driver-core/src/main/com/mongodb/client/model/Indexes.java
public static Bson ascending(final List<String> fieldNames) {
        notNull("fieldNames", fieldNames);
        return compoundIndex(fieldNames, new BsonInt32(1));
    }
public static Bson ascending(final List<String> fieldNames) {
        notNull("fieldNames", fieldNames);
        return compoundIndex(fieldNames, new BsonInt32(1));
    }
Create an index key for an ascending index on the given fields.

@param fieldNames the field names, which must contain at least one
@return the index specification
@mongodb.driver.manual core/indexes indexes