Getting / setting by value / reference

package com.tvdhout;

public class Main {

    public static void main(String[] args) {

        Car car20 = new Car(20);
        Person thijs = new Person("Thijs", car20);

        Car thijsCar = thijs.getCar();
        thijsCar = new Car(9000);
        System.out.println(thijsCar);  // prints Car{serialNr=9000}
        System.out.println(thijs.getCar());  // prints Car{serialNr=20}
        // So replacing the object thijsCar we got with a getter does not update the Car in the Person object
        // For that we need a setter

        thijsCar = thijs.getCar();
        thijsCar.setSerialNr(9000);
        System.out.println(thijs.getCar());  // prints Car{serialNr=9000}
        // But we CAN change things inside thijsCar and it will update the Car in the Person object because
        // That is the car that is referred to.
    }
}

class Person {
    private String name;
    private Car car;

    public Person(String name, Car car) {
        this.name = name;
        this.car = car;
    }

    public Car getCar() {
        return car;
    }
}

class Car {
    private int serialNr;

    public Car(int serialNr) {
        this.serialNr = serialNr;
    }

    public int getSerialNr() {
        return serialNr;
    }

    public void setSerialNr(int serialNr) {
        this.serialNr = serialNr;
    }

    @Override
    public String toString() {
        return "Car{" +
                "serialNr=" + serialNr +
                '}';
    }
}