Functional interfaces should be annotated as functional interfaces

Mage.Client/src/main/java/mage/client/util/Command.java

@@ -3,7 +3,7 @@ package mage.client.util;
 /**
  * @author nantuko
  */
+@FunctionalInterface
 public interface Command {
-
     void execute();
 }

Mage.Client/src/main/java/mage/client/util/Listener.java

@@ -34,6 +34,7 @@ import java.io.Serializable;
  * @author BetaSteward_at_googlemail.com
  * @param <E>
  */
+@FunctionalInterface
 public interface Listener<E extends Event> extends Serializable {
 
     void event(E event);

Mage.Client/src/main/java/org/mage/card/arcane/TextboxRule.java

@@ -23,6 +23,7 @@ public class TextboxRule {
 
     // An attributed region in the text, which can be applied to an
     // attributed string.
+    @FunctionalInterface
     public interface AttributeRegion {
 
         void applyToAttributedString(AttributedString str, Font normal, Font italic);