CODEWITHSUNDEEP
javainheritancecomposition
MaxxD17
MaxxD17

Reputation: 57

How do you create a child object with the constructor of parent object? (Java)

As an example, I have a Car class. When I create a car object I would like to pass the number of wheels I would like that car to have and the constructor would create Wheel objects from 1-x number of wheel objects I passed to the Car's constructor.

Any help you can provide would be greatly appreciated!

Parent is Car Child is Wheel

Upvotes: 1

Views: 1058

Answers (2)

Amal K
Amal K

Reputation: 4899

The relationship of a car with its wheels is a has a relationship. A car has wheels. This being modelled through inheritance is semantically incorrect as inheritance is an is a relationship. A car is not a wheel. Has a relationship must be modelled through composition. A car contains multiple wheels. Use a collection like an array that holds the wheels for each car:

class Wheel {
        
}


class Car {
    // Use an array to hold the wheel objects
    private Wheel[] wheels;
    private String name;

    public Car(String name, int wheelCount) {
        this.name = name;
        wheels = new Wheel[wheelCount];
        for(int i = 0; i < wheelCount; i++) {
            wheels[i] = new Wheel();
        }
    }

    public void changeWheelAt(int index, Wheel wheel) {
        wheels[index] = wheel;
    }

    public Wheel getWheelAt(int index) {
        return wheels[index];
    }

    public Wheel[] getWheels() {
        return wheels;
    }
    
    
    public String getCarName() {
        return name;
    }

    public int getNumberOfWheels() {
        return wheels.length;
    }
}

Upvotes: 2

Ashutosh Thakur
Ashutosh Thakur

Reputation: 94

I hope you do not actually mean that the Car class is the parent of Wheel. That would be an IS-A relationship. We do know that a wheel IS-NOT-A car.

What we might want to use here is the many-to-one composition (HAS-A) relationship between Wheel and Car. Most cars HAVE wheels!

So something like:-

public class Car {
      ...
      Set<Wheel> wheels;
}

Then you could have a constructor wherein you could pass the number of wheels and the set would be initialized.

public class Car {
      Car(int numOfWheels) {
        wheels = new HashSet<>();
        for(int i=0; i<numOfWheels; i++) {
           wheels.add(new Wheel());
           ... // Other Wheel properties
        }
      }
      ...
      Set<Wheel> wheels;
}

Upvotes: 1

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