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<div class="subTitle">org.apache.mahout.cf.taste.impl.similarity</div>
<h2 title="Class PearsonCorrelationSimilarity" class="title">Class PearsonCorrelationSimilarity</h2>
</div>
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<ul class="inheritance">
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<li>
<ul class="inheritance">
<li><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/AbstractItemSimilarity.html" title="class in org.apache.mahout.cf.taste.impl.similarity">org.apache.mahout.cf.taste.impl.similarity.AbstractItemSimilarity</a></li>
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<li>org.apache.mahout.cf.taste.impl.similarity.PearsonCorrelationSimilarity</li>
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<dt>All Implemented Interfaces:</dt>
<dd><a href="../../../../../../../org/apache/mahout/cf/taste/common/Refreshable.html" title="interface in org.apache.mahout.cf.taste.common">Refreshable</a>, <a href="../../../../../../../org/apache/mahout/cf/taste/similarity/ItemSimilarity.html" title="interface in org.apache.mahout.cf.taste.similarity">ItemSimilarity</a>, <a href="../../../../../../../org/apache/mahout/cf/taste/similarity/UserSimilarity.html" title="interface in org.apache.mahout.cf.taste.similarity">UserSimilarity</a></dd>
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<pre>public final class <span class="typeNameLabel">PearsonCorrelationSimilarity</span>
extends <a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/AbstractItemSimilarity.html" title="class in org.apache.mahout.cf.taste.impl.similarity">AbstractItemSimilarity</a></pre>
<div class="block"><p>
An implementation of the Pearson correlation. For users X and Y, the following values are calculated:
</p>
<ul>
<li>sumX2: sum of the square of all X's preference values</li>
<li>sumY2: sum of the square of all Y's preference values</li>
<li>sumXY: sum of the product of X and Y's preference value for all items for which both X and Y express a
preference</li>
</ul>
<p>
The correlation is then:
<p>
<code>sumXY / sqrt(sumX2 * sumY2)</code>
</p>
<p>
Note that this correlation "centers" its data, shifts the user's preference values so that each of their
means is 0. This is necessary to achieve expected behavior on all data sets.
</p>
<p>
This correlation implementation is equivalent to the cosine similarity since the data it receives
is assumed to be centered -- mean is 0. The correlation may be interpreted as the cosine of the angle
between the two vectors defined by the users' preference values.
</p>
<p>
For cosine similarity on uncentered data, see <a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/UncenteredCosineSimilarity.html" title="class in org.apache.mahout.cf.taste.impl.similarity"><code>UncenteredCosineSimilarity</code></a>.
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<th class="colOne" scope="col">Constructor and Description</th>
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<tr class="altColor">
<td class="colOne"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/PearsonCorrelationSimilarity.html#PearsonCorrelationSimilarity-org.apache.mahout.cf.taste.model.DataModel-">PearsonCorrelationSimilarity</a></span>(<a href="../../../../../../../org/apache/mahout/cf/taste/model/DataModel.html" title="interface in org.apache.mahout.cf.taste.model">DataModel</a>&nbsp;dataModel)</code>&nbsp;</td>
</tr>
<tr class="rowColor">
<td class="colOne"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/PearsonCorrelationSimilarity.html#PearsonCorrelationSimilarity-org.apache.mahout.cf.taste.model.DataModel-org.apache.mahout.cf.taste.common.Weighting-">PearsonCorrelationSimilarity</a></span>(<a href="../../../../../../../org/apache/mahout/cf/taste/model/DataModel.html" title="interface in org.apache.mahout.cf.taste.model">DataModel</a>&nbsp;dataModel,
<a href="../../../../../../../org/apache/mahout/cf/taste/common/Weighting.html" title="enum in org.apache.mahout.cf.taste.common">Weighting</a>&nbsp;weighting)</code>&nbsp;</td>
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<th class="colFirst" scope="col">Modifier and Type</th>
