Mean.java
/*
* Java Genetic Algorithm Library (@__identifier__@).
* Copyright (c) @__year__@ Franz Wilhelmstötter
*
* Licensed 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.
*
* Author:
* Franz Wilhelmstötter (franz.wilhelmstoetter@gmx.at)
*/
package org.jenetics.stat;
import static java.lang.String.format;
import static org.jenetics.internal.util.object.eq;
import org.jenetics.internal.util.HashBuilder;
import org.jenetics.util.MappedAccumulator;
/**
* <p>Calculate the Arithmetic mean from a finite sample of <i>N</i>
* observations.</p>
* <p><img src="doc-files/arithmetic-mean.gif"
* alt="\bar{x}=\frac{1}{N}\sum_{i=1}^{N}x_i"
* />
* </p>
*
* <p/>
* <strong>Note that this implementation is not synchronized.</strong> If
* multiple threads access this object concurrently, and at least one of the
* threads modifies it, it must be synchronized externally.
*
* @see <a href="http://mathworld.wolfram.com/ArithmeticMean.html">Wolfram MathWorld: Artithmetic Mean</a>
* @see <a href="http://en.wikipedia.org/wiki/Arithmetic_mean">Wikipedia: Arithmetic Mean</a>
*
* @author <a href="mailto:franz.wilhelmstoetter@gmx.at">Franz Wilhelmstötter</a>
* @since 1.0
* @version 1.0 — <em>$Date: 2014-03-01 $</em>
*/
public class Mean<N extends Number> extends MappedAccumulator<N> {
protected double _mean = Double.NaN;
public Mean() {
}
/**
* Return the mean value of the accumulated values.
*
* @return the mean value of the accumulated values, or {@link java.lang.Double#NaN}
* if {@code getSamples() == 0}.
*/
public double getMean() {
return _mean;
}
/**
* Return the
* <a href="https://secure.wikimedia.org/wikipedia/en/wiki/Standard_error_%28statistics%29">
* Standard error
* </a> of the calculated mean.
*
* @return the standard error of the calculated mean.
*/
public double getStandardError() {
double sem = Double.NaN;
if (_samples > 0) {
sem = _mean/Math.sqrt(_samples);
}
return sem;
}
/**
* @throws NullPointerException if the given {@code value} is {@code null}.
*/
@Override
public void accumulate(final N value) {
if (_samples == 0) {
_mean = 0;
}
_mean += (value.doubleValue() - _mean)/(++_samples);
}
@Override
public int hashCode() {
return HashBuilder.of(getClass()).and(super.hashCode()).and(_mean).value();
}
@Override
public boolean equals(final Object obj) {
if (obj == this) {
return true;
}
if (obj == null || getClass() != obj.getClass()) {
return false;
}
final Mean<?> mean = (Mean<?>)obj;
return eq(_mean, mean._mean) && super.equals(mean);
}
@Override
public String toString() {
return format(
"%s[samples=%d, mean=%f, stderr=%f]",
getClass().getSimpleName(),
getSamples(),
getMean(),
getStandardError()
);
}
@Override
public Mean<N> clone() {
return (Mean<N>)super.clone();
}
}