Casting an object to an interface which is not implemented

casting, interface, java

Solution

The correct answer is `3`: The compiler sees only that a `Phone` is being cast to a `Roamable` and that `Phone` is not final, so it thinks that the object being cast, although referred to as `Phone` may be a subclass of `Phone` that does implement `Roamable`, so no compile-time error or warning is issued.

According to JLS chapter 5

5.5.1. Reference Type Casting

Given a compile-time reference type S (source) and a compile-time reference type T (target), a casting conversion exists from S to T if no compile-time errors occur due to the following rules. If T is an interface type:

If S is not a final class (§8.1.1), then, if there exists a supertype X of T, and a supertype Y of S, such that both X and Y are provably distinct parameterized types, and that the erasures of X and Y are the same, a compile-time error occurs.

Otherwise, the cast is always legal at compile time (because even if S does not implement T, a subclass of S might).

If S is a final class (§8.1.1), then S must implement T, or a compile-time error occurs.

The following code compiles:

interface Roamable{}
class Phone {}
class Tablet extends Phone implements Roamable {
    Roamable var = (Roamable)new Phone(); // Compiles
}

Problem

I found following question in my study book and a bit confused: Given the following code, which option, if used to replace `/* INSERT CODE HERE */`, will enable a reference variable of type `Roamable` to refer to an object of the `Phone` class? (Select 1 option.) ``` interface Roamable{} class Phone {} class Tablet extends Phone implements Roamable { //INSERT CODE HERE } ``` Options are: - `Roamable var = new Phone();` - `Roamable var = (Roamable)Phone();` - `Roamable var = (Roamable)new Phone();` - Because interface `Roamable` and class `Phone` are unrelated, a reference variable of type `Roamable` can’t refer to an object of class `Phone`. I thought the correct option is 4, however it says it is 3. But, `Phone` doesn't implement `Roamable` interface, so you can't cast, can you?

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