Add Nullable annotation to parameter of overridden equals method.

Closes #1374
Original pull request #1375

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/convert/Identifier.java

@@ -208,7 +208,7 @@ public final class Identifier {
 		}
 
 		@Override
-		public boolean equals(Object o) {
+		public boolean equals(@Nullable Object o) {
 
 			if (this == o)
 				return true;
@@ -272,7 +272,7 @@ public final class Identifier {
 	}
 
 	@Override
-	public boolean equals(Object o) {
+	public boolean equals(@Nullable Object o) {
 
 		if (this == o) return true;
 		if (o == null || getClass() != o.getClass()) return false;

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/convert/InsertSubject.java

@@ -17,6 +17,8 @@ package org.springframework.data.jdbc.core.convert;
 
 import java.util.Objects;
 
+import org.springframework.lang.Nullable;
+
 /**
  * The subject of an insert, described by the entity instance and its {@link Identifier}, where identifier contains
  * information about data that needs to be considered for the insert but which is not part of the entity. Namely
@@ -50,7 +52,7 @@ public final class InsertSubject<T> {
 	}
 
 	@Override
-	public boolean equals(Object o) {
+	public boolean equals(@Nullable Object o) {
 
 		if (this == o)
 			return true;

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/convert/SqlGenerator.java

@@ -1001,7 +1001,7 @@ class SqlGenerator {
 		}
 
 		@Override
-		public boolean equals(Object o) {
+		public boolean equals(@Nullable Object o) {
 
 			if (this == o) {
 				return true;

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/mapping/AggregateReference.java

@@ -65,7 +65,7 @@ public interface AggregateReference<T, ID> {
 		}
 
 		@Override
-		public boolean equals(Object o) {
+		public boolean equals(@Nullable Object o) {
 
 			if (this == o)
 				return true;

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/mapping/JdbcValue.java

@@ -55,7 +55,7 @@ public class JdbcValue {
 	}
 
 	@Override
-	public boolean equals(Object o) {
+	public boolean equals(@Nullable Object o) {
 
 		if (this == o)
 			return true;

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/repository/query/JdbcQueryCreator.java

@@ -352,7 +352,7 @@ class JdbcQueryCreator extends RelationalQueryCreator<ParametrizedQuery> {
 		}
 
 		@Override
-		public boolean equals(Object o) {
+		public boolean equals(@Nullable Object o) {
 
 			if (this == o)
 				return true;