当超类没有实现接口而是子类时，在接口子类和超类之间进行转换

Question

In the following code

abstract class Vehicle { }
class Car extends Vehicle implements LandMover { }
interface LandMover { }
     Car porsche=new Car();
     LandMover lmv;
     lmv = porsche;
     Vehicle vec = (Vehicle)lmv;

Shouldnt there be a compiler error in 4th line as there is no relation between class vehicle and Interface LandMover?? and if there isnt what could be the reason.Thanks!!

Answer 1

The compiler only checks if there is a possible relationship, and there is one:

A LandMover could be a Car , which in turn IS-A Vehicle . Since you promise that this conversion is ok by using an explicit cast, the compiler is happy.

Answer 2

No, since

    LandMover lmv = porsche;

doesn't declare an object of type LandMover, but refers to an object of type 'Car' via reference 'LandMover'. Compiler knows that this is an object of type 'Car' (note: 'Car' and "Vehicale' are in the same inheritance tree).

Answer 3

By explicitly casting to Vehicle , the compiler will not tell you anything. You might just get an Exception at runtime if the relationships finally don't quite work out and you are casting something that is actually not a Vehicle

Answer 4

Compilation will be fine with the code you have provided. lmv = porsche; is same as saying

public class Car extends Vehicle implements LandMover {

    public static void main(String args[]){

        LandMover car = new Car();
    }
}

A Car implements LandMover so you can use it as an polymorphic reference. So no problem so far.

Vehicle vec = (Vehicle)lmv;

Now you are saying the LandMover(reference) which can very well be a Car is fact a Vehicle. So compiler will not complaint.

But note after this type cast if you try to call some function on vec that is not defined in Vehicle Class(Not even the ones in Car class though infact it is an runtime instance of Car class) compiler will complaint.

This is because at runtime compiler only checks the class of reference type and if the function that is called is defined(may not be implemented) in it then there will not be any compilation errors.

Answer 5

There won't be any compile errors. This is because a class can implement many interfaces therefore a given 'LandMover' instance can be a Vehicle. However since a class can inherit from a single super class you know that you can't cast a 'java.lang.String' to 'Vehicle' because they don't share a common superclass other than object.

Answer 6

Considering the hierarchy

Car extends Vehicle

&

Car implements LandMover

For the casting Vehicle vec = (Vehicle)lmv; the compiler can check statically (compile time), the type of lmv which is LandMover and which in turn also CAN be Car (as Car implements LandMover ). Now as Car extends Vehicle the compiler does not complain.

to Confirm it, try commenting the code as follows

        //Car porsche=new Car();
         LandMover lmv;
         //lmv= porsche;
         Vehicle vec = (Vehicle)lmv; //Compiler error

Here, compiler is sure that lmv CANNOT be Vehicle for sure and hence flags error.

Answer 7

由于抽象类无法实例化以便更多地引用此帖子，因此会出现错误