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Using Stored Collections
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Using Stored Collections
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<th width="60%" align="center">Appendix A.
API Notes and Details
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="UsingStoredCollections"></a>
Using Stored Collections
</h2>
</div>
</div>
<div></div>
</div>
<p>
When a stored collection is created it is based on either a
<a href="../../java/com/sleepycat/db/Database.html" target="_top">Database</a>
or a
<span>
<a href="../../java/com/sleepycat/db/SecondaryDatabase.html" target="_top">SecondaryDatabase</a>.
</span>
When a database is used, the primary key of the database is used as
the collection key. When a secondary database is used, the index
key is used as the collection key. Indexed collections can be used
for reading elements and removing elements but not for adding or
updating elements.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="StoredCollectionAccessMethods"></a>
Stored Collection and Access Methods
</h3>
</div>
</div>
<div></div>
</div>
<p>
The use of stored collections is constrained in certain respects as
described below.
<span>
Most of these restrictions have to do with
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/List.html" target="_top">List</a>
interfaces; for
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html" target="_top">Map</a>
interfaces, most all access modes are fully supported since the
Berkeley DB model is map-like.
</span>
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedSet.html" target="_top">SortedSet</a>
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedMap.html" target="_top">SortedMap</a>
interfaces may only be used if keys are ordered. This means ordered keys are required for creating a
<a href="../../java/com/sleepycat/collections/StoredSortedEntrySet.html" target="_top">StoredSortedEntrySet</a>,
<a href="../../java/com/sleepycat/collections/StoredSortedKeySet.html" target="_top">StoredSortedKeySet</a>,
<a href="../../java/com/sleepycat/collections/StoredSortedMap.html" target="_top">StoredSortedMap</a>, or
<a href="../../java/com/sleepycat/collections/StoredSortedValueSet.html" target="_top">StoredSortedValueSet</a>.
</p>
</li>
<li>
<p>
All iterators for stored collections implement the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html" target="_top">ListIterator</a>
interface as well as the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Iterator.html" target="_top">Iterator</a>
interface.
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#hasPrevious()" target="_top">ListIterator.hasPrevious()</a>
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#previous()" target="_top">ListIterator.previous()</a>
work in all cases.
<span>
However, the following
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html" target="_top">ListIterator</a>
method behavior is dependent on the access method.
</span>
</p>
<div class="itemizedlist">
<ul type="circle">
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#nextIndex()" target="_top">ListIterator.nextIndex()</a>
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#previousIndex()" target="_top">ListIterator.previousIndex()</a>
only work when record number keys are used, and throw
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/UnsupportedOperationException.html" target="_top">UnsupportedOperationException</a>
otherwise.
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#add()" target="_top">ListIterator.add()</a>
inserts before the current position and renumbers following keys if the RECNO-RENUMBER
access method is used.
</p>
</li>
<li>
<p>
For all access methods other than RECNO-RENUMBER:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#add()" target="_top">ListIterator.add()</a>
throws
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/UnsupportedOperationException.html" target="_top">UnsupportedOperationException</a>
if duplicates are not allowed.
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#add()" target="_top">ListIterator.add()</a>
inserts a duplicate before the current position
if duplicates are unsorted.
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#add()" target="_top">ListIterator.add()</a>
inserts a duplicate in sorted order if
duplicates are sorted.
</p>
</li>
</ul>
</div>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#set()" target="_top">ListIterator.set()</a>
throws
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/UnsupportedOperationException.html" target="_top">UnsupportedOperationException</a>
if sorted duplicates are configured, since updating with sorted duplicates would change the
iterator position.
</p>
</li>
</ul>
</div>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.Entry.html#setValue()" target="_top">Map.Entry.setValue()</a>
throws
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/UnsupportedOperationException.html" target="_top">UnsupportedOperationException</a>
if duplicates are sorted.
</p>
</li>
<li>
<p>
Only the access methods that use a record number key may be used
with a
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/List.html" target="_top">List</a>
<tt class="classname">List</tt>
view.
</p>
</li>
<li>
<p>
To create a stored List that supports the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/List.html#add()" target="_top">List.add()</a>
<tt class="methodname">List.add()</tt>
method, only the RECNO-RENUMBER access method may be used.