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<td class="colFirst"><code>double[]</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/PearsonCorrelationSimilarity.html#itemSimilarities-long-long:A-">itemSimilarities</a></span>(long&nbsp;itemID1,
long[]&nbsp;itemID2s)</code>
<div class="block">A bulk-get version of <a href="../../../../../../../org/apache/mahout/cf/taste/similarity/ItemSimilarity.html#itemSimilarity-long-long-"><code>ItemSimilarity.itemSimilarity(long, long)</code></a>.</div>
</td>
</tr>
<tr id="i1" class="rowColor">
<td class="colFirst"><code>double</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/PearsonCorrelationSimilarity.html#itemSimilarity-long-long-">itemSimilarity</a></span>(long&nbsp;itemID1,
long&nbsp;itemID2)</code>
<div class="block">
Returns the degree of similarity, of two items, based on the preferences that users have expressed for
the items.</div>
</td>
</tr>
<tr id="i2" class="altColor">
<td class="colFirst"><code>void</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/PearsonCorrelationSimilarity.html#refresh-java.util.Collection-">refresh</a></span>(<a href="http://docs.oracle.com/javase/7/docs/api/java/util/Collection.html?is-external=true" title="class or interface in java.util">Collection</a>&lt;<a href="../../../../../../../org/apache/mahout/cf/taste/common/Refreshable.html" title="interface in org.apache.mahout.cf.taste.common">Refreshable</a>&gt;&nbsp;alreadyRefreshed)</code>
<div class="block">
Triggers "refresh" -- whatever that means -- of the implementation.</div>
</td>
</tr>
<tr id="i3" class="rowColor">
<td class="colFirst"><code>void</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/PearsonCorrelationSimilarity.html#setPreferenceInferrer-org.apache.mahout.cf.taste.similarity.PreferenceInferrer-">setPreferenceInferrer</a></span>(<a href="../../../../../../../org/apache/mahout/cf/taste/similarity/PreferenceInferrer.html" title="interface in org.apache.mahout.cf.taste.similarity">PreferenceInferrer</a>&nbsp;inferrer)</code>
<div class="block">
Attaches a <a href="../../../../../../../org/apache/mahout/cf/taste/similarity/PreferenceInferrer.html" title="interface in org.apache.mahout.cf.taste.similarity"><code>PreferenceInferrer</code></a> to the <a href="../../../../../../../org/apache/mahout/cf/taste/similarity/UserSimilarity.html" title="interface in org.apache.mahout.cf.taste.similarity"><code>UserSimilarity</code></a> implementation.</div>
</td>
</tr>
<tr id="i4" class="altColor">
<td class="colFirst"><code><a href="http://docs.oracle.com/javase/7/docs/api/java/lang/String.html?is-external=true" title="class or interface in java.lang">String</a></code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/PearsonCorrelationSimilarity.html#toString--">toString</a></span>()</code>&nbsp;</td>
</tr>
<tr id="i5" class="rowColor">
<td class="colFirst"><code>double</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/PearsonCorrelationSimilarity.html#userSimilarity-long-long-">userSimilarity</a></span>(long&nbsp;userID1,
long&nbsp;userID2)</code>
<div class="block">
Returns the degree of similarity, of two users, based on the their preferences.</div>
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<h3>Methods inherited from class&nbsp;org.apache.mahout.cf.taste.impl.similarity.<a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/AbstractItemSimilarity.html" title="class in org.apache.mahout.cf.taste.impl.similarity">AbstractItemSimilarity</a></h3>
<code><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/AbstractItemSimilarity.html#allSimilarItemIDs-long-">allSimilarItemIDs</a>, <a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/AbstractItemSimilarity.html#getDataModel--">getDataModel</a></code></li>
