Thread: Should Java have a "this" type parameter?

It could look something like this:

public interface Vehicle {
    public <T:this> T copy();
}
public class Car implements Vehicle {
    public <T:this> T copy() { ... }
}
public class Boat implements Vehicle {
    public <T:this> T copy() { ... }
}

As it is now, you'll have to either declare the return type as Vehicle, so you have to cast your copy (Car copy = (Car) car.copy();), or use non-interface copy methods so that copying a Vehicle becomes a PITA (Vehicle copy = vehicle instanceof Car car ? car.copy() : vehicle instanceof Boat boat ? boat.copy() : ...).

I see no reason why you couldn't have the best of both worlds: if you have a Vehicle, you know it will have a copy method which returns a type implementing Vehicle, and if you have a Car, you know the return type will be Car (or a subclass of Car).

a class can use the this keyword to refer to itself. The this keyword is also required when a local variable within a method has the same name as a class member variable, to distinguish them both.

I don't see how my suggested use of this within the type parameter declaration conflicts with any current use of it. But some other keyword could be used, or some other notation entirely.

Hi,
The interface definition is incorrect IMHO. It should look like this.

public interface Vehicle<T> {
        public T copy();
    }
 
    public class Car implements Vehicle<Car> {
 
        @Override
        public Car copy() {
            return null;
        }
    }
 
    public class Boat implements Vehicle<Boat> {
 
        @Override
        public Boat copy() {
            return null;
        }
    }

But it is even better if you separate the interfaces.

public interface Vehicle {}
 
public interface Copyable<T extends Vehicle>{
    T copy();
}
 
public class Car implements Vehicle, Copyable<Car> {
 
    @Override
    public Car copy() {
        return null;
    }
}
 
public class Boat implements Vehicle, Copyable<Boat> {
 
    @Override
    public Boat copy() {
        return null;
    }
}

Then you can use
Car c = new Car()
Vehicle v = c.copy()