Github user dineshjoshi commented on a diff in the pull request:
https://github.com/apache/cassandra/pull/244#discussion_r207697381
--- Diff: src/java/org/apache/cassandra/metrics/FrequencySampler.java ---
@@ -0,0 +1,105 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.cassandra.metrics;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.stream.Collectors;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import com.clearspring.analytics.stream.StreamSummary;
+
+/**
+ * Find the most frequent sample. A sample adds to the sum of its key ie
+ * <p>add("x", 10); and add("x", 20); will result in "x" = 30</p> This uses
StreamSummary to only store the
+ * approximate cardinality (capacity) of keys. If the number of distinct keys exceed
the capacity, the error of the
+ * sample may increase depending on distribution of keys among the total set.
+ *
+ * @param <T>
+ */
+public abstract class FrequencySampler<T> extends Sampler<T>
+{
+ private static final Logger logger = LoggerFactory.getLogger(FrequencySampler.class);
+ private boolean enabled = false;
+
+ private StreamSummary<T> summary;
+
+ /**
+ * Start to record samples
+ *
+ * @param capacity
+ * Number of sample items to keep in memory, the lower this is
+ * the less accurate results are. For best results use value
+ * close to cardinality, but understand the memory trade offs.
+ */
+ public synchronized void beginSampling(int capacity)
+ {
+ if (!enabled)
+ {
+ summary = new StreamSummary<T>(capacity);
+ enabled = true;
+ }
+ }
+
+ /**
+ * Call to stop collecting samples, and gather the results
+ * @param count Number of most frequent items to return
+ */
+ public synchronized List<Sample<T>> finishSampling(int count)
+ {
+ List<Sample<T>> results = Collections.EMPTY_LIST;
--- End diff --
`Collections.emptyList()` is safer.
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