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<code><a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true#clone--" title="class or interface in java.lang">clone</a>, <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true#equals-java.lang.Object-" title="class or interface in java.lang">equals</a>, <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true#finalize--" title="class or interface in java.lang">finalize</a>, <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true#getClass--" title="class or interface in java.lang">getClass</a>, <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true#hashCode--" title="class or interface in java.lang">hashCode</a>, <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true#notify--" title="class or interface in java.lang">notify</a>, <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true#notifyAll--" title="class or interface in java.lang">notifyAll</a>, <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true#wait--" title="class or interface in java.lang">wait</a>, <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true#wait-long-" title="class or interface in java.lang">wait</a>, <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true#wait-long-int-" title="class or interface in java.lang">wait</a></code></li>
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<h4>PearsonCorrelationSimilarity</h4>
<pre>public&nbsp;PearsonCorrelationSimilarity(<a href="../../../../../../../org/apache/mahout/cf/taste/model/DataModel.html" title="interface in org.apache.mahout.cf.taste.model">DataModel</a>&nbsp;dataModel)
throws <a href="../../../../../../../org/apache/mahout/cf/taste/common/TasteException.html" title="class in org.apache.mahout.cf.taste.common">TasteException</a></pre>
<dl>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="http://docs.oracle.com/javase/7/docs/api/java/lang/IllegalArgumentException.html?is-external=true" title="class or interface in java.lang">IllegalArgumentException</a></code> - if <a href="../../../../../../../org/apache/mahout/cf/taste/model/DataModel.html" title="interface in org.apache.mahout.cf.taste.model"><code>DataModel</code></a> does not have preference values</dd>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/common/TasteException.html" title="class in org.apache.mahout.cf.taste.common">TasteException</a></code></dd>
</dl>
</li>
</ul>
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<h4>PearsonCorrelationSimilarity</h4>
<pre>public&nbsp;PearsonCorrelationSimilarity(<a href="../../../../../../../org/apache/mahout/cf/taste/model/DataModel.html" title="interface in org.apache.mahout.cf.taste.model">DataModel</a>&nbsp;dataModel,
<a href="../../../../../../../org/apache/mahout/cf/taste/common/Weighting.html" title="enum in org.apache.mahout.cf.taste.common">Weighting</a>&nbsp;weighting)
throws <a href="../../../../../../../org/apache/mahout/cf/taste/common/TasteException.html" title="class in org.apache.mahout.cf.taste.common">TasteException</a></pre>
<dl>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="http://docs.oracle.com/javase/7/docs/api/java/lang/IllegalArgumentException.html?is-external=true" title="class or interface in java.lang">IllegalArgumentException</a></code> - if <a href="../../../../../../../org/apache/mahout/cf/taste/model/DataModel.html" title="interface in org.apache.mahout.cf.taste.model"><code>DataModel</code></a> does not have preference values</dd>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/common/TasteException.html" title="class in org.apache.mahout.cf.taste.common">TasteException</a></code></dd>
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<h3>Method Detail</h3>
<a name="setPreferenceInferrer-org.apache.mahout.cf.taste.similarity.PreferenceInferrer-">
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<h4>setPreferenceInferrer</h4>
<pre>public final&nbsp;void&nbsp;setPreferenceInferrer(<a href="../../../../../../../org/apache/mahout/cf/taste/similarity/PreferenceInferrer.html" title="interface in org.apache.mahout.cf.taste.similarity">PreferenceInferrer</a>&nbsp;inferrer)</pre>
<div class="block"><span class="descfrmTypeLabel">Description copied from interface:&nbsp;<code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/UserSimilarity.html#setPreferenceInferrer-org.apache.mahout.cf.taste.similarity.PreferenceInferrer-">UserSimilarity</a></code></span></div>
<div class="block"><p>
Attaches a <a href="../../../../../../../org/apache/mahout/cf/taste/similarity/PreferenceInferrer.html" title="interface in org.apache.mahout.cf.taste.similarity"><code>PreferenceInferrer</code></a> to the <a href="../../../../../../../org/apache/mahout/cf/taste/similarity/UserSimilarity.html" title="interface in org.apache.mahout.cf.taste.similarity"><code>UserSimilarity</code></a> implementation.
