Interfaces and Comparable

Question

i'm beginner and just learning abstract classes and interfaces but really struggeling to get it right.

MY Class structure.

Abstract Class Vehicle
Class Car (extends Vehicle, implements ITuning)
Class Motorcycle (extending Vehicle)
Interface ITuning

I want to have an abstract method doRace() that I can use in my Car class to compare the Car Objects (car.getHp()) and prints the winner. I try to implement a doRace(object o) into the Interface ITuning and a doRace(Car o) into my Car class but get the error. How can I implement that correctly ?

The type Car must implement the inherited abstract method ITuning.doRace(Object)

But if do chage it to Object, i can't use it in my Car class…

public interface ITuning
{
abstract void doRace(Object o1);
}

public Car extends Vehicle implements ITuning
{
 public void doRace(Car o1){
 if(this.getHp() > o1.getHp())
 };
}

Any Idea what i'm doing wrong? I assume its a comprehension error

Hint: `doRace(Object o1)` and `doRace(Car o1)`. Why are you using `Object` in your interface? — Marvin, Nov 10 '22 at 13:41
Where is `getHp()` defined? In `Car` or in `Vehicle`? The basic idea would be to use that class in your `doRace()` method in the interface declaration. — Marvin, Nov 10 '22 at 13:47
Tbh I tried "Object" as a placeholder in doRace(Obeject) as I wasn't sure what to use. Whatever i tried, it does not work. get(Hp) is an abstract method in Vehicle Class and implemented in Car & Motorcylce class, as I wanted to use it in both. — , Nov 10 '22 at 14:26

score 1 · Answer 1 · answered Nov 10 '22 at 13:56

1

You can make ITuning generic.

public interface ITuning<T> {
  
  void doRace(T other);
}

Implementation will be like this:

public class Car extends Vehicle implements ITuning<Car> {

  @Override
  public void doRace(Car other) {
    //do comparison
  }
}

Implementation in other classes will be quite similar, just change the generic parameter.

As a side note, i would rename the interface to something more fitting. Considering that tuning a vehicle is the act of modifying it to optimise its' performance, ITuning providing functionality to do the actual racing is counter intuitive.

answered Nov 10 '22 at 13:56

Chaosfire

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Thanks for your answer! Just for clarification does that mean if i want my "interface" to interact with multiple Objects (like comparing) I need to change it to generic ? – Nov 10 '22 at 14:38
1

@EddiOne That depends. If you mean like `Comparator` and `Comparable`, which compare objects of the **same** type, yes, it has to be generic. If you want to race, let's say car vs motorcycle, **different** types, then generic is not necessary, although it can be done with generic as well, by providing bound to it. See [Marvin's answer](https://stackoverflow.com/a/74390783/17795888) for non generic solution. – Chaosfire Nov 10 '22 at 14:52

score 0 · Accepted Answer · answered Nov 10 '22 at 14:37

0

You can change your ITuning interface to

public interface ITuning
{
    void doRace(Vehicle other);
}

Since Vehicle defines the getHp() method this ensures that your actual implementation can access it.

The difference to Chaosfire's answer is that this will allow you to do a race between different types of vehicles, e.g. a Car and a Motorcycle, while the generic ITuning<T> class will be limited to races between equal types*.

Chaosfire's point regarding the interface name is still valid.

*This is a bit over-simplified to make my point clearer. Of course Car could implement ITuning<Car> and ITuning<Motorcycle> but this may not be what you want.

answered Nov 10 '22 at 14:37

Marvin

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Thanks Marvin, that is exactly what i was tying to do. However If I change this as you said where do I implement the actual method? If I do in the abstract Vehicles class, i don't need the Interface ITuning anymore… – Nov 10 '22 at 14:55
Okay i think i found my Issue. My main class was following `public static void main(String[]args){. Car car1 = new Car("bmw", 90) Motorcycle moto1 = new Motorcyle("Ducati", 100) Vehicle moto2 = new Motorcycle("Yamaha", 96) } ` Thats why it wasn't working. Now i changed the last Object to Motorcylce moto = new Motorcycle("Yamaha", 96) and implemented the method doRace into Motorcylce.java and Car.java. Now I can race all kinds. Thank you – Nov 10 '22 at 15:37
1

I'd argue that you probably indeed do not need the `ITuning` interface. If you are unsure about the differences between an interface and an abstract class, maybe a look at [Interface vs Abstract Class (general OO)](https://stackoverflow.com/q/761194/4616087) helps. – Marvin Nov 10 '22 at 16:10

Interfaces and Comparable

2 Answers2