@@ -694,6 +697,7 @@ public class Window extends Panel
      *
      * @see com.vaadin.ui.Window.ResizeEvent
      */
+    @FunctionalInterface
     public interface ResizeListener extends Serializable {
         public void windowResized(ResizeEvent e);
     }

server/src/main/java/com/vaadin/ui/declarative/DesignContext.java

@@ -674,6 +674,7 @@ public class DesignContext implements Serializable {
      *
      * @author Vaadin Ltd
      */
+    @FunctionalInterface
     public interface ComponentCreationListener extends Serializable {
 
         /**

server/src/main/java/com/vaadin/ui/declarative/ShouldWriteDataDelegate.java

@@ -28,6 +28,7 @@ import com.vaadin.ui.Component;
  * @since 7.5.0
  * @author Vaadin Ltd
  */
+@FunctionalInterface
 public interface ShouldWriteDataDelegate extends Serializable {
 
     /**

server/src/main/java/com/vaadin/ui/renderers/ClickableRenderer.java

@@ -48,6 +48,7 @@ public abstract class ClickableRenderer<T, V> extends AbstractRenderer<T, V> {
      *
      * @see ButtonRenderer#addClickListener(RendererClickListener)
      */
+    @FunctionalInterface
     public interface RendererClickListener<T> extends ConnectorEventListener {
 
         static final Method CLICK_METHOD = ReflectTools.findMethod(