EntityIndex (Oracle - Berkeley DB Java API)
Date: 2017-04-13

com.sleepycat.persist

## Interface EntityIndex<K,V>

- All Known Implementing Classes:
    PrimaryIndex, SecondaryIndex

      public interface EntityIndex<K,V>

  The interface for accessing keys and entities via a primary or secondary index.

  `EntityIndex` objects are thread-safe. Multiple threads may safely call the methods of a shared `EntityIndex` object.

  An index is conceptually a map. {key:value} mappings are stored in the index and accessed by key. In fact, for interoperability with other libraries that use the standard Java `Map` or `SortedMap` interfaces, an `EntityIndex` may be accessed via these standard interfaces by calling the `map()` or `sortedMap()` methods.

  `EntityIndex` is an interface that is implemented by several classes in this package for different purposes. Depending on the context, the key type (K) and value type (V) of the index take on different meanings. The different classes that implement `EntityIndex` are:

  - `PrimaryIndex` maps primary keys to entities.
  - `SecondaryIndex` maps secondary keys to entities.
  - `SecondaryIndex.keysIndex` maps secondary keys to primary keys.
  - `SecondaryIndex.subIndex(SK)` maps primary keys to entities, for the subset of entities having a specified secondary key.

  In all cases, the index key type (K) is a primary or secondary key class. The index value type (V) is an entity class in all cases except for a `SecondaryIndex.keysIndex`, when it is a primary key class.

  In the following example, a `Employee` entity with a `MANY_TO_ONE` secondary key is defined.

       @Entity
       class Employee {

           @PrimaryKey
           long id;

           @SecondaryKey(relate=MANY_TO_ONE)
           String department;

           String name;

           private Employee() {}
       }

  Consider that we have stored the entities below:

  Entities
  ID | Department | Name
  --- | --- | ---
  1 | Engineering | Jane Smith
  2 | Sales | Joan Smith
  3 | Engineering | John Smith
  4 | Sales | Jim Smith

  `PrimaryIndex` maps primary keys to entities:

       PrimaryIndex<Long, Employee> primaryIndex =
           store.getPrimaryIndex(Long.class, Employee.class);

  primaryIndex
  Primary Key | Entity
  --- | ---
  1 | 1 | Engineering | Jane Smith
  2 | 2 | Sales | Joan Smith
  3 | 3 | Engineering | John Smith
  4 | 4 | Sales | Jim Smith

  `SecondaryIndex` maps secondary keys to entities:

       SecondaryIndex<String, Long, Employee> secondaryIndex =
           store.getSecondaryIndex(primaryIndex, String.class, "department");

  secondaryIndex
  Secondary Key | Entity
  --- | ---
  Engineering | 1 | Engineering | Jane Smith
  Engineering | 3 | Engineering | John Smith
  Sales | 2 | Sales | Joan Smith
  Sales | 4 | Sales | Jim Smith

  `SecondaryIndex.keysIndex` maps secondary keys to primary keys:

       EntityIndex<String, Long> keysIndex = secondaryIndex.keysIndex();

  keysIndex
  Secondary Key | Primary Key
  --- | ---
  Engineering | 1
  Engineering | 3
  Sales | 2
  Sales | 4

  `SecondaryIndex.subIndex(SK)` maps primary keys to entities, for the subset of entities having a specified secondary key:

       EntityIndex<Long, Entity> subIndex = secondaryIndex.subIndex("Engineering");

  subIndex
  Primary Key | Entity
  --- | ---
  1 | 1 | Engineering | Jane Smith
  3 | 3 | Engineering | John Smith

  ### Accessing the Index

  An `EntityIndex` provides a variety of methods for retrieving entities from an index. It also provides methods for deleting entities. However, it does not provide methods for inserting and updating. To insert and update entities, use the `PrimaryIndex.put(E)` family of methods in the `PrimaryIndex` class.

