Update javadoc & switch method defaulting.

Closes #5010
Original pull request: #5015

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

@@ -116,7 +116,7 @@ public class DefaultIndexOperations implements IndexOperations {
 
 	@Override
 	@SuppressWarnings("NullAway")
-	public String ensureIndex(IndexDefinition indexDefinition) {
+	public String createIndex(IndexDefinition indexDefinition) {
 
 		return execute(collection -> {
 

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

@@ -84,7 +84,7 @@ public class DefaultReactiveIndexOperations implements ReactiveIndexOperations {
 
 	@Override
 	@SuppressWarnings("NullAway")
-	public Mono<String> ensureIndex(IndexDefinition indexDefinition) {
+	public Mono<String> createIndex(IndexDefinition indexDefinition) {
 
 		return mongoOperations.execute(collectionName, collection -> {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/index/IndexOperations.java

@@ -36,19 +36,20 @@ public interface IndexOperations {
 	 * @deprecated since 4.5, in favor of {@link #createIndex(IndexDefinition)}.
 	 */
 	@Deprecated(since = "4.5", forRemoval = true)
-	String ensureIndex(IndexDefinition indexDefinition);
+	default String ensureIndex(IndexDefinition indexDefinition) {
+		return createIndex(indexDefinition);
+	}
 
 	/**
-	 * Create the index for the provided {@link IndexDefinition} exists for the collection indicated by the entity class.
-	 * If not it will be created.
+	 * Create the index for the provided {@link IndexDefinition} for the collection indicated by the entity class. If the
+	 * index does not exist it will be created. Might error if the collection already defines an index with the same name
+	 * but different settings.
 	 *
 	 * @param indexDefinition must not be {@literal null}.
 	 * @return the index name.
 	 * @since 4.5
 	 */
-	default String createIndex(IndexDefinition indexDefinition) {
-		return ensureIndex(indexDefinition);
-	}
+	String createIndex(IndexDefinition indexDefinition);
 
 	/**
 	 * Alters the index with given {@literal name}.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/index/IndexOperationsAdapter.java

@@ -41,8 +41,8 @@ public interface IndexOperationsAdapter extends IndexOperations {
 		return new IndexOperationsAdapter() {
 
 			@Override
-			public String ensureIndex(IndexDefinition indexDefinition) {
-				return reactiveIndexOperations.ensureIndex(indexDefinition).block();
+			public String createIndex(IndexDefinition indexDefinition) {
+				return reactiveIndexOperations.createIndex(indexDefinition).block();
 			}
 
 			@Override

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/index/ReactiveIndexOperations.java

@@ -36,19 +36,20 @@ public interface ReactiveIndexOperations {
 	 * @deprecated since 4.5, in favor of {@link #createIndex(IndexDefinition)}.
 	 */
 	@Deprecated(since = "4.5", forRemoval = true)
-	Mono<String> ensureIndex(IndexDefinition indexDefinition);
+	default Mono<String> ensureIndex(IndexDefinition indexDefinition) {
+		return createIndex(indexDefinition);
+	}
 
 	/**
-	 * Create the index for the provided {@link IndexDefinition} exists for the collection indicated by the entity class.
-	 * If not it will be created.
+	 * Create the index for the provided {@link IndexDefinition} for the collection indicated by the entity class. If the
+	 * index does not exist it will be created. Might error if the collection already defines an index with the same name
+	 * but different settings.
 	 *
 	 * @param indexDefinition must not be {@literal null}.
 	 * @return the index name.
 	 * @since 4.5
 	 */
-	default Mono<String> createIndex(IndexDefinition indexDefinition) {
-		return ensureIndex(indexDefinition);
-	}
+	Mono<String> createIndex(IndexDefinition indexDefinition);
 
 	/**
 	 * Alters the index with given {@literal name}.