Polishing.

Extract duplicates into peek method.

See #4312
Original pull request: #4323

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MappingMongoConverter.java

@@ -679,8 +679,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		 * in between.
 		 */
 		if (value instanceof Document document) {
-			if(property.isMap()) {
+			if (property.isMap()) {
-				if(document.isEmpty() || document.values().iterator().next() instanceof DBRef) {
+				if (document.isEmpty() || peek(document.values()) instanceof DBRef) {
 					accessor.setProperty(property, dbRefResolver.resolveDbRef(property, null, callback, handler));
 				} else {
 					accessor.setProperty(property, readMap(context, document, property.getTypeInformation()));
@@ -688,8 +688,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			} else {
 				accessor.setProperty(property, read(property.getActualType(), document));
 			}
-		} else if (value instanceof Collection<?> collection && collection.size() > 0
+		} else if (value instanceof Collection<?> collection && !collection.isEmpty()
-				&& collection.iterator().next() instanceof Document) {
+				&& peek(collection) instanceof Document) {
 			accessor.setProperty(property, readCollectionOrArray(context, collection, property.getTypeInformation()));
 		} else {
 			accessor.setProperty(property, dbRefResolver.resolveDbRef(property, null, callback, handler));
@@ -722,8 +722,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		}
 
 		// DATAMONGO-913
-		if (object instanceof LazyLoadingProxy) {
+		if (object instanceof LazyLoadingProxy proxy) {
-			return ((LazyLoadingProxy) object).toDBRef();
+			return proxy.toDBRef();
 		}
 
 		return createDBRef(object, referringProperty);
@@ -1022,11 +1022,13 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 							.getPointer();
 				}).collect(Collectors.toList());
 
-				return writeCollectionInternal(targetCollection, TypeInformation.of(DocumentPointer.class), new ArrayList<>(targetCollection.size()));
+				return writeCollectionInternal(targetCollection, TypeInformation.of(DocumentPointer.class),
+						new ArrayList<>(targetCollection.size()));
 			}
 
 			if (property.hasExplicitWriteTarget()) {
-				return writeCollectionInternal(collection, new FieldTypeInformation<>(property), new ArrayList<>(collection.size()));
+				return writeCollectionInternal(collection, new FieldTypeInformation<>(property),
+						new ArrayList<>(collection.size()));
 			}
 
 			return writeCollectionInternal(collection, property.getTypeInformation(), new ArrayList<>(collection.size()));
@@ -1674,7 +1676,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		}
 
 		List<Object> result = bulkReadAndConvertDBRefs(context, Collections.singletonList(dbref), type);
-		return CollectionUtils.isEmpty(result) ? null : result.iterator().next();
+		return CollectionUtils.isEmpty(result) ? null : peek(result);
 	}
 
 	@SuppressWarnings({ "unchecked", "rawtypes" })
@@ -1699,10 +1701,9 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			return Collections.emptyList();
 		}
 
-		List<Document> referencedRawDocuments = dbrefs.size() == 1
+		List<Document> referencedRawDocuments = dbrefs.size() == 1 ? Collections.singletonList(readRef(peek(dbrefs)))
-				? Collections.singletonList(readRef(dbrefs.iterator().next()))
 				: bulkReadRefs(dbrefs);
-		String collectionName = dbrefs.iterator().next().getCollectionName();
+		String collectionName = peek(dbrefs).getCollectionName();
 
 		List<T> targetList = new ArrayList<>(dbrefs.size());
 
@@ -1847,6 +1848,10 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		return true;
 	}
 
+	private static <T> T peek(Iterable<T> result) {
+		return result.iterator().next();
+	}
+
 	static Predicate<MongoPersistentProperty> isIdentifier(PersistentEntity<?, ?> entity) {
 		return entity::isIdProperty;
 	}
@@ -1925,7 +1930,8 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 		public MongoDbPropertyValueProvider withContext(ConversionContext context) {
 
-			return context == this.context ? this : new MongoDbPropertyValueProvider(context, accessor, evaluator);
+			return context == this.context ? this
+					: new MongoDbPropertyValueProvider(context, accessor, evaluator);
 		}
 	}