  An `EntityIndex` supports two mechanisms for retrieving entities:

  1. The `get(K)` method returns a single value for a given key. If there are multiple values with the same secondary key (duplicates), it returns the first entity in the duplicate set.
  2. An `EntityCursor` can be obtained using the `keys()` and `entities()` family of methods. A cursor can be used to return all values in the index, including duplicates. A cursor can also be used to return values within a specified range of keys.

  Using the example entities above, calling `get(K)` on the primary index will always return the employee with the given ID, or null if no such ID exists. But calling `get(K)` on the secondary index will retrieve the first employee in the given department, which may not be very useful:

       Employee emp = primaryIndex.get(1);      // Returns by unique ID
       emp = secondaryIndex.get("Engineering"); // Returns first in department

  Using a cursor, you can iterate through all duplicates in the secondary index:

       EntityCursor<Employee> cursor = secondaryIndex.entities();
       try {
           for (Employee entity : cursor) {
               if (entity.department.equals("Engineering")) {
                   // Do something with the entity...
               }
           }
       } finally {
           cursor.close();
       }

  But for a large database it is much more efficient to iterate over only those entities with the secondary key you're searching for. This could be done by restricting a cursor to a range of keys:

       EntityCursor<Employee> cursor =
           secondaryIndex.entities("Engineering", true, "Engineering", true);
       try {
           for (Employee entity : cursor) {
               // Do something with the entity...
           }
       } finally {
           cursor.close();
       }

  However, when you are interested only in the entities with a particular secondary key value, it is more convenient to use a sub-index:

       EntityIndex<Long, Entity> subIndex = secondaryIndex.subIndex("Engineering");
       EntityCursor<Employee> cursor = subIndex.entities();
       try {
           for (Employee entity : cursor) {
               // Do something with the entity...
           }
       } finally {
           cursor.close();
       }

  In addition to being more convenient than a cursor range, a sub-index allows retrieving by primary key:

       Employee emp = subIndex.get(1);

  When using a sub-index, all operations performed on the sub-index are restricted to the single key that was specified when the sub-index was created. For example, the following returns null because employee 2 is not in the Engineering department and therefore is not part of the sub-index:

       Employee emp = subIndex.get(2);

  For more information on using cursors and cursor ranges, see `EntityCursor`.

  Note that when using an index, keys and values are stored and retrieved by value not by reference. In other words, if an entity object is stored and then retrieved, or retrieved twice, each object will be a separate instance. For example, in the code below the assertion will always fail.

       MyKey key = ...;
       MyEntity entity1 = index.get(key);
       MyEntity entity2 = index.get(key);
       assert entity1 == entity2; // always fails!

  ### Deleting from the Index

  Any type of index may be used to delete entities with a specified key by calling `delete(K)`. The important thing to keep in mind is that all entities with the specified key are deleted. In a primary index, at most a single entity is deleted:

       primaryIndex.delete(1); // Deletes a single employee by unique ID

  But in a secondary index, multiple entities may be deleted:

       secondaryIndex.delete("Engineering"); // Deletes all Engineering employees

  This begs this question: How can a single entity be deleted without knowing its primary key? The answer is to use cursors. After locating an entity using a cursor, the entity can be deleted by calling `EntityCursor.delete()`.

  ### Transactions

  Transactions can be used to provide standard ACID (Atomicity, Consistency, Integrity and Durability) guarantees when retrieving, storing and deleting entities. This section provides a brief overview of how to use transactions with the Direct Persistence Layer. For more information on using transactions, see Writing Transactional Applications.

  Transactions may be used only with a transactional `EntityStore`, which is one for which `StoreConfig.setTransactional(true)` has been called. Likewise, a transactional store may only be used with a transactional `Environment`, which is one for which `EnvironmentConfig.setTransactional(true)` has been called. For example:

       EnvironmentConfig envConfig = new EnvironmentConfig();
       envConfig.setTransactional(true);
       envConfig.setAllowCreate(true);
       Environment env = new Environment(new File("/my/data"), envConfig);

       StoreConfig storeConfig = new StoreConfig();
       storeConfig.setTransactional(true);
       storeConfig.setAllowCreate(true);
       EntityStore store = new EntityStore(env, "myStore", storeConfig);

  Transactions are represented by `Transaction` objects, which are part of the `Base API`. Transactions are created using the `Environment.beginTransaction` method.

