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<title>KeyField (Oracle - Berkeley DB Java Edition API)</title>
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<div class="aboutLanguage"><b>Berkeley DB Java Edition</b><br><font size=\"-1\"> version 7.5.11</font>
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<li>Summary: </li>
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<div class="subTitle">com.sleepycat.persist.model</div>
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<h2 title="Annotation Type KeyField" class="title">Annotation Type KeyField</h2>
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<pre>@Documented
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@Retention(value=RUNTIME)
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@Target(value=FIELD)
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public @interface <span class="memberNameLabel">KeyField</span></pre>
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<div class="block">Indicates the sorting position of a key field in a composite key class when
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the <code>Comparable</code> interface is not implemented. The <code>KeyField</code>
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integer value specifies the sort order of this field within the set of
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fields in the composite key.
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<p>If the field type of a <a href="../../../../com/sleepycat/persist/model/PrimaryKey.html" title="annotation in com.sleepycat.persist.model"><code>PrimaryKey</code></a> or <a href="../../../../com/sleepycat/persist/model/SecondaryKey.html" title="annotation in com.sleepycat.persist.model"><code>SecondaryKey</code></a> is a
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composite key class containing more than one key field, then a <code>KeyField</code> annotation must be present on each non-transient instance field of
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the composite key class. The <code>KeyField</code> value must be a number
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between one and the number of non-transient instance fields declared in the
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composite key class.</p>
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<p>Note that a composite key class is a flat container for one or more
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simple type fields. All non-transient instance fields in the composite key
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class are key fields, and its superclass must be <code>Object</code>.</p>
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<p>For example:</p>
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<pre class="code">
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@Entity
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class Animal {
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@PrimaryKey
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Classification classification;
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...
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}
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@Persistent
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class Classification {
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@KeyField(1) String kingdom;
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@KeyField(2) String phylum;
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@KeyField(3) String clazz;
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@KeyField(4) String order;
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@KeyField(5) String family;
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@KeyField(6) String genus;
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@KeyField(7) String species;
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@KeyField(8) String subspecies;
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...
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}</pre>
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<p>This causes entities to be sorted first by <code>kingdom</code>, then by
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<code>phylum</code> within <code>kingdom</code>, and so on.</p>
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<p>The fields in a composite key class may not be null.</p>
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<p><a name="comparable"><strong>Custom Sort Order</strong></a></p>
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<p>To override the default sort order, a composite key class may implement
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the <code>Comparable</code> interface. This allows overriding the sort order and
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is therefore useful even when there is only one key field in the composite
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key class. For example, the following class sorts Strings using a Canadian
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collator:</p>
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<pre class="code">
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import java.text.Collator;
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import java.util.Locale;
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@Entity
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class Animal {
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...
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@SecondaryKey(relate=ONE_TO_ONE)
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CollatedString canadianName;
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...
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}
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@Persistent
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class CollatedString implements Comparable<CollatedString> {
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static Collator collator = Collator.getInstance(Locale.CANADA);
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@KeyField(1)
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String value;
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CollatedString(String value) { this.value = value; }
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private CollatedString() {}
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public int compareTo(CollatedString o) {
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return collator.compare(value, o.value);
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}
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}</pre>
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<p>Several important rules should be considered when implementing a custom
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comparison method. Failure to follow these rules may result in the primary
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or secondary index becoming unusable; in other words, the store will not be
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able to function.</p>
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<ol>
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<li>The comparison method must always return the same result, given the same
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inputs. The behavior of the comparison method must not change over
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time.</li>
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<li>A corollary to the first rule is that the behavior of the comparison
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method must not be dependent on state which may change over time. For
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example, if the above collation method used the default Java locale, and the
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default locale is changed, then the sort order will change.</li>
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<li>The comparison method must not assume that it is called after the store
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has been opened. With Berkeley DB Java Edition, the comparison method is
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called during database recovery, which occurs in the <a href="../../../../com/sleepycat/je/Environment.html" title="class in com.sleepycat.je"><code>Environment</code></a>
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constructor.</li>
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<li>The comparison method must not assume that it will only be called with
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keys that are currently present in the database. The comparison method will
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occasionally be called with deleted keys or with keys for records that were
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not part of a committed transaction.</li>
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</ol></div>
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<dl>
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<dt><span class="simpleTagLabel">Author:</span></dt>
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<dd>Mark Hayes</dd>
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</dl>
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<!-- =========== ANNOTATION TYPE REQUIRED MEMBER SUMMARY =========== -->
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<h3>Required Element Summary</h3>
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<table class="memberSummary" border="0" cellpadding="3" cellspacing="0" summary="Required Element Summary table, listing required elements, and an explanation">
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<caption><span>Required Elements</span><span class="tabEnd"> </span></caption>
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<tr>
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<th class="colFirst" scope="col">Modifier and Type</th>
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<th class="colLast" scope="col">Required Element and Description</th>
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</tr>
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<tr class="altColor">
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<td class="colFirst"><code>int</code></td>
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<td class="colLast"><code><span class="memberNameLink"><a href="../../../../com/sleepycat/persist/model/KeyField.html#value--">value</a></span></code> </td>
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<h4>value</h4>
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<pre>public abstract int value</pre>
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<div class="aboutLanguage"><b>Berkeley DB Java Edition</b><br><font size=\"-1\"> version 7.5.11</font>
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