How to use generics and inherit from parent class without causing name clash?

Question

I have a parent class in Java called Flight. I have children classes: JetFlight, NormalFlight, etc. which inherit from Flight.

I want all the children classes to implement compareTo from the Comparable interface. I want them to inherit from Flight because I want to use polymorphism (for example, initiate a Flight array and fill it with objects of JetFlight, NormalFlight, etc.).

This is my code for the parent class:

public abstract class Flight  {
    public abstract int compareTo(Object o);
}

and this is the code for one of the children classes:

public class JetFlight extends Flight implements Comparable<JetFlight> {
    private int flightTime;
    public JetFlight(int flightTime) {
        this.flightTime = flightTime;
    }
    public int compareTo(JetFlight j) {
        return this.flightTime - j.flightTime;
    }
}

This code won't compile because of 2 errors:

1) compareTo(T) in 'java.lang.Comparable' clashes with 'compareTo(Object)' in 'Flight'; both objects have the same erasure, yet neither overrides the other.

2) Class 'JetFlight' must either be declared abstract or implement abstract method 'compareTo(Object)' in 'Flight'

How do I go about the situation when I want to inherit from a class and at the same time use generics on the child class?

Answer 1

Option 1: Since your comparison is based upon flight time, and as far as I know, the variable flightTime can be pushed up in the parent class as all flights will have this feature. Then implement your compareTo() method in parent class itself.

Option 2: in case you want to keep your current code the way it is:

    public abstract class Flight implements Comparable<Flight> {
    public abstract int compareTo(Flight o);
}

---

public class JetFlight extends Flight {
private int flightTime;
public JetFlight(int flightTime) {
    this.flightTime = flightTime;
}
public int compareTo(Flight f) {
    if(!(f instanceof JetFlight)){
        throw new IllegalArgumentException();
    }
    return this.flightTime - ((JetFlight)f).flightTime;
}

Answer 2

You can parameterize the Comparable on a type T bounded by Flight:

abstract class Flight<T extends Flight<T>> implements Comparable<T>  {
    public abstract int compareTo(T o);
}

class JetFlight extends Flight<JetFlight> {
    private int flightTime;
    public JetFlight(int flightTime) {
        this.flightTime = flightTime;
    }

    public int compareTo(JetFlight j) {
        return this.flightTime - j.flightTime;
    }
}

Answer 3

If you use Generics, you will get rid of the problem.

You need to define

public abstract int compareTo(JetFlight o);

and also use

public class JetFlight extends Flight implements Comparable<? extends JetFlight>