Classes as an instance field in another class

Question

I have tried turning to google and youtube for answers and looked at various possibilities such as nested classes, inner classes, and local inner classes. They don't seem to provide the answers I'm looking for.

I am following a program online and one of the exercises we have to make a carpet cost calculator. We times the width and the length of the floor and then times that by the carpet cost.

To do this we were instructed to create 3 classes.

"Floor" class and calculate the area of the floor:

public class Floor {

    private double length;
    private double width;

    public Floor(double width, double length) {

        this.width = width;
        this.length = length;
    }

    public double getArea() {
        return this.width * this.length;
    }
}

"Carpet" class and it holds the cost of the carpet:

public class Carpet {

    private double cost;

    public Carpet(double cost) {
        this.cost = cost;
    }

    public double getCost() {
        return this.cost;
    }
}

"Calculator" class which formulates the two classes methods together to produce the cost

public class Calculator {

    private Floor floor;
    private Carpet carpet;

    public Calculator(Floor floor, Carpet carpet) {
        this.floor = floor;
        this.carpet = carpet;

    }

    public double getTotalCost() {
        return floor.getArea() * carpet.getCost();
    }
}

what I don't understand is that in the calculator class we make 2 instance fields called:

private Floor floor;

Private Carpet carpet;

So 2 classes have become instance fields in another class. I don't really understand how that works or what is going on here. I would really like to understand because I want to learn. I will greatly appreciate any explanations that will assist this process.

https://www.geeksforgeeks.org/classes-objects-java/ might help you understand the concepts. — prembhaskal, Jun 12 '20 at 11:33
Does this answer your question? [The difference between Classes, Objects, and Instances](https://stackoverflow.com/questions/1215881/the-difference-between-classes-objects-and-instances) — Janez Kuhar, Jun 12 '20 at 11:47
Well, this is the whole point of object-oriented programming. A class is a blueprint of an object. We make classes to model objects, but an object may be consisting of other objects. For example, a `Car` has probably a `Wheel`, which in turn has a radius. `Car` probably also has an `Engine`. — MC Emperor, Jun 12 '20 at 18:36

score 1 · Answer 1 · answered Jun 12 '20 at 11:39

There is nothing hard in it. When you create an instance of Calculator you pass Carpet and Floor objects into constructor of Calculator like this:

Floor floor1 = new Floor(100, 100);
Carpet carpet1 = new Carpet(1000);

and then you pass these two objects into Calculator:

Calculator calculator = new Calculator(floor1, carpet1);

Now in Calculator class you have access to instance methods of Floor and Carpet like floor.getArea() or carpet.getCost() and you can calc the total cost. These two private fields let you use your floor and carpet objects in every place in this class. You don't have to write new Floor(100, 100) and new Carpet(1000) in every method. Also you you follow the encapsulation rules.

If you dont like to create private fields here, you can use static method in Calculator like this:

public class Calculator {

public static double getTotalCost(Floor floor, Calculator calculator) {
    return floor.getArea() * carpet.getCost();
}

and call it from other class:

double totalCost = Calculator.getTotalCost(floor1, carpet1);

Classes as an instance field in another class

1 Answers1