  A transaction will include all operations for which the transaction object is passed as a method argument. All retrieval, storage and deletion methods have an optional `Transaction` parameter for this purpose. When a transaction is passed to a method that opens a cursor, all retrieval, storage and deletion operations performed using that cursor will be included in the transaction.

  A transaction may be committed by calling `Transaction.commit()` or aborted by calling `Transaction.abort()`. For example, two employees may be deleted atomically with a transaction; other words, either both are deleted or neither is deleted:

       Transaction txn = env.beginTransaction(null, null);
       try {
           primaryIndex.delete(txn, 1);
           primaryIndex.delete(txn, 2);
           txn.commit();
           txn = null;
       } finally {
           if (txn != null) {
               txn.abort();
           }
       }

  WARNING: Transactions must always be committed or aborted to prevent resource leaks which could lead to the index becoming unusable or cause an `OutOfMemoryError`. To ensure that a transaction is aborted in the face of exceptions, call `Transaction.abort()` in a finally block.

  For a transactional store, storage and deletion operations are always transaction protected, whether or not a transaction is explicitly used. A null transaction argument means to perform the operation using auto-commit, or the implied thread transaction if an XAEnvironment is being used. A transaction is automatically started as part of the operation and is automatically committed if the operation completes successfully. The transaction is automatically aborted if an exception occurs during the operation, and the exception is re-thrown to the caller. For example, each employee is deleted using a an auto-commit transaction below, but it is possible that employee 1 will be deleted and employee 2 will not be deleted, if an error or crash occurs while deleting employee 2:

       primaryIndex.delete(null, 1);
       primaryIndex.delete(null, 2);

  When retrieving entities, a null transaction argument means to perform the operation non-transactionally. The operation is performed outside the scope of any transaction, without providing transactional ACID guarantees. If an implied thread transaction is present (i.e. if an XAEnvironment is being used), that transaction is used. When a non-transactional store is used, transactional ACID guarantees are also not provided.

  For non-transactional and auto-commit usage, overloaded signatures for retrieval, storage and deletion methods are provided to avoid having to pass a null transaction argument. For example, `delete(K)` may be called instead of `delete(Transaction,Object)`. For example, the following code is equivalent to the code above where null was passed for the transaction:

       primaryIndex.delete(1);
       primaryIndex.delete(2);

  For retrieval methods the overloaded signatures also include an optional `LockMode` parameter, and overloaded signatures for opening cursors include an optional `CursorConfig` parameter. These parameters are described further below in the Locking and Lock Modes section.

  ### Transactions and Cursors

  There are two special consideration when using cursors with transactions. First, for a transactional store, a non-null transaction must be passed to methods that open a cursor if that cursor will be used to delete or update entities. Cursors do not perform auto-commit when a null transaction is explicitly passed or implied by the method signature. For example, the following code will throw `DatabaseException` when the `EntityCursor.delete()` method is called:

       // Does not work with a transactional store!
       EntityCursor<Employee> cursor = primaryIndex.entities();
       try {
           for (Employee entity : cursor) {
               cursor.delete(); // Will throw DatabaseException.
           }
       } finally {
           cursor.close();
       }

  Instead, the `entities(Transaction,CursorConfig)` signature must be used and a non-null transaction must be passed:

       EntityCursor<Employee> cursor = primaryIndex.entities(txn, null);
       try {
           for (Employee entity : cursor) {
               cursor.delete();
           }
       } finally {
           cursor.close();
       }

  The second consideration is that error handling is more complex when using both transactions and cursors, for the following reasons:

  1. When an exception occurs, the transaction should be aborted.
  2. Cursors must be closed whether or not an exception occurs.
  3. Cursors must be closed before committing or aborting the transaction.

