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9.8 KiB
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<!--$Id: dbc_get.so,v 10.46 2001/01/19 17:29:46 bostic Exp $-->
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<!--Copyright 1997, 1998, 1999, 2000 by Sleepycat Software, Inc.-->
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<!--All rights reserved.-->
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<html>
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<head>
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<title>Berkeley DB: Dbc.get</title>
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<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,java,C,C++">
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</head>
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<body bgcolor=white>
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<a name="2"><!--meow--></a>
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<table><tr valign=top>
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<td>
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<h1>Dbc.get</h1>
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</td>
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<td width="1%">
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<a href="../api_java/java_index.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a>
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</td></tr></table>
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<hr size=1 noshade>
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<tt>
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<h3><pre>
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import com.sleepycat.db.*;
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<p>
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public int get(Dbt key, Dbt data, int flags)
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throws DbException;
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</pre></h3>
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<h1>Description</h1>
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<p>The Dbc.get method retrieves key/data pairs from the database. The
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byte array and length of the key
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are returned in the object referenced by <b>key</b> (except for the case
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of the Db.DB_SET flag where the <b>key</b> object is unchanged),
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and the byte array and length of
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the data are returned in the object referenced by <b>data</b>.
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<p>Modifications to the database during a sequential scan will be reflected
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in the scan, i.e. records inserted behind a cursor will not be returned
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while records inserted in front of a cursor will be returned.
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<p>In Queue and Recno databases, missing entries (i.e., entries that were
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never explicitly created or that were created and then deleted), will be
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skipped during a sequential scan.
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<p>If multiple threads or processes insert items into the same database file
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without using locking, the results are undefined.
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For more detail,
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see <a href="../ref/am/stability.html">Cursor stability</a>.
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<p>The <b>flags</b> parameter must be set to one of the following values:
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<p><dl compact>
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<p><dt><a name="Db.DB_CURRENT">Db.DB_CURRENT</a><dd>Return the key/data pair currently referenced by the cursor.
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<p>If the cursor key/data pair was deleted, Dbc.get will return
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<a href="../ref/program/errorret.html#DB_KEYEMPTY">Db.DB_KEYEMPTY</a>.
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<p>If the cursor is not yet initialized, the Dbc.get method throws an exception that encapsulates EINVAL.
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<p><dt><a name="Db.DB_FIRST">Db.DB_FIRST</a>, <a name="Db.DB_LAST">Db.DB_LAST</a><dd>The cursor is set to reference the first (last) key/data pair of the
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database, and that pair is returned. In the presence of duplicate key
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values, the first (last) data item in the set of duplicates is returned.
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<p>If the database is a Queue or Recno database, Dbc.get using the
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Db.DB_FIRST (Db.DB_LAST) flags will ignore any keys that exist
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but were never explicitly created by the application or were created and
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later deleted.
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<p>If the database is empty, Dbc.get will return <a href="../ref/program/errorret.html#DB_NOTFOUND">Db.DB_NOTFOUND</a>.
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<p><dt><a name="Db.DB_GET_BOTH">Db.DB_GET_BOTH</a><dd>The Db.DB_GET_BOTH flag is identical to the Db.DB_SET flag,
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except that both the key and the data arguments must be matched by the
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key and data item in the database.
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<p><dt><a name="Db.DB_GET_RECNO">Db.DB_GET_RECNO</a><dd>Return the record number associated with the cursor. The record number
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will be returned in <b>data</b> as described in <a href="../api_java/dbt_class.html">Dbt</a>. The
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<b>key</b> parameter is ignored.
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<p>For Db.DB_GET_RECNO to be specified, the underlying database must be
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of type Btree and it must have been created with the <a href="../api_java/db_set_flags.html#DB_RECNUM">Db.DB_RECNUM</a>
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flag.
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<p><dt><a name="Db.DB_JOIN_ITEM">Db.DB_JOIN_ITEM</a><dd>Do not use the data value found in all of the cursors as a lookup key for
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the primary database, but simply return it in the key parameter instead.
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The data parameter is left unchanged.
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<p>For Db.DB_JOIN_ITEM to be specified, the underlying cursor must have
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been returned from the <a href="../api_java/db_join.html">Db.join</a> method.
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<p><dt><a name="Db.DB_NEXT">Db.DB_NEXT</a>, <a name="Db.DB_PREV">Db.DB_PREV</a><dd>If the cursor is not yet initialized, Db.DB_NEXT (Db.DB_PREV)
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is identical to Db.DB_FIRST (Db.DB_LAST). Otherwise, the cursor
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is moved to the next (previous) key/data pair of the database, and that
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pair is returned. In the presence of duplicate key values, the value of
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the key may not change.
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<p>If the database is a Queue or Recno database, Dbc.get using the
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Db.DB_NEXT (Db.DB_PREV) flag will skip any keys that exist but
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were never explicitly created by the application or were created and later
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deleted.
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<p>If the cursor is already on the last (first) record in the database,
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Dbc.get will return <a href="../ref/program/errorret.html#DB_NOTFOUND">Db.DB_NOTFOUND</a>.
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<p><dt><a name="Db.DB_NEXT_DUP">Db.DB_NEXT_DUP</a><dd>If the next key/data pair of the database is a duplicate record for the
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current key/data pair, the cursor is moved to the next key/data pair of
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the database, and that pair is returned. Otherwise, Dbc.get will
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return <a href="../ref/program/errorret.html#DB_NOTFOUND">Db.DB_NOTFOUND</a>.
