public Individual pickRandomIndividual(List<Individual> pool) {
    Double totalScore = 0d;
    Double runningScore = 0d;
    for (Individual ind : pool) {
        totalScore += ind.fitnessValue;
    }

    Double rnd = Math.random() * totalScore;

    for (Individual ind : pool) {
        if (rnd >= runningScore &&
                rnd <= runningScore + ind.fitnessValue) {
            return ind;
        }
        runningScore += ind.fitnessValue;
    }
    return null;
}

Double fitnessSum = genePool.stream()
                .mapToDouble(individual -> individual.fitnessValue)
                .sum();
Double runningScore = 0d;

Double rnd = random() * fitnessSum;

for (Individual g : genePool) {
    if (rnd >= runningScore && rnd <= runningScore + g.fitnessValue) {
            return g;
    }
    runningScore += g.fitnessValue;
}
return null;