public class Main {

    public static void main(String[] args) {
        House house = new House();

        house.sethouseNbr(1);
        house.setaddress("Main Street 2");
        house.setbuyingPrice(25500000);

        Apartment a1 = new Apartment(),
                  a2 = new Apartment();

        a1.setNbr(1);
        a1.setRentPerMonth(6300);
        a1.setArea(40);
        a2.setNbr(2);
        a2.setRentPerMonth(8400);
        a2.setArea(65);

        house.addApartment(a1);
        house.addApartment(a2);

        house.updateRentPerMonth(1, 7000);
    }
}

import java.util.ArrayList;

public class House {

    private int houseNbr;
    private String address;
    private double buyingPrice;
    private ArrayList<Apartment> myList = new ArrayList<Apartment>();

    public void updateRentPerMonth(int apartmenNbr, double newRent) {
        Apartment a = findApartment(apartmenNbr);
        if (a != null)
            a.setRentPerMonth(newRent);
    }

    public Apartment findApartment(int nbr) {
        for (int i = 0; i < myList.size(); i++) {
            Apartment temp = myList.get(i);
            if (nbr == temp.getNbr())
                return temp;
        }
        return null;
    }

    public double totalRentPerMonth() {
        double totalRent = 0;
        for (int i=0; i < myList.size(); i++)
            totalRent += myList.get(i).getRentPerMonth();
        return totalRent;
    }

    public int area() {
        int area = 0;
        for (int i=0; i < myList.size(); i++)
            area += myList.get(i).getArea();

        return area;
    }

    public void addApartment(Apartment a) {
        myList.add(a);
    }

    public int gethouseNbr() {
        return this.houseNbr;
    }

    public String getaddress() {
        return this.address;
    }

    public double getbuyingPrice() {
        return this.buyingPrice;
    }

    public void sethouseNbr(int hnr) {
        this.houseNbr = hnr;
    }

    public void setaddress(String adr) {
        this.address = adr;
    }

    public void setbuyingPrice(double bpr) {
        this.buyingPrice = bpr;
    }
}

public class Apartment {

    private int nbr;
    private double rentPerMonth = 0;
    private int area = 0;

    public double rentPerSqrm() {
        return this.rentPerMonth / area;
    }

    public double rentPerYear() {
        return this.rentPerMonth * 12;
    }

    public double getRentPerMonth() {
        return this.rentPerMonth;
    }

    public int getNbr() {
        return this.nbr;
    }

    public int getArea() {
        return this.area;
    }

    public void setNbr(int i) {
        this.nbr = i;
    }

    public void setRentPerMonth(double d) {
        this.rentPerMonth = d;
    }

    public void setArea(int a) {
        this.area = a;
    }

}