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Java example source code file (TournamentSelection.java)

This example Java source code file (TournamentSelection.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

arraylist, chromosomepair, listpopulation, mathillegalargumentexception, population, selectionpolicy, tournamentselection, util

The TournamentSelection.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.MathIllegalArgumentException;
import org.apache.commons.math3.exception.util.LocalizedFormats;

/**
 * Tournament selection scheme. Each of the two selected chromosomes is selected
 * based on n-ary tournament -- this is done by drawing {@link #arity} random
 * chromosomes without replacement from the population, and then selecting the
 * fittest chromosome among them.
 *
 * @since 2.0
 */
public class TournamentSelection implements SelectionPolicy {

    /** number of chromosomes included in the tournament selections */
    private int arity;

    /**
     * Creates a new TournamentSelection instance.
     *
     * @param arity how many chromosomes will be drawn to the tournament
     */
    public TournamentSelection(final int arity) {
        this.arity = arity;
    }

    /**
     * Select two chromosomes from the population. Each of the two selected
     * chromosomes is selected based on n-ary tournament -- this is done by
     * drawing {@link #arity} random chromosomes without replacement from the
     * population, and then selecting the fittest chromosome among them.
     *
     * @param population the population from which the chromosomes are chosen.
     * @return the selected chromosomes.
     * @throws MathIllegalArgumentException if the tournament arity is bigger than the population size
     */
    public ChromosomePair select(final Population population) throws MathIllegalArgumentException {
        return new ChromosomePair(tournament((ListPopulation) population),
                                  tournament((ListPopulation) population));
    }

    /**
     * Helper for {@link #select(Population)}. Draw {@link #arity} random chromosomes without replacement from the
     * population, and then select the fittest chromosome among them.
     *
     * @param population the population from which the chromosomes are chosen.
     * @return the selected chromosome.
     * @throws MathIllegalArgumentException if the tournament arity is bigger than the population size
     */
    private Chromosome tournament(final ListPopulation population) throws MathIllegalArgumentException {
        if (population.getPopulationSize() < this.arity) {
            throw new MathIllegalArgumentException(LocalizedFormats.TOO_LARGE_TOURNAMENT_ARITY,
                                                   arity, population.getPopulationSize());
        }
        // auxiliary population
        ListPopulation tournamentPopulation = new ListPopulation(this.arity) {
            /** {@inheritDoc} */
            public Population nextGeneration() {
                // not useful here
                return null;
            }
        };

        // create a copy of the chromosome list
        List<Chromosome> chromosomes = new ArrayList (population.getChromosomes());
        for (int i=0; i<this.arity; i++) {
            // select a random individual and add it to the tournament
            int rind = GeneticAlgorithm.getRandomGenerator().nextInt(chromosomes.size());
            tournamentPopulation.addChromosome(chromosomes.get(rind));
            // do not select it again
            chromosomes.remove(rind);
        }
        // the winner takes it all
        return tournamentPopulation.getFittestChromosome();
    }

    /**
     * Gets the arity (number of chromosomes drawn to the tournament).
     *
     * @return arity of the tournament
     */
    public int getArity() {
        return arity;
    }

    /**
     * Sets the arity (number of chromosomes drawn to the tournament).
     *
     * @param arity arity of the tournament
     */
    public void setArity(final int arity) {
        this.arity = arity;
    }

}

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