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diff --git a/src/main/java/org/apache/commons/math3/stat/descriptive/moment/FirstMoment.java b/src/main/java/org/apache/commons/math3/stat/descriptive/moment/FirstMoment.java
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+/*
+ * 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.commons.math3.stat.descriptive.moment;
+
+import java.io.Serializable;
+
+import org.apache.commons.math3.exception.NullArgumentException;
+import org.apache.commons.math3.stat.descriptive.AbstractStorelessUnivariateStatistic;
+import org.apache.commons.math3.util.MathUtils;
+
+/**
+ * Computes the first moment (arithmetic mean).  Uses the definitional formula:
+ * <p>
+ * mean = sum(x_i) / n </p>
+ * <p>
+ * where <code>n</code> is the number of observations. </p>
+ * <p>
+ * To limit numeric errors, the value of the statistic is computed using the
+ * following recursive updating algorithm: </p>
+ * <p>
+ * <ol>
+ * <li>Initialize <code>m = </code> the first value</li>
+ * <li>For each additional value, update using <br>
+ *   <code>m = m + (new value - m) / (number of observations)</code></li>
+ * </ol></p>
+ * <p>
+ *  Returns <code>Double.NaN</code> if the dataset is empty. Note that
+ *  Double.NaN may also be returned if the input includes NaN and / or infinite
+ *  values.</p>
+ * <p>
+ * <strong>Note that this implementation is not synchronized.</strong> If
+ * multiple threads access an instance of this class concurrently, and at least
+ * one of the threads invokes the <code>increment()</code> or
+ * <code>clear()</code> method, it must be synchronized externally.</p>
+ *
+ */
+class FirstMoment extends AbstractStorelessUnivariateStatistic
+    implements Serializable {
+
+    /** Serializable version identifier */
+    private static final long serialVersionUID = 6112755307178490473L;
+
+
+    /** Count of values that have been added */
+    protected long n;
+
+    /** First moment of values that have been added */
+    protected double m1;
+
+    /**
+     * Deviation of most recently added value from previous first moment.
+     * Retained to prevent repeated computation in higher order moments.
+     */
+    protected double dev;
+
+    /**
+     * Deviation of most recently added value from previous first moment,
+     * normalized by previous sample size.  Retained to prevent repeated
+     * computation in higher order moments
+     */
+    protected double nDev;
+
+    /**
+     * Create a FirstMoment instance
+     */
+    FirstMoment() {
+        n = 0;
+        m1 = Double.NaN;
+        dev = Double.NaN;
+        nDev = Double.NaN;
+    }
+
+    /**
+     * Copy constructor, creates a new {@code FirstMoment} identical
+     * to the {@code original}
+     *
+     * @param original the {@code FirstMoment} instance to copy
+     * @throws NullArgumentException if original is null
+     */
+     FirstMoment(FirstMoment original) throws NullArgumentException {
+         super();
+         copy(original, this);
+     }
+
+    /**
+     * {@inheritDoc}
+     */
+     @Override
+    public void increment(final double d) {
+        if (n == 0) {
+            m1 = 0.0;
+        }
+        n++;
+        double n0 = n;
+        dev = d - m1;
+        nDev = dev / n0;
+        m1 += nDev;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void clear() {
+        m1 = Double.NaN;
+        n = 0;
+        dev = Double.NaN;
+        nDev = Double.NaN;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public double getResult() {
+        return m1;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public long getN() {
+        return n;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public FirstMoment copy() {
+        FirstMoment result = new FirstMoment();
+        // No try-catch or advertised exception because args are guaranteed non-null
+        copy(this, result);
+        return result;
+    }
+
+    /**
+     * Copies source to dest.
+     * <p>Neither source nor dest can be null.</p>
+     *
+     * @param source FirstMoment to copy
+     * @param dest FirstMoment to copy to
+     * @throws NullArgumentException if either source or dest is null
+     */
+    public static void copy(FirstMoment source, FirstMoment dest)
+        throws NullArgumentException {
+        MathUtils.checkNotNull(source);
+        MathUtils.checkNotNull(dest);
+        dest.setData(source.getDataRef());
+        dest.n = source.n;
+        dest.m1 = source.m1;
+        dest.dev = source.dev;
+        dest.nDev = source.nDev;
+    }
+}