</p>
</li>
<li>
<p>
For List access methods that do not support
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/List.html#add()" target="_top">List.add()</a>
<tt class="methodname">List.add()</tt>
(RECNO, QUEUE, and BTREE-RECNUM):
</p>
<div class="itemizedlist">
<ul type="circle">
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/List.html#add()" target="_top">List.add()</a>
<tt class="methodname">List.add()</tt>
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#add()" target="_top">ListIterator.add()</a>
<tt class="methodname">ListIterator.add()</tt>
always throw
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/UnsupportedOperationException.html" target="_top">UnsupportedOperationException</a> .
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/List.html#remove()" target="_top">List.remove()</a>
<tt class="methodname">List.remove()</tt>
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#remove()" target="_top">ListIterator.remove()</a>
<tt class="methodname">ListIterator.remove()</tt>
do not cause list indices to be renumbered. However, iterators will skip
the removed values.
</p>
</li>
</ul>
</div>
<p>
For these access methods, stored Lists are most useful as
read-only collections where indices are not required to be
sequential.
</p>
</li>
<li>
<p>
When duplicates are allowed the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collection.html" target="_top">Collection</a>
interfaces are modified in several ways as described in the next
section.
</p>
</li>
</ul>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="StoredVersusStandardCollections"></a>
Stored Collections Versus Standard Java Collections
</h3>
</div>
</div>
<div></div>
</div>
<p>
Stored collections have the following differences with the
standard Java collection interfaces. Some of these are interface
contract violations.
</p>
<p>
The Java collections interface does not support duplicate keys
(multi-maps or multi-sets). When the access method allows duplicate
keys, the collection interfaces are defined as follows.
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html#entrySet()" target="_top">Map.entrySet()</a>
may contain multiple
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.Entry.html" target="_top">Map.Entry</a>
objects with the same key.
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html#keySet()" target="_top">Map.keySet()</a>
always contains unique keys, it does not contain duplicates.
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html#values()" target="_top">Map.values()</a>
contains all values including the values
associated with duplicate keys.
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html#put()" target="_top">Map.put()</a>
appends a duplicate if the key already exists rather than replacing
the existing value, and always returns null.
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html#remove()" target="_top">Map.remove()</a>
removes all duplicates for the specified key.
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html#get()" target="_top">Map.get()</a>
returns the first duplicate for the specified key.
</p>
</li>
<li>
<p>
<a href="../../java/com/sleepycat/collections/StoredMap.html#duplicates(java.lang.Object)" target="_top">StoredMap.duplicates()</a>
is an additional method for returning the values for a given key as a
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collection.html" target="_top">Collection</a>.
</p>
</li>
</ul>
</div>
<p>
Other differences are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Collection.size() and Map.size() always throws
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/UnsupportedOperationException.html" target="_top">UnsupportedOperationException</a>.
This is because the number of
records in a database cannot be determined reliably or
cheaply.
</p>
</li>
<li>
<p>
Because the size() method cannot be used, the bulk operation
methods of standard Java collections cannot be passed stored
collections as parameters, since the implementations rely on
size(). However, the bulk operation methods of stored collections
can be passed standard Java collections as parameters.
<tt class="literal">storedCollection.addAll(standardCollection)</tt> is allowed
while <tt class="literal">standardCollection.addAll(storedCollection)</tt> is
<span class="emphasis"><em>not</em></span> allowed. This restriction applies to the standard
collection constructors that take a Collection parameter (copy
constructors), the Map.putAll() method, and the following
Collection methods: addAll(), containsAll(), removeAll() and
retainAll().
</p>
</li>
<li>
<p>
The <tt class="methodname">ListIterator.nextIndex()</tt> method
returns <tt class="literal">Integer.MAX_VALUE</tt>
for stored lists when positioned at the end of the list, rather
than returning the list size as specified by the ListIterator
interface. Again, this is because the database size is not
available.
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Comparator.html" target="_top">Comparator</a>
objects cannot be used and the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedMap.html#comparator()" target="_top">SortedMap.comparator()</a>
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedSet.html#comparator()" target="_top">SortedSet.comparator()</a>
methods always return null. The
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Comparable.html" target="_top">Comparable</a>
interface is not supported. However, Comparators that operate on
byte arrays may be specified using
<span>
<a href="../../java/com/sleepycat/db/DatabaseConfig.html#setBtreeComparator(java.util.Comparator)" target="_top">DatabaseConfig.setBtreeComparator</a>.
