The Weasle-program with Jenetics (https://en.wikipedia.org/wiki/Weasel_program)

WeaselProgram.java

class WeaselSelector<
	G extends Gene<?, G>,
	C extends Comparable<? super C>
>
	implements Selector<G, C>
{
	@Override
	public Population<G, C> select(
		final Population<G, C> population,
		final int count,
		final Optimize opt
	) {
		requireNonNull(population, "Population");
		requireNonNull(opt, "Optimization");
		if (count < 0) {
			throw new IllegalArgumentException(format(
				"Selection count must be greater or equal then zero, but was %s",
				count
			));
		}

		final MinMax<Phenotype<G, C>> minMax = population.stream()
			.collect(MinMax.toMinMax(opt.ascending()));

		return new Population<G, C>(count).fill(minMax::getMax, count);
	}
}

class WeaselMutator<
	G extends Gene<?, G>,
	C extends Comparable<? super C>
>
	extends Mutator<G, C>
{

	public WeaselMutator(final double probability) {
		super(probability);
	}

	public WeaselMutator() {
		this(0.05);
	}

	@Override
	public int alter(final Population<G, C> population, final long generation) {
		final IntRef alterations = new IntRef(0);

		for (int i = 0; i < population.size(); ++i) {
			final Phenotype<G, C> pt = population.get(i);

			final Genotype<G> gt = pt.getGenotype();
			final Genotype<G> mgt = mutate(gt, alterations);
			final Phenotype<G, C> mpt = pt.newInstance(mgt, generation);
			population.set(i, mpt);
		}

		return alterations.value;
	}

	private Genotype<G> mutate(
		final Genotype<G> genotype,
		final IntRef alterations
	) {
		final MSeq<Chromosome<G>> chromosomes = genotype.toSeq().copy();

		alterations.value += IntStream.range(0, chromosomes.size())
			.map(i -> mutate(chromosomes, i))
			.sum();

		return Genotype.of(chromosomes);
	}

	private int mutate(final MSeq<Chromosome<G>> c, final int index) {
		final Chromosome<G> chromosome = c.get(index);
		final MSeq<G> genes = chromosome.toSeq().copy();

		final int mutations = mutate(genes, _probability);
		if (mutations > 0) {
			c.set(index, chromosome.newInstance(genes.toISeq()));
		}
		return mutations;
	}
}

// See: https://en.wikipedia.org/wiki/Weasel_program
public class WeaselProgram {

	private static final String TARGET_STRING = "methinks it is like a weasel";

	private static Integer evaluate(final Genotype<CharacterGene> gt) {
		final CharSequence source = (CharSequence)gt.getChromosome();

		return IntStream.range(0, TARGET_STRING.length())
			.map(i -> source.charAt(i) == TARGET_STRING.charAt(i) ? 1 : 0)
			.sum();
	}

	public static void main(String[] args) throws Exception {
		final CharSeq chars = CharSeq.of("a-z ");
		final Factory<Genotype<CharacterGene>> gtf = Genotype.of(
			new CharacterChromosome(chars, TARGET_STRING.length())
		);

		final Engine<CharacterGene, Integer> engine = Engine
			.builder(WeaselProgram::evaluate, gtf)
			.populationSize(200)
			.selector(new WeaselSelector<>())
			.offspringFraction(1)
			.alterers(new WeaselMutator<>(0.05))
			.build();

		final Phenotype<CharacterGene, Integer> result = engine.stream()
			.limit(50)
			.peek(r -> System.out.println(r.getBestPhenotype()))
			.collect(toBestPhenotype());

		System.out.println(result);
	}
}

/*
Output:

[mjtyx rj ibqqnxgatyrhsdkabex] --> 6
[mjtyx rj ibqqnxgakyrhsdeabex] --> 8
[mjtye rj ibqqnxgakyrhsdeasex] --> 9
[metye rs ibqqnxggkgrhsyeasex] --> 11
[metye rs ibqqsxggkgrhsaeasex] --> 12
[metye rs ibqqsxggkgrhsaeasel] --> 13
[metye rs ibdisxggkgrhsaeasel] --> 14
[metye rs ibdisxggkg hsaeasel] --> 15
[metye hs ibdiszggkg esweasel] --> 16
[metye hs ibdiszggkg asweasel] --> 17
[metye hs ibdiszggkg asweasel] --> 17
[metye hs ib iszggkg asweasel] --> 18
[metye hs ib iszggkg asweasel] --> 18
[metye hs ib iszggkg asweasel] --> 18
[metye hs ib iszggkg a weasel] --> 19
[metye hs ib iszggkg a weasel] --> 19
[metye hs ib iszgikg a weasel] --> 20
[metye hs ib iszgikg a weasel] --> 20
[metye hs ib iszgikg a weasel] --> 20
[metyenhs ib iszgikg a weasel] --> 21
[metyenhs ib is gikg a weasel] --> 22
[methenhs ib is gikg a weasel] --> 23
[methenhs ib is gike a weasel] --> 24
[methenhs it is gike a weasel] --> 25
[methenhs it is gike a weasel] --> 25
[methenhs it is gike a weasel] --> 25
[methenhs it is gike a weasel] --> 25
[methenhs it is gike a weasel] --> 25
[methenhs it is gike a weasel] --> 25
[methenhs it is like a weasel] --> 26
[methinhs it is like a weasel] --> 27
[methinhs it is like a weasel] --> 27
[methinhs it is like a weasel] --> 27
[methinhs it is like a weasel] --> 27
[methinhs it is like a weasel] --> 27
[methinhs it is like a weasel] --> 27
[methinhs it is like a weasel] --> 27
[methinhs it is like a weasel] --> 27
[methinhs it is like a weasel] --> 27
[methinhs it is like a weasel] --> 27
[methinhs it is like a weasel] --> 27
[methinhs it is like a weasel] --> 27
[methinks it is like a weasel] --> 28
*/