DATACMNS-1141 - Add missing NonNullApi and Nullable annotations.

src/main/java/org/springframework/data/domain/Persistable.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2008-2011 the original author or authors.
+ * Copyright 2008-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -15,25 +15,29 @@
  */
 package org.springframework.data.domain;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Simple interface for entities.
  * 
  * @param <ID> the type of the identifier
  * @author Oliver Gierke
+ * @author Christoph Strobl
  */
 public interface Persistable<ID> {
 
 	/**
 	 * Returns the id of the entity.
 	 * 
-	 * @return the id
+	 * @return the id. Can be {@literal null}.
 	 */
+	@Nullable
 	ID getId();
 
 	/**
 	 * Returns if the {@code Persistable} is new or was persisted already.
 	 * 
-	 * @return if the object is new
+	 * @return if {@literal true} the object is new.
 	 */
 	boolean isNew();
 }