</span>
</p>
</li>
<li>
<p>
The
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Object.html#equals()" target="_top">Object.equals()</a>
method is not used to determine whether a key
or value is contained in a collection, to locate a value by key,
etc. Instead the byte array representation of the keys and values
are used. However, the equals() method <span class="emphasis"><em>is</em></span> called for each
key and value when comparing two collections for equality. It is
the responsibility of the application to make sure that the
equals() method returns true if and only if the byte array
representations of the two objects are equal. Normally this occurs
naturally since the byte array representation is derived from the
object's fields.
</p>
</li>
</ul>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="StoredCollectionCharacteristics"></a>
Other Stored Collection Characteristics
</h3>
</div>
</div>
<div></div>
</div>
<p>
The following characteristics of stored collections are
extensions of the definitions in the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/package-summary.html" target="_top">java.util</a>
package. These differences do not violate the Java
collections interface contract.
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
All stored collections are thread safe (can be used by multiple
threads concurrently)
<span>
whenever the Berkeley DB Concurrent Data Store or
Transactional Data Store environment is used.
</span>
Locking is handled by the Berkeley DB
environment. To access a collection from multiple threads, creation
of synchronized collections using the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collections.html" target="_top">Collections</a>
class is not necessary
<span>
except when using the Data Store environment.
</span>
Iterators, however, should always be used only by a single thread.
</p>
</li>
<li>
<p>
All stored collections may be read-only if desired by passing
false for the writeAllowed parameter of their constructor. Creation
of immutable collections using the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collections.html" target="_top">Collections</a>
class is not necessary.
</p>
</li>
<li>
<p>
A stored collection is partially read-only if a secondary
index is used. Specifically, values may be removed but may not be
added or updated. The following methods will throw
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/UnsupportedOperationException.html" target="_top">UnsupportedOperationException</a>
when an index is used:
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collection.html#add()" target="_top">Collection.add()</a>,
<span><a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/List.html#set()" target="_top">List.set()</a>,</span>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#set()" target="_top">ListIterator.set()</a>
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.Entry.html#setValue()" target="_top">Map.Entry.setValue()</a>.
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedMap.html#entrySet()" target="_top">SortedMap.entrySet()</a>
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedMap.html#keySet()" target="_top">SortedMap.keySet()</a>
return a
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedSet.html" target="_top">SortedSet</a>,
not just a
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Set.html" target="_top">Set</a>
as specified in Java collections interface. This allows using the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedSet.html" target="_top">SortedSet</a>
methods on the returned collection.
</p>
</li>
<li>
<p>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedMap.html#values()" target="_top">SortedMap.values()</a>
returns a
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedSet.html" target="_top">SortedSet</a>,
not just a
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collection.html" target="_top">Collection</a>,
whenever the keys of the map can be derived from the values using
an entity binding. Note that the sorted set returned is not really
a set if duplicates are allowed, since it is technically a
collection; however, the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedSet.html" target="_top">SortedSet</a>
methods (for example, subSet()), can still be used.
</p>
</li>
<li>
<p>
For
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedSet.html" target="_top">SortedSet</a>
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/SortedMap.html" target="_top">SortedMap</a>
views, additional subSet() and subMap() methods are provided that
allow control over whether keys are treated as inclusive or
exclusive values in the key range.
</p>
</li>
<li>
<p>
Keys and values are stored by value, not by reference. This is
because objects that are added to collections are converted to byte
arrays (by bindings) and stored in the database. When they are
retrieved from the collection they are read from the database and
converted from byte arrays to objects. Therefore, the object
reference added to a collection will not be the same as the
reference later retrieved from the collection.
</p>
</li>
<li>
<p>
A runtime exception,
<a href="../../java/com/sleepycat/util/RuntimeExceptionWrapper.html" target="_top">RuntimeExceptionWrapper</a>,
is thrown whenever database exceptions occur which are not runtime
exceptions. The
<a href="../../java/com/sleepycat/util/RuntimeExceptionWrapper.html#getCause()" target="_top">RuntimeExceptionWrapper.getCause()</a>
method can be called to get the underlying exception.
</p>
</li>
<li>
<p>
All iterators for stored collections implement the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html" target="_top">ListIterator</a>
interface as well as the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Iterator.html" target="_top">Iterator</a>
interface. This is to allow use of the
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#hasPrevious()" target="_top">ListIterator.hasPrevious()</a>
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#previous()" target="_top">ListIterator.previous()</a>
methods, which work for all collections
since Berkeley DB provides bidirectional cursors.
