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Berkeley DB Java Edition is a open source, transactional storage solution for Java applications. The Direct Persistence Layer (DPL) API is faster and easier to develop, deploy, and manage than serialized object files or ORM-based Java persistence solutions. The Collections API enhances the standard java.util.collections classes allowing them to be persisted to a local file system and accessed concurrently while protected by ACID transactions. Data is stored by serializing objects and managing class and instance data separately so as not to waste space. Berkeley DB Java Edition is the reliable drop-in solution for complex, fast, and scalable storage. Source for this release is in 'je-4.0.92-sources.jar', the Javadoc is located at 'http://download.oracle.com/berkeley-db/docs/je/4.0.92/'.

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/*-
 * Copyright (C) 2002, 2018, Oracle and/or its affiliates. All rights reserved.
 *
 * This file was distributed by Oracle as part of a version of Oracle Berkeley
 * DB Java Edition made available at:
 *
 * http://www.oracle.com/technetwork/database/database-technologies/berkeleydb/downloads/index.html
 *
 * Please see the LICENSE file included in the top-level directory of the
 * appropriate version of Oracle Berkeley DB Java Edition for a copy of the
 * license and additional information.
 */

package com.sleepycat.persist;

import com.sleepycat.je.DatabaseEntry;

/**
 * An adapter that translates between database entries (key, primary key, data)
 * and a "value", which may be either the key, primary key, or entity.  This
 * interface is used to implement a generic index and cursor (BasicIndex and
 * BasicCursor).  If we didn't use this approach, we would need separate index
 * and cursor implementations for each type of value that can be returned.  In
 * other words, this interface is used to reduce class explosion.
 *
 * @author Mark Hayes
 */
interface ValueAdapter {

    /**
     * Creates a DatabaseEntry for the key or returns null if the key is not
     * needed.
     */
    DatabaseEntry initKey();

    /**
     * Creates a DatabaseEntry for the primary key or returns null if the
     * primary key is not needed.
     */
    DatabaseEntry initPKey();

    /**
     * Creates a DatabaseEntry for the data or returns null if the data is not
     * needed.  BasicIndex.NO_RETURN_ENTRY may be returned if the data argument
     * is required but we don't need it.
     */
    DatabaseEntry initData();

    /**
     * Sets the data array of the given entries to null, based on knowledge of
     * which entries are non-null and are not NO_RETURN_ENTRY.
     */
    void clearEntries(DatabaseEntry key,
                      DatabaseEntry pkey,
                      DatabaseEntry data);

    /**
     * Returns the appropriate "value" (key, primary key, or entity) using the
     * appropriate bindings for that purpose.
     */
    V entryToValue(DatabaseEntry key,
                   DatabaseEntry pkey,
                   DatabaseEntry data);

    /**
     * Converts an entity value to a data entry using an entity binding, or
     * throws UnsupportedOperationException if this is not appropriate.  Called
     * by BasicCursor.update.
     */
    void valueToData(V value, DatabaseEntry data);
}