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diff --git a/statistics/src/main/java/com/yahoo/statistics/Callback.java b/statistics/src/main/java/com/yahoo/statistics/Callback.java
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+// Copyright 2016 Yahoo Inc. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.statistics;
+
+/**
+ * Definition of the callback interface for the statistics API. It is a common
+ * use case to need a reference to the Handle (e.g. Value or Counter) which a
+ * callback is related to. Since everything in a Handle since 5.1.4 is fully
+ * initialized from the constructor, it became cumbersome to use Runnable for
+ * the callback and this interface came into use.
+ *
+ * @author <a href="mailto:steinar@yahoo-inc.com">Steinar Knutsen</a>
+ * @since 5.1.4
+ */
+public interface Callback {
+    /**
+     * Invoked each logging cycle right before the events for a Handle are
+     * emitted to the log.
+     *
+     * @param h
+     *            the handle which invoked this callback
+     * @param firstTime
+     *            true the first time the method is invoked from h, false later
+     */
+    public void run(Handle h, boolean firstTime);
+}