In Part 5 Primitive and Reference variables section, there is a Person class defined and then another class called TimeMachine which has a reference variable defined for type Person.

public class TimeMachine {

    private Person traveler;

    public TimeMachine(Person person) {
        this.traveler = person;
    }

    public void travelInTime(int years) {
        this.traveler.setYear(this.traveler.getYear() + years);
    }
}

Driver class:

public class TimeMachineMain {

    public static void main(String[] args){

        Person lorraine = new Person();
        TimeMachine tardis = new TimeMachine(lorraine);
        tardis.travelInTime(6);
        System.out.println(lorraine.getYear());

    }
}

In the next section, there is another class called AmusementParkRide which doesn’t have an instance variable of type Person but is passing Person as method parameter to the method allowedToRide directly.

public class AmusementParkRide {

    private String name;
    private int lowestHeight;

    public AmusementParkRide(String name, int lowestHeight){
        this.name = name;
        this.lowestHeight = lowestHeight;
    }   

    public boolean allowedToRide(Person person){
        if (person.getHeight() < this.lowestHeight){
            return false;     
        }

        return true;
    }

    public String toString(){
        return this.name + ", minimum height: " + this.lowestHeight;
    }
}

Driver class:

public class AmusementParkRideMain {

    public static void main(String[] args) {

        Person matt = new Person("Matt");
        matt.setWeight(86);
        matt.setHeight(180);    
    
        Person jasper = new Person("Jasper");
        jasper.setWeight(34);
        jasper.setHeight(132);

        AmusementParkRide waterTrack = new AmusementParkRide("Water track", 140);

        if (waterTrack.allowedToRide(matt)) {
            System.out.println(matt.getName() + " may enter the ride");
        } else {
            System.out.println(matt.getName() + " may not enter the ride");
        }

        if (waterTrack.allowedToRide(jasper)) {
            System.out.println(jasper.getName() + " may enter the ride");
        } else {
            System.out.println(jasper.getName() + " may not enter the ride");
        }

        System.out.println(waterTrack);

    }
}

Question: How do you know when to declare an object as a reference variable in another object? Based on the above examples, TimeMachine doesn’t really have a HAS A relationship with the class person, intuitively, so not sure why it was implemented like that. Would this same implementation have been better like this, similar to how it’s done in the AmusementParkRide class?

public class NewTimeMachine {

    public void travelInTime(Person person, int years) {
        person.setYear(person.getYear() + years);
    }

}

Driver class:

public class NewTimeMachineMain {

    public static void main(String[] args){

        Person lorraine = new Person("Lorraine");
        NewTimeMachine tardis = new NewTimeMachine();
        tardis.travelInTime(lorraine, 6);
        System.out.println(lorraine.getYear());

    }

}

Tried the approach above by creating the NewTimeMachine class and got the same result as the TimeMachine example. Just need to understand why/when to declare an object as a reference variable in another object vs how it was implemented in the AmusementParkRide example.