  For example:

       Transaction txn = env.beginTransaction(null, null);
       EntityCursor<Employee> cursor = null;
       try {
           cursor = primaryIndex.entities(txn, null);
           for (Employee entity : cursor) {
               cursor.delete();
           }
           cursor.close();
           cursor = null;
           txn.commit();
           txn = null;
       } finally {
           if (cursor != null) {
               cursor.close();
           }
           if (txn != null) {
               txn.abort();
           }
       }

  ### Locking and Lock Modes

  This section provides a brief overview of locking and describes how lock modes are used with the Direct Persistence Layer. For more information on locking, see Writing Transactional Applications.

  When using transactions, locks are normally acquired on each entity that is retrieved or stored. The locks are used to isolate one transaction from another. Locks are normally released only when the transaction is committed or aborted.

  When not using transactions, locks are also normally acquired on each entity that is retrieved or stored. However, these locks are released when the operation is complete. When using cursors, in order to provide cursor stability locks are held until the cursor is moved to a different entity or closed.

  This default locking behavior provides full transactional ACID guarantees and cursor stability. However, application performance can sometimes be improved by compromising these guarantees. As described in Writing Transactional Applications, the `LockMode` and `CursorConfig` parameters are two of the mechanisms that can be used to make compromises.

  For example, imagine that you need an approximate count of all entities matching certain criterion, and it is acceptable for entities to be changed by other threads or other transactions while performing this query. `LockMode.READ_UNCOMMITTED` can be used to perform the retrievals without acquiring any locks. This reduces memory consumption, does less processing, and improves concurrency.

       EntityCursor<Employee> cursor = primaryIndex.entities(txn, null);
       try {
           Employee entity;
           while ((entity = cursor.next(LockMode.READ_UNCOMMITTED)) != null) {
               // Examine the entity and accumulate totals...
           }
       } finally {
           cursor.close();
       }

  The `LockMode` parameter specifies locking behavior on a per-operation basis. If null or `LockMode.DEFAULT` is specified, the default lock mode is used.

  It is also possible to specify the default locking behavior for a cursor using `CursorConfig`. The example below is equivalent to the example above:

       CursorConfig config = new CursorConfig();
       config.setReadUncommitted(true);
       EntityCursor<Employee> cursor = primaryIndex.entities(txn, config);
       try {
           Employee entity;
           while ((entity = cursor.next()) != null) {
               // Examine the entity and accumulate totals...
           }
       } finally {
           cursor.close();
       }

  The use of other lock modes, cursor configuration, and transaction configuration are discussed in Writing Transactional Applications.

  ### Performing Transaction Retries

  Lock conflict handling is another important topic discussed in Writing Transactional Applications. To go along with that material, here we show a lock conflict handling loop in the context of the Direct Persistence Layer. The example below shows deleting all entities in a primary index in a single transaction. If a lock conflict occurs, the transaction is aborted and the operation is retried.

        void doTransaction(final Environment env,
                           final PrimaryIndex<Long, Employee> primaryIndex,
                           final int maxTries)
            throws DatabaseException {

            boolean success = false;
            long sleepMillis = 0;
            for (int i = 0; i < maxTries; i++) {
                // Sleep before retrying.
                if (sleepMillis != 0) {
                    Thread.sleep(sleepMillis);
                    sleepMillis = 0;
                }
                Transaction txn = null;
                try {
                    txn = env.beginTransaction(null, null);
                    final EntityCursor<Employee> cursor =
                        primaryIndex.entities(txn, null);
                    try {
                        // INSERT APP-SPECIFIC CODE HERE:
                        // Perform read and write operations, for example:
                        for (Employee entity : cursor) {
                            cursor.delete();
                        }
                    } finally {
                        cursor.close();
                    }
                    txn.commit();
                    success = true;
                    return;
                } catch (DeadlockException e) {
                    sleepMillis = LOCK_CONFLICT_RETRY_SEC * 1000;
                    continue;
                } finally {
                    if (!success) {
                        if (txn != null) {
                            txn.abort();
                        }
                    }
                }
            }
            // INSERT APP-SPECIFIC CODE HERE:
            // Transaction failed, despite retries.
            // Take some app-specific course of action.
        }

