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Java example source code file (NPointCrossover.java)

This example Java source code file (NPointCrossover.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

abstractlistchromosome, arraylist, chromosomepair, crossoverpolicy, dimensionmismatchexception, list, mathillegalargumentexception, notstrictlypositiveexception, npointcrossover, numberistoolargeexception, randomgenerator, suppresswarnings, util

The NPointCrossover.java Java example source code

/*
 * 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.genetics;

import java.util.ArrayList;
import java.util.List;

import org.apache.commons.math3.exception.DimensionMismatchException;
import org.apache.commons.math3.exception.MathIllegalArgumentException;
import org.apache.commons.math3.exception.NotStrictlyPositiveException;
import org.apache.commons.math3.exception.NumberIsTooLargeException;
import org.apache.commons.math3.exception.util.LocalizedFormats;
import org.apache.commons.math3.random.RandomGenerator;

/**
 * N-point crossover policy. For each iteration a random crossover point is
 * selected and the first part from each parent is copied to the corresponding
 * child, and the second parts are copied crosswise.
 *
 * Example (2-point crossover):
 * <pre>
 * -C- denotes a crossover point
 *           -C-       -C-                         -C-        -C-
 * p1 = (1 0  | 1 0 0 1 | 0 1 1)    X    p2 = (0 1  | 1 0 1 0  | 1 1 1)
 *      \----/ \-------/ \-----/              \----/ \--------/ \-----/
 *        ||      (*)       ||                  ||      (**)       ||
 *        VV      (**)      VV                  VV      (*)        VV
 *      /----\ /--------\ /-----\             /----\ /--------\ /-----\
 * c1 = (1 0  | 1 0 1 0  | 0 1 1)    X   c2 = (0 1  | 1 0 0 1  | 0 1 1)
 * </pre>
 *
 * This policy works only on {@link AbstractListChromosome}, and therefore it
 * is parameterized by T. Moreover, the chromosomes must have same lengths.
 *
 * @param <T> generic type of the {@link AbstractListChromosome}s for crossover
 * @since 3.1
 */
public class NPointCrossover<T> implements CrossoverPolicy {

    /** The number of crossover points. */
    private final int crossoverPoints;

    /**
     * Creates a new {@link NPointCrossover} policy using the given number of points.
     * <p>
     * <b>Note: the number of crossover points must be < chromosome length - 1.
     * This condition can only be checked at runtime, as the chromosome length is not known in advance.
     *
     * @param crossoverPoints the number of crossover points
     * @throws NotStrictlyPositiveException if the number of {@code crossoverPoints} is not strictly positive
     */
    public NPointCrossover(final int crossoverPoints) throws NotStrictlyPositiveException {
        if (crossoverPoints <= 0) {
            throw new NotStrictlyPositiveException(crossoverPoints);
        }
        this.crossoverPoints = crossoverPoints;
    }

    /**
     * Returns the number of crossover points used by this {@link CrossoverPolicy}.
     *
     * @return the number of crossover points
     */
    public int getCrossoverPoints() {
        return crossoverPoints;
    }

    /**
     * Performs a N-point crossover. N random crossover points are selected and are used
     * to divide the parent chromosomes into segments. The segments are copied in alternate
     * order from the two parents to the corresponding child chromosomes.
     *
     * Example (2-point crossover):
     * <pre>
     * -C- denotes a crossover point
     *           -C-       -C-                         -C-        -C-
     * p1 = (1 0  | 1 0 0 1 | 0 1 1)    X    p2 = (0 1  | 1 0 1 0  | 1 1 1)
     *      \----/ \-------/ \-----/              \----/ \--------/ \-----/
     *        ||      (*)       ||                  ||      (**)       ||
     *        VV      (**)      VV                  VV      (*)        VV
     *      /----\ /--------\ /-----\             /----\ /--------\ /-----\
     * c1 = (1 0  | 1 0 1 0  | 0 1 1)    X   c2 = (0 1  | 1 0 0 1  | 0 1 1)
     * </pre>
     *
     * @param first first parent (p1)
     * @param second second parent (p2)
     * @return pair of two children (c1,c2)
     * @throws MathIllegalArgumentException iff one of the chromosomes is
     *   not an instance of {@link AbstractListChromosome}
     * @throws DimensionMismatchException if the length of the two chromosomes is different
     */
    @SuppressWarnings("unchecked") // OK because of instanceof checks
    public ChromosomePair crossover(final Chromosome first, final Chromosome second)
        throws DimensionMismatchException, MathIllegalArgumentException {

        if (!(first instanceof AbstractListChromosome<?> && second instanceof AbstractListChromosome)) {
            throw new MathIllegalArgumentException(LocalizedFormats.INVALID_FIXED_LENGTH_CHROMOSOME);
        }
        return mate((AbstractListChromosome<T>) first, (AbstractListChromosome) second);
    }

    /**
     * Helper for {@link #crossover(Chromosome, Chromosome)}. Performs the actual crossover.
     *
     * @param first the first chromosome
     * @param second the second chromosome
     * @return the pair of new chromosomes that resulted from the crossover
     * @throws DimensionMismatchException if the length of the two chromosomes is different
     * @throws NumberIsTooLargeException if the number of crossoverPoints is too large for the actual chromosomes
     */
    private ChromosomePair mate(final AbstractListChromosome<T> first,
                                final AbstractListChromosome<T> second)
        throws DimensionMismatchException, NumberIsTooLargeException {

        final int length = first.getLength();
        if (length != second.getLength()) {
            throw new DimensionMismatchException(second.getLength(), length);
        }
        if (crossoverPoints >= length) {
            throw new NumberIsTooLargeException(crossoverPoints, length, false);
        }

        // array representations of the parents
        final List<T> parent1Rep = first.getRepresentation();
        final List<T> parent2Rep = second.getRepresentation();
        // and of the children
        final List<T> child1Rep = new ArrayList(length);
        final List<T> child2Rep = new ArrayList(length);

        final RandomGenerator random = GeneticAlgorithm.getRandomGenerator();

        List<T> c1 = child1Rep;
        List<T> c2 = child2Rep;

        int remainingPoints = crossoverPoints;
        int lastIndex = 0;
        for (int i = 0; i < crossoverPoints; i++, remainingPoints--) {
            // select the next crossover point at random
            final int crossoverIndex = 1 + lastIndex + random.nextInt(length - lastIndex - remainingPoints);

            // copy the current segment
            for (int j = lastIndex; j < crossoverIndex; j++) {
                c1.add(parent1Rep.get(j));
                c2.add(parent2Rep.get(j));
            }

            // swap the children for the next segment
            List<T> tmp = c1;
            c1 = c2;
            c2 = tmp;

            lastIndex = crossoverIndex;
        }

        // copy the last segment
        for (int j = lastIndex; j < length; j++) {
            c1.add(parent1Rep.get(j));
            c2.add(parent2Rep.get(j));
        }

        return new ChromosomePair(first.newFixedLengthChromosome(child1Rep),
                                  second.newFixedLengthChromosome(child2Rep));
    }
}

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