Java - Implement subtypes for interface methods Java

Implement subtypes for interface methods Java… here is a solution to the problem.

Implement subtypes for interface methods Java

I’ve been working on generic classes and methods in Java and ran into this issue in a past paper

I tried to implement the interface and class proposed in the problem, including the refuel method, and found no problem passing the Car parameter as a parameter

Motorized interface

public interface Motorized {

public int getFuel();

}

Vehicle class

public abstract class Vehicle{

private int fuel;

public Vehicle(int fuel) {

this.fuel = fuel;
    }

public String toString(){

return("Fuel is: " + getFuel());
    }

public int getFuel() {

return fuel;
    }
}

Automotive category

public class Car extends Vehicle implements Motorized {

int seats;

public Car(int fuel, int seats) {

super(fuel);
        this.seats = seats;
    }

public int getSeats() {

return seats;
    }

@Override
    public String toString(){

return("Fuel is: " + getFuel() + "and the car has" + getSeats() + "seats.");
    }
}

Test methodology

public class VehicleTest {

public static Motorized refuel(Motorized v) {

return v;
    }

public static void main(String[] args) {

Car car = new Car(15, 5);

System.out.println(refuel(car));
    }
}

Can someone explain to me what this problem should be and why my implementation doesn’t reflect out of it?

Solution

The problem is the return value of the method:

public static Motorized refuel(Motorized v)

You said that you had no problem passing the Car, which is a completely valid statement. But you haven’t tried to get the value back from the refuel method:

Car car = ...
Car refueled = refuel(car);  compiler error: Motorized is not assignable to Car!

Although the Car extends Motorized return type is Motorized, you cannot be sure that the returned Motorized instance will always be a car. See this simplified example:

public static Motorized refuel(Motorized v) {
     try to refuel
     uh oh... the gas station exploded
     you have to change to a motorbike which just stands around
    return new Motorbike();
}

Now you might expect a car, but you get a motorcycle, so even if the conversion fails:

Car refueled = (Car) refuel(car);  ClassCastException, Motorbike is not a Car

You can do this using generics though:

public static <M extends Motorized> M refuel(M m) {
     refuel the motorized vehicle
    return m;
}

But if the gas station explodes again, then there is a problem with this method. It doesn’t know what M actually is. So, this can be a lifesaver for your headache.

Java – Implement subtypes for interface methods Java

Implement subtypes for interface methods Java

Solution

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