  ### Low Level Access

  Each Direct Persistence Layer index is associated with an underlying `Database` or `SecondaryDatabase` defined in the `Base API`. At this level, an index is a Btree managed by the Berkeley DB Java Edition transactional storage engine. Although you may never need to work at the `Base API` level, keep in mind that some types of performance tuning can be done by configuring the underlying databases. See the `EntityStore` class for more information on database and sequence configuration.

  If you wish to access an index using the `Base API`, you may call the `BasicIndex.getDatabase()` or `SecondaryIndex.getDatabase()` method to get the underlying database. To translate between entity or key objects and `DatabaseEntry` objects at this level, use the bindings returned by `PrimaryIndex.getEntityBinding()`, `PrimaryIndex.getKeyBinding()`, and `SecondaryIndex.getKeyBinding()`.

- - ### Method Summary

    All Methods Instance Methods Abstract Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `contains(K key)` Checks for existence of a key in this index.
    `boolean` | `contains(Transaction txn, K key, LockMode lockMode)` Checks for existence of a key in this index.
    `long` | `count()` Returns a non-transactional count of the entities in this index.
    `boolean` | `delete(K key)` Deletes all entities with a given index key.
    `boolean` | `delete(Transaction txn, K key)` Deletes all entities with a given index key.
    `EntityCursor<V>` | `entities()` Opens a cursor for traversing all entities in this index.
    `EntityCursor<V>` | `entities(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive)` Opens a cursor for traversing entities in a key range.
    `EntityCursor<V>` | `entities(Transaction txn, CursorConfig config)` Opens a cursor for traversing all entities in this index.
    `EntityCursor<V>` | `entities(Transaction txn, K fromKey, boolean fromInclusive, K toKey, boolean toInclusive, CursorConfig config)` Opens a cursor for traversing entities in a key range.
    `V` | `get(K key)` Gets an entity via a key of this index.
    `V` | `get(Transaction txn, K key, LockMode lockMode)` Gets an entity via a key of this index.
    `Database` | `getDatabase()` Returns the underlying database for this index.
    `EntityCursor<K>` | `keys()` Opens a cursor for traversing all keys in this index.
    `EntityCursor<K>` | `keys(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive)` Opens a cursor for traversing keys in a key range.
    `EntityCursor<K>` | `keys(Transaction txn, CursorConfig config)` Opens a cursor for traversing all keys in this index.
    `EntityCursor<K>` | `keys(Transaction txn, K fromKey, boolean fromInclusive, K toKey, boolean toInclusive, CursorConfig config)` Opens a cursor for traversing keys in a key range.
    `java.util.Map<K,V>` | `map()` Returns a standard Java map based on this entity index.
    `java.util.SortedMap<K,V>` | `sortedMap()` Returns a standard Java sorted map based on this entity index.

- - ### Method Detail

    - #### getDatabase

          Database getDatabase()

      Returns the underlying database for this index.
      Returns:
        the database.

    - #### contains

          boolean contains(K key)
                    throws DatabaseException

      Checks for existence of a key in this index.

      The operation will not be transaction protected, and `LockMode.DEFAULT` is used implicitly.

      Parameters:
        `key` - the key to search for.
      Returns:
        whether the key exists in the index.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### contains

          boolean contains(Transaction txn,
                           K key,
                           LockMode lockMode)
                    throws DatabaseException

      Checks for existence of a key in this index.
      Parameters:
        `txn` - the transaction used to protect this operation, or null if the operation should not be transaction protected.
        `key` - the key to search for.
        `lockMode` - the lock mode to use for this operation, or null to use `LockMode.DEFAULT`.
      Returns:
        whether the key exists in the index.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### get

          V get(K key)
          throws DatabaseException

      Gets an entity via a key of this index.