</p>
</li>
<li>
<p>
All stored collections have a
<a href="../../java/com/sleepycat/collections/StoredCollection.html#iterator(boolean)" target="_top">StoredCollection.iterator(boolean)</a>
method that allows creating
a read-only iterator for a writable collection. For the standard
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Collection.html#iterator()" target="_top">Collection.iterator()</a>
method, the iterator is read-only only
when the collection is read-only.
<span>Read-only iterators are important
for using the Berkeley DB Concurrent Data Store environment, since
only one write cursors may be open at one time.</span>
</p>
</li>
<li>
<p>
Iterator stability for stored collections is greater than the
iterator stability defined by the Java collections interfaces.
Stored iterator stability is the same as the cursor stability
defined by Berkeley DB.
</p>
</li>
<li>
<p>
When an entity binding is used, updating (setting) a value is
not allowed if the key in the entity is not equal to the original
key. For example, calling
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html#put()" target="_top">Map.put()</a>
is not allowed when the key parameter is not equal to the key of
the entity parameter.
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.html#put()" target="_top">Map.put()</a>,
<span><a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/List.html#set()" target="_top">List.set()</a>,</span>
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#set()" target="_top">ListIterator.set()</a>,
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/Map.Entry.html#setValue()" target="_top">Map.Entry.setValue()</a>
will throw
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/IllegalArgumentException.html" target="_top">IllegalArgumentException</a>
in this situation.
</p>
</li>
<li>
<p>
Adding and removing items from stored lists is not allowed for
sublists. This is simply an unimplemented feature and may be
changed in the future. Currently for sublists the following
methods throw
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/UnsupportedOperationException.html" target="_top">UnsupportedOperationException</a>:
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/List.html#add()" target="_top">List.add()</a><tt class="methodname">List.add()</tt>,
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/List.html#remove()" target="_top">List.remove()</a><tt class="methodname">List.remove()</tt>,
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#add()" target="_top">ListIterator.add()</a>
and
<a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/ListIterator.html#remove()" target="_top">ListIterator.remove()</a><tt class="methodname">ListIterator.remove()</tt>.
</p>
</li>
<li>
<p>
The
<span>
<a href="../../java/com/sleepycat/collections/StoredList.html#append(java.lang.Object)" target="_top">StoredList.append(java.lang.Object)</a>
and
</span>
<a href="../../java/com/sleepycat/collections/StoredMap.html#append(java.lang.Object)" target="_top">StoredMap.append(java.lang.Object)</a>
extension method<span>s</span> allows
adding a new record with an automatically assigned key.
<span>
Record number assignment by the database itself is supported
for QUEUE, RECNO and RECNO-RENUMBER databases.
</span>
An application-defined
<a href="../../java/com/sleepycat/collections/PrimaryKeyAssigner.html" target="_top">PrimaryKeyAssigner</a>
is used to assign the key value.
</p>
</li>
</ul>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="WhyJavaCollections"></a>
Why Java Collections for Berkeley DB
</h3>
</div>
</div>
<div></div>
</div>
<p>
The Java collections interface was chosen as the best Java API
for DB given these requirements:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Provide the Java developer with an API that is as familiar and
easy to use as possible.
</p>
</li>
<li>
<p>
Provide access to all, or a large majority, of the features of
the underlying Berkeley DB storage system.
</p>
</li>
<li>
<p>
Compared to the DB API, provide a higher-level API
that is oriented toward Java developers.
</p>
</li>
<li>
<p>
For ease of use, support object-to-data bindings, per-thread
transactions, and some traditional database features such as
foreign keys.
</p>
</li>
<li>
<p>
Provide a thin layer that can be thoroughly tested and which
does not significantly impact the reliability and performance of
DB.
</p>
</li>
</ol>
</div>
<p>
Admittedly there are several things about the Java Collections
API that don't quite fit with DB or with any transactional
database, and therefore there are some new rules for applying the
Java Collections API. However, these disadvantages are considered
to be smaller than the disadvantages of the alternatives:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
A new API not based on the Java Collections API could have been
designed that maps well to DB but is higher-level.
However, this would require designing an entirely new model. The
exceptions for using the Java Collections API are considered easier
to learn than a whole new model. A new model would also require a
long design stabilization period before being as complete and
understandable as either the Java Collections API or the DB
API.
</p>
</li>
<li>
<p>
The ODMG API or another object persistence API could have been
implemented on top of DB. However, an object persistence
implementation would add much code and require a long stabilization
period. And while it may work well for applications that require
object persistence, it would probably never perform well enough for
many other applications.
</p>
</li>
</ul>
</div>
</div>
</div>
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