Java Tutorials: Example of Co-variant return type

package project1;
class Fuel

{
int power;
int smoothness;
int roots;
}

class Hydrogon extends Fuel

{
int gasUnit;
}

class Car

{
Fuel run()

{
return new Fuel();
}
}

class FutureCar extends Car

{
Hydrogon run()

{
return new Hydrogon();
}
}

public class Class4

{
public Class4()

{
}

public static void main(String[] args)

{
FutureCar futureCar = new FutureCar();
futureCar.run();
}
}