      The operation will not be transaction protected, and `LockMode.DEFAULT` is used implicitly.

      Parameters:
        `key` - the key to search for.
      Returns:
        the value mapped to the given key, or null if the key is not present in the index.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### get

          V get(Transaction txn,
                K key,
                LockMode lockMode)
          throws DatabaseException

      Gets an entity via a key of this index.
      Parameters:
        `txn` - the transaction used to protect this operation, or null if the operation should not be transaction protected.
        `key` - the key to search for.
        `lockMode` - the lock mode to use for this operation, or null to use `LockMode.DEFAULT`.
      Returns:
        the value mapped to the given key, or null if the key is not present in the index.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### count

          long count()
              throws DatabaseException

      Returns a non-transactional count of the entities in this index.
      Returns:
        the number of entities in this index.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### delete

          boolean delete(K key)
                  throws DatabaseException

      Deletes all entities with a given index key.

      Auto-commit is used implicitly if the store is transactional.

      Parameters:
        `key` - the key to search for.
      Returns:
        whether any entities were deleted.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### delete

          boolean delete(Transaction txn,
                         K key)
                  throws DatabaseException

      Deletes all entities with a given index key.
      Parameters:
        `txn` - the transaction used to protect this operation, null to use auto-commit, or null if the store is non-transactional.
        `key` - the key to search for.
      Returns:
        whether any entities were deleted.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### keys

          EntityCursor<K> keys()
                        throws DatabaseException

      Opens a cursor for traversing all keys in this index.

      The operations performed with the cursor will not be transaction protected, and `CursorConfig.DEFAULT` is used implicitly. If the store is transactional, the cursor may not be used to update or delete entities.

      Returns:
        the cursor.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### keys

          EntityCursor<K> keys(Transaction txn,
                               CursorConfig config)
                        throws DatabaseException

      Opens a cursor for traversing all keys in this index.
      Parameters:
        `txn` - the transaction used to protect all operations performed with the cursor, or null if the operations should not be transaction protected. If the store is non-transactional, null must be specified. For a transactional store the transaction is optional for read-only access and required for read-write access.
        `config` - the cursor configuration that determines the default lock mode used for all cursor operations, or null to implicitly use `CursorConfig.DEFAULT`.
      Returns:
        the cursor.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### entities

          EntityCursor<V> entities()
                            throws DatabaseException

      Opens a cursor for traversing all entities in this index.

      The operations performed with the cursor will not be transaction protected, and `CursorConfig.DEFAULT` is used implicitly. If the store is transactional, the cursor may not be used to update or delete entities.

      Returns:
        the cursor.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### entities

          EntityCursor<V> entities(Transaction txn,
                                   CursorConfig config)
                            throws DatabaseException

      Opens a cursor for traversing all entities in this index.
      Parameters:
        `txn` - the transaction used to protect all operations performed with the cursor, or null if the operations should not be transaction protected. If the store is non-transactional, null must be specified. For a transactional store the transaction is optional for read-only access and required for read-write access.
        `config` - the cursor configuration that determines the default lock mode used for all cursor operations, or null to implicitly use `CursorConfig.DEFAULT`.
      Returns:
        the cursor.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### keys

          EntityCursor<K> keys(K fromKey,
                               boolean fromInclusive,
                               K toKey,
                               boolean toInclusive)
                        throws DatabaseException

      Opens a cursor for traversing keys in a key range.

      The operations performed with the cursor will not be transaction protected, and `CursorConfig.DEFAULT` is used implicitly. If the store is transactional, the cursor may not be used to update or delete entities.