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<p>If the cursor is not yet initialized, the Dbc.get method throws an exception that encapsulates EINVAL.
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<p><dt><a name="Db.DB_NEXT_NODUP">Db.DB_NEXT_NODUP</a>, <a name="Db.DB_PREV_NODUP">Db.DB_PREV_NODUP</a><dd>If the cursor is not yet initialized, Db.DB_NEXT_NODUP
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(Db.DB_PREV_NODUP) is identical to Db.DB_FIRST
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(Db.DB_LAST). Otherwise, the cursor is moved to the next (previous)
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non-duplicate key/data pair of the database, and that pair is returned.
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<p>If the database is a Queue or Recno database, Dbc.get using the
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Db.DB_NEXT_NODUP (Db.DB_PREV_NODUP) flags will ignore any keys
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that exist but were never explicitly created by the application or were
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created and later deleted.
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<p>If no non-duplicate key/data pairs occur after (before) the cursor
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position in the database, Dbc.get will return <a href="../ref/program/errorret.html#DB_NOTFOUND">Db.DB_NOTFOUND</a>.
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<p><dt><a name="Db.DB_SET">Db.DB_SET</a><dd>Move the cursor to the specified key/data pair of the database, and
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return the datum associated with the given key.
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<p>In the presence of duplicate key values, Dbc.get will return the
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first data item for the given key.
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<p>If the database is a Queue or Recno database and the requested key exists,
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but was never explicitly created by the application or was later deleted,
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Dbc.get will return <a href="../ref/program/errorret.html#DB_KEYEMPTY">Db.DB_KEYEMPTY</a>.
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<p>If no matching keys are found, Dbc.get will return
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<a href="../ref/program/errorret.html#DB_NOTFOUND">Db.DB_NOTFOUND</a>.
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<p><dt><a name="Db.DB_SET_RANGE">Db.DB_SET_RANGE</a><dd>The Db.DB_SET_RANGE flag is identical to the Db.DB_SET flag,
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except that the key is returned as well as the data item, and, in the case
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of the Btree access method, the returned key/data pair is the smallest
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key greater than or equal to the specified key (as determined by the
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comparison method), permitting partial key matches and range
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searches.
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<p><dt><a name="Db.DB_SET_RECNO">Db.DB_SET_RECNO</a><dd>Move the cursor to the specific numbered record of the database, and
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return the associated key/data pair. The <b>data</b> field of the
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specified <b>key</b>
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must be a byte array containing a record number, as described in
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<a href="../api_java/dbt_class.html">Dbt</a>. This determines the record to be retrieved.
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<p>For Db.DB_SET_RECNO to be specified, the underlying database must be
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of type Btree and it must have been created with the <a href="../api_java/db_set_flags.html#DB_RECNUM">Db.DB_RECNUM</a>
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flag.
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</dl>
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<p>In addition, the following flag may be set by bitwise inclusively <b>OR</b>'ing it into the
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<b>flags</b> parameter:
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<p><dl compact>
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<p><dt><a name="Db.DB_RMW">Db.DB_RMW</a><dd>Acquire write locks instead of read locks when doing the retrieval.
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Setting this flag may decrease the likelihood of deadlock during a
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read-modify-write cycle by immediately acquiring the write lock during
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the read part of the cycle so that another thread of control acquiring
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a read lock for the same item, in its own read-modify-write cycle, will
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not result in deadlock.
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</dl>
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<p>Otherwise, the Dbc.get method throws an exception that encapsulates a non-zero error value on
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failure.
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<p>If Dbc.get fails for any reason, the state of the cursor will be
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unchanged.
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<h1>Errors</h1>
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<p>The Dbc.get method may fail and throw an exception encapsulating a non-zero error for the following conditions:
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<p><dl compact>
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<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
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<p>The specified cursor was not currently initialized.
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</dl>
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<p>If the operation was selected to resolve a deadlock, the
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Dbc.get method will fail and
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throw a <a href="../api_java/deadlock_class.html">DbDeadlockException</a> exception.
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<p>If the requested item could not be returned due to insufficient memory,
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the Dbc.get method will fail and
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throw a <a href="../api_java/mem_class.html">DbMemoryException</a> exception.
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<p>The Dbc.get method may fail and throw an exception for errors specified for other Berkeley DB and C library or system methods.
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If a catastrophic error has occurred, the Dbc.get method may fail and throw
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a <a href="../api_java/runrec_class.html">DbRunRecoveryException</a>, in which case all subsequent Berkeley DB calls
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will fail in the same way.
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<h3>Class</h3>
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<a href="../api_java/dbc_class.html">Dbc</a>
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<h1>See Also</h1>
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<a href="../api_java/dbc_close.html">Dbc.close</a>,
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<a href="../api_java/dbc_count.html">Dbc.count</a>,
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<a href="../api_java/dbc_del.html">Dbc.del</a>,
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<a href="../api_java/dbc_dup.html">Dbc.dup</a>,
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<a href="../api_java/dbc_get.html">Dbc.get</a>
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and
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<a href="../api_java/dbc_put.html">Dbc.put</a>.
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</tt>
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<table><tr><td><br></td><td width="1%">
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<a href="../api_java/java_index.html"><img src="../images/api.gif" alt="API"></a><a href="../ref/toc.html"><img src="../images/ref.gif" alt="Ref"></a>
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</td></tr></table>
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<p><font size=1><a href="http://www.sleepycat.com">Copyright Sleepycat Software</a></font>
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</body>
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</html>
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