Question

Consider the following code :

public class Main implements Vehicle, Car {
  public static void main(String[] args) {
    Main generalConcreteClass = new Main();
    System.out.println(((Vehicle) generalConcreteClass).TYPE); //**Line 1**

    Vehicle vehicle = new Main();  //**Line 2**
    System.out.println(vehicle.TYPE);//  Here there is no Ambiguity since vehicle is of TYPE vehicle

    System.out.println(((Car)vehicle).TYPE);  // **Line3** This doesn't throw ClassCastException..
  }
}

Here, both the interfaces Vehicle and Car have the same constant TYPE with different value.

There will be ambiguity with generalConcreteClass at Line1, so type-cast is necessary and any one of TYPE can be accessed.

Line2 : vehicle object has reference of Vehicle interface.

Line3 : How can I cast my vehicle object to Car type and still access the constant TYPE of Car. How is becomes visible to vehicle object. Or how does it work internally??

Now, if I dont make my class implement Car interface, then a similar type-casting of vehicle object to type Car throws a ClassCastException.

Answer 1

Static methods and fields are not polymorphic. You should never use an instance to access a static field. Use Vehicle.TYPE or Car.TYPE.

If you want to access the type of an object polymorphically, then use a getType() instance method.

Line 3 doesn’t throw a ClassCastException because the object’s concrete type is Main, and a Main is a Car, so casting works.