      Parameters:
        `fromKey` - is the lower bound of the key range, or null if the range has no lower bound.
        `fromInclusive` - is true if keys greater than or equal to fromKey should be included in the key range, or false if only keys greater than fromKey should be included.
        `toKey` - is the upper bound of the key range, or null if the range has no upper bound.
        `toInclusive` - is true if keys less than or equal to toKey should be included in the key range, or false if only keys less than toKey should be included.
      Returns:
        the cursor.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### keys

          EntityCursor<K> keys(Transaction txn,
                               K fromKey,
                               boolean fromInclusive,
                               K toKey,
                               boolean toInclusive,
                               CursorConfig config)
                        throws DatabaseException

      Opens a cursor for traversing keys in a key range.
      Parameters:
        `txn` - the transaction used to protect all operations performed with the cursor, or null if the operations should not be transaction protected. If the store is non-transactional, null must be specified. For a transactional store the transaction is optional for read-only access and required for read-write access.
        `fromKey` - is the lower bound of the key range, or null if the range has no lower bound.
        `fromInclusive` - is true if keys greater than or equal to fromKey should be included in the key range, or false if only keys greater than fromKey should be included.
        `toKey` - is the upper bound of the key range, or null if the range has no upper bound.
        `toInclusive` - is true if keys less than or equal to toKey should be included in the key range, or false if only keys less than toKey should be included.
        `config` - the cursor configuration that determines the default lock mode used for all cursor operations, or null to implicitly use `CursorConfig.DEFAULT`.
      Returns:
        the cursor.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### entities

          EntityCursor<V> entities(K fromKey,
                                   boolean fromInclusive,
                                   K toKey,
                                   boolean toInclusive)
                            throws DatabaseException

      Opens a cursor for traversing entities in a key range.

      The operations performed with the cursor will not be transaction protected, and `CursorConfig.DEFAULT` is used implicitly. If the store is transactional, the cursor may not be used to update or delete entities.

      Parameters:
        `fromKey` - is the lower bound of the key range, or null if the range has no lower bound.
        `fromInclusive` - is true if keys greater than or equal to fromKey should be included in the key range, or false if only keys greater than fromKey should be included.
        `toKey` - is the upper bound of the key range, or null if the range has no upper bound.
        `toInclusive` - is true if keys less than or equal to toKey should be included in the key range, or false if only keys less than toKey should be included.
      Returns:
        the cursor.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### entities

          EntityCursor<V> entities(Transaction txn,
                                   K fromKey,
                                   boolean fromInclusive,
                                   K toKey,
                                   boolean toInclusive,
                                   CursorConfig config)
                            throws DatabaseException

      Opens a cursor for traversing entities in a key range.
      Parameters:
        `txn` - the transaction used to protect all operations performed with the cursor, or null if the operations should not be transaction protected. If the store is non-transactional, null must be specified. For a transactional store the transaction is optional for read-only access and required for read-write access.
        `fromKey` - is the lower bound of the key range, or null if the range has no lower bound.
        `fromInclusive` - is true if keys greater than or equal to fromKey should be included in the key range, or false if only keys greater than fromKey should be included.
        `toKey` - is the upper bound of the key range, or null if the range has no upper bound.
        `toInclusive` - is true if keys less than or equal to toKey should be included in the key range, or false if only keys less than toKey should be included.
        `config` - the cursor configuration that determines the default lock mode used for all cursor operations, or null to implicitly use `CursorConfig.DEFAULT`.
      Returns:
        the cursor.
      Throws:
        `DatabaseException` - the base class for all BDB exceptions.

    - #### map

          java.util.Map<K,V> map()

      Returns a standard Java map based on this entity index. The `StoredMap` returned is defined by the Collections API. Stored collections conform to the standard Java collections framework interface.
      Returns:
        the map.

    - #### sortedMap

          java.util.SortedMap<K,V> sortedMap()

      Returns a standard Java sorted map based on this entity index. The `StoredSortedMap` returned is defined by the Collections API. Stored collections conform to the standard Java collections framework interface.
      Returns:
        the map.

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