</p></div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/UserSimilarity.html#setPreferenceInferrer-org.apache.mahout.cf.taste.similarity.PreferenceInferrer-">setPreferenceInferrer</a></code>&nbsp;in interface&nbsp;<code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/UserSimilarity.html" title="interface in org.apache.mahout.cf.taste.similarity">UserSimilarity</a></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>inferrer</code> - <a href="../../../../../../../org/apache/mahout/cf/taste/similarity/PreferenceInferrer.html" title="interface in org.apache.mahout.cf.taste.similarity"><code>PreferenceInferrer</code></a></dd>
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<h4>userSimilarity</h4>
<pre>public&nbsp;double&nbsp;userSimilarity(long&nbsp;userID1,
long&nbsp;userID2)
throws <a href="../../../../../../../org/apache/mahout/cf/taste/common/TasteException.html" title="class in org.apache.mahout.cf.taste.common">TasteException</a></pre>
<div class="block"><span class="descfrmTypeLabel">Description copied from interface:&nbsp;<code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/UserSimilarity.html#userSimilarity-long-long-">UserSimilarity</a></code></span></div>
<div class="block"><p>
Returns the degree of similarity, of two users, based on the their preferences.
</p></div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/UserSimilarity.html#userSimilarity-long-long-">userSimilarity</a></code>&nbsp;in interface&nbsp;<code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/UserSimilarity.html" title="interface in org.apache.mahout.cf.taste.similarity">UserSimilarity</a></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>userID1</code> - first user ID</dd>
<dd><code>userID2</code> - second user ID</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>similarity between the users, in [-1,1] or <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Double.html?is-external=true#NaN" title="class or interface in java.lang"><code>Double.NaN</code></a> similarity is unknown</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/common/NoSuchUserException.html" title="class in org.apache.mahout.cf.taste.common">NoSuchUserException</a></code> - if either user is known to be non-existent in the data</dd>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/common/TasteException.html" title="class in org.apache.mahout.cf.taste.common">TasteException</a></code> - if an error occurs while accessing the data</dd>
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<h4>itemSimilarity</h4>
<pre>public final&nbsp;double&nbsp;itemSimilarity(long&nbsp;itemID1,
long&nbsp;itemID2)
throws <a href="../../../../../../../org/apache/mahout/cf/taste/common/TasteException.html" title="class in org.apache.mahout.cf.taste.common">TasteException</a></pre>
<div class="block"><span class="descfrmTypeLabel">Description copied from interface:&nbsp;<code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/ItemSimilarity.html#itemSimilarity-long-long-">ItemSimilarity</a></code></span></div>
<div class="block"><p>
Returns the degree of similarity, of two items, based on the preferences that users have expressed for
the items.
</p></div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/ItemSimilarity.html#itemSimilarity-long-long-">itemSimilarity</a></code>&nbsp;in interface&nbsp;<code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/ItemSimilarity.html" title="interface in org.apache.mahout.cf.taste.similarity">ItemSimilarity</a></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>itemID1</code> - first item ID</dd>
<dd><code>itemID2</code> - second item ID</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>similarity between the items, in [-1,1] or <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Double.html?is-external=true#NaN" title="class or interface in java.lang"><code>Double.NaN</code></a> similarity is unknown</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/common/NoSuchItemException.html" title="class in org.apache.mahout.cf.taste.common">NoSuchItemException</a></code> - if either item is known to be non-existent in the data</dd>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/common/TasteException.html" title="class in org.apache.mahout.cf.taste.common">TasteException</a></code> - if an error occurs while accessing the data</dd>
</dl>
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</ul>
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<h4>itemSimilarities</h4>
<pre>public&nbsp;double[]&nbsp;itemSimilarities(long&nbsp;itemID1,
long[]&nbsp;itemID2s)
