Question

I have two classes: Apartments and Building. The Apartment class:

public class Apartment {
    private String familyName;
    private int rent;
    private boolean isRented;
}

The Building class has an Apartment array, and a numOfApartments variable which indicates how many apartments the building has, and also a FINAL variable who indicates the max number for apartments in one building.

private Apartment[] apartments;
private int numOfApartments;
private final int MAX_APARTMENTS=20;

public Building() {
    this.apartments=new Apartment[MAX_APARTMENTS];
    this.numOfApartments=0;
}

Now I want to make a method which raises the rent for an apartment by an amount which is taken as parameter:

public void raiseRent(int amount) {
    for (int i=0; i<numOfApartments; i++) {
        apartments[i].setRent(apartments[i].getRent() + amount);
    }
}

the addAppartment method:

public boolean addApartment(Apartment a){ //adds an appartment to the building if
                                           //there's enough space in the array.
        if (this.numOfApartments<this.MAX_APARTMENTS)
        {
            this.apartments[this.numOfApartments]=new Apartment(a);
            this.numOfApartments++;
            return true;
        }
        return false;
    }

The apartment class string ToString method:

public String toString()
{
    if (this.isRented) //if the apartment is rented, prints:
    {
        return "Apartment rent: " + this.rent + ". rented to " + this.familyName + " family.";
    }
    else
        return "Apartment rent: " + this.rent  + ". not rented yet.";
}

The main is:

String familyName; int rent;
System.out.println("enter family name and rent by this order");
familyName=scan.next();
rent=scan.nextInt();

Apartment a=new Apartment(familyName,rent);  //build new apartment object.
Building b=new Building();
System.out.println(b.addApartment(a));
b.raiseRent(200); //initiate the raiseRent method.

Everything here works, but when I write this:

 System.out.println(a);

The rent is shown as the old rent, not the updated one.
When I write this:

System.out.println(b.apartments[0]);

It works.

Why when I write System.out.println(a); it doesn’t work?

Thank you for the answer.

Answer 1

In your

public boolean addApartment(Apartment a) method:

this.apartments[this.numOfApartments]=new Apartment(a);

Creates a new apartment which appears to be constructed from an existing apartment. This is a copy.

After you raise the rent, the

Apartment a=new Apartment(familyName,rent); //build new apartment object.

That you passed in to

b.addApartment(a)

Will not have an updated rent. This is because Java is pass-by-value. It’s a “new” int in the new Apartment your Building Apartment[] collection contains, even if you set it from an existing one (this.rent = other.rent)