Java converts objects to unimplemented interfaces

I found the following problems in my study book and was a little confused:

Given the following code, which option, if used to replace / * insert code here * /, will enable the reference variable of roamable type to refer to the object? (select 1 option.)

interface Roamable{}
class Phone {}
class Tablet extends Phone implements Roamable {
    //INSERT CODE HERE
}

Options include:

> Roamable var = new Phone(); > Roamable var = (roamable) telephone (); > Roamable var = (roamable) new phone (); > Because the interface roamable is not related to the phone class, it is a reference variable, and the roamable type cannot reference the object of the phone class

I think the correct option is 4, but it says it is 3

However, the phone does not implement the roamable interface, so you can't use it, can you?

Solution

The correct answer is 3: the compiler only sees that a phone is cast to roamable and the phone is not final. Therefore, it thinks that although the object being converted is called phone, it may be a subclass of the phone implementing roamable, so it will not issue compile time errors or warnings

According to JLS Chapter 5

5.5. 1. Reference type casting

The following code is compiled:

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