Null：价值 10 亿美元的错误

final class City {    public function interactionBetween($somePerson, $anotherPerson) {        if ($this->meetingProbability() < random()) {                return null; //no interaction            } else {                return new PersonToPersonInteraction($somePerson, $anotherPerson);        }    }}final class PersonToPersonInteraction {    public function propagate($aVirus) {            if ($this->somePerson->isInfectedWith($aVirus) && $aVirus->infectionProbability() > random()) {                $this->anotherPerson->getInfectedWith($aVirus);        }    }}$covid19 = new Virus();    $janeDoe = new Person();$johnSmith = new Person();    $wuhan = new City();$interaction = $wuhan->interactionBetween($johnSmith, $janeDoe);if ($interaction != null) {        $interaction->propagate($covid19);}/* In this example we modeled the interaction between an infected person and a healthy one.Jane is healthy but might be infected if Virus R0 applies to her.*/

Interface SocialInteraction{    public function propagate($aVirus);}final class SocialDistancing implements SocialInteraction {    public function propagate($aVirus) { //Do nothing !!!!    }}final class PersonToPersonInteraction implements SocialInteraction {    public function propagate($aVirus) {            if ($this->somePerson->isInfectedWith($aVirus) && $aVirus->infectionProbability() > random()) {                $this->anotherPerson->getInfectedWith($aVirus);        }    }}final class City {    public function interactionBetween($aPerson, $anotherPerson) {        return new SocialDistancing(); // The cities are smart enough to implement social distancing to model Person to Person interactions    }}$covid19 = new Virus();    $janeDoe = new Person();$johnSmith = new Person();    $wuhan = new City();$interaction = $wuhan->interactionBetween($johnSmith, $janeDoe);$interaction->propagate($covid19);/* Jane will not be affected since the interaction prevents from propagating the virus

Interface Visitable {    public function accept($aVisitor);}final class Date implements Visitable {    public function accept($aVisitor) {            $aVisitor->visitDate($this);    }}final class DateNotPresent implements Visitable {    public function accept($aVisitor) {            $aVisitor->visitDateNotPresent($this);    }}final class AverageCalculator {    private $count = 0;        private $ageSum = 0;    public function visitDate($aDate) {            $this->count++;            $this->ageSum += today() - $aDate;    }    public function visitDateNotPresent($aDate) {    }    public function average() {        if ($this->count == 0)                return 0;            else                return $this->ageSum / $this->count;    }}function averagePatientsAge($patients) {        $calculator = new AverageCalculator();        foreach ($patients as $patient)            $patient->birthDate()->accept($calculator);        return $calculator->average();}