throws <a href="../../../../../../../org/apache/mahout/cf/taste/common/TasteException.html" title="class in org.apache.mahout.cf.taste.common">TasteException</a></pre>
<div class="block"><span class="descfrmTypeLabel">Description copied from interface:&nbsp;<code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/ItemSimilarity.html#itemSimilarities-long-long:A-">ItemSimilarity</a></code></span></div>
<div class="block"><p>A bulk-get version of <a href="../../../../../../../org/apache/mahout/cf/taste/similarity/ItemSimilarity.html#itemSimilarity-long-long-"><code>ItemSimilarity.itemSimilarity(long, long)</code></a>.</p></div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/ItemSimilarity.html#itemSimilarities-long-long:A-">itemSimilarities</a></code>&nbsp;in interface&nbsp;<code><a href="../../../../../../../org/apache/mahout/cf/taste/similarity/ItemSimilarity.html" title="interface in org.apache.mahout.cf.taste.similarity">ItemSimilarity</a></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>itemID1</code> - first item ID</dd>
<dd><code>itemID2s</code> - second item IDs to compute similarity with</dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>similarity between itemID1 and other items</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/common/NoSuchItemException.html" title="class in org.apache.mahout.cf.taste.common">NoSuchItemException</a></code> - if any item is known to be non-existent in the data</dd>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/common/TasteException.html" title="class in org.apache.mahout.cf.taste.common">TasteException</a></code> - if an error occurs while accessing the data</dd>
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<a name="refresh-java.util.Collection-">
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<h4>refresh</h4>
<pre>public final&nbsp;void&nbsp;refresh(<a href="http://docs.oracle.com/javase/7/docs/api/java/util/Collection.html?is-external=true" title="class or interface in java.util">Collection</a>&lt;<a href="../../../../../../../org/apache/mahout/cf/taste/common/Refreshable.html" title="interface in org.apache.mahout.cf.taste.common">Refreshable</a>&gt;&nbsp;alreadyRefreshed)</pre>
<div class="block"><span class="descfrmTypeLabel">Description copied from interface:&nbsp;<code><a href="../../../../../../../org/apache/mahout/cf/taste/common/Refreshable.html#refresh-java.util.Collection-">Refreshable</a></code></span></div>
<div class="block"><p>
Triggers "refresh" -- whatever that means -- of the implementation. The general contract is that any
<a href="../../../../../../../org/apache/mahout/cf/taste/common/Refreshable.html" title="interface in org.apache.mahout.cf.taste.common"><code>Refreshable</code></a> should always leave itself in a consistent, operational state, and that the refresh
atomically updates internal state from old to new.
</p></div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/common/Refreshable.html#refresh-java.util.Collection-">refresh</a></code>&nbsp;in interface&nbsp;<code><a href="../../../../../../../org/apache/mahout/cf/taste/common/Refreshable.html" title="interface in org.apache.mahout.cf.taste.common">Refreshable</a></code></dd>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/AbstractItemSimilarity.html#refresh-java.util.Collection-">refresh</a></code>&nbsp;in class&nbsp;<code><a href="../../../../../../../org/apache/mahout/cf/taste/impl/similarity/AbstractItemSimilarity.html" title="class in org.apache.mahout.cf.taste.impl.similarity">AbstractItemSimilarity</a></code></dd>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>alreadyRefreshed</code> - <a href="../../../../../../../org/apache/mahout/cf/taste/common/Refreshable.html" title="interface in org.apache.mahout.cf.taste.common"><code>Refreshable</code></a>s that are known to have already been
refreshed as a result of an initial call to a {#refresh(Collection)} method on some
object. This ensure that objects in a refresh dependency graph aren't refreshed twice
needlessly.</dd>
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<h4>toString</h4>
<pre>public final&nbsp;<a href="http://docs.oracle.com/javase/7/docs/api/java/lang/String.html?is-external=true" title="class or interface in java.lang">String</a>&nbsp;toString()</pre>
<dl>
<dt><span class="overrideSpecifyLabel">Overrides:</span></dt>
<dd><code><a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true#toString--" title="class or interface in java.lang">toString</a></code>&nbsp;in class&nbsp;<code><a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a></code></dd>
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