Java Interfaces Casting

Question

Imagine we have class C that implements interfaces I and J declared as ;

I i = new C();

why do these 3 not give errors (especially the last one):

1) C c = (C)i;
2) J j = (J)i;
3) i   = (I)null;

thanks in advance.

@JordiCastilla Based on question I think we can safely assume that it is `class C implements I,J{}`. — Pshemo, May 24 '15 at 11:04

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

C c = (C)i;

Since i can hold instance of type C there is no problem with allowing by compiler casting I to C type reference (if casting will succeed at runtime we will be sure that all methods from C reference will be supported by instance which will also be of type C or its subtype).
J j = (J)i;

Since there is a possibility (like in our case) that instance stored in i will also implement J interface compilers allows us to cast between references of unrelated interfaces. But if instance stored in i will not implement J then we will get ClassCastException at runtime.
i = (I)null;

Well, null can be assigned to any reference so there is no problem with allowing it to be assigned to i or cast it.

score 0 · Answer 2 · answered May 24 '15 at 10:56

The first one works, because i is an instance of C (even if the reference is of type I, but for the casting the reference type is irrelevant).

The second works, because i is an instance of C and thus also an instance of J (since C implements J). The reference type is irrelevant for casting.

score 0 · Answer 3 · edited May 23 '17 at 10:26

0

The interfaces is a different "view" from the object. When you debug it, there is always the "C" object.

For Null you can look here: No Exception while type casting with a null in java

edited May 23 '17 at 10:26

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answered May 24 '15 at 10:57

pL4Gu33

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score 0 · Answer 4 · answered May 24 '15 at 11:03

Works because i is a reference variable of I, but that holds an instance of C. You're allowed to downcast an object if the object is an instance of the class you're casting to.
C implements J and you're always allowed to upcast an object.
I guess it's becuase i is a reference variable, which is allowed to have a null value, and you're giving it a value of null.

score 0 · Answer 5 · answered May 24 '15 at 11:04

Once you have created the object and assigned it to i with the statement.

    I i = new C();

The object is still a C even though the variable refering to it is defined as I.

SO

    C c = (C)i;

just assigns the same object to a new variable that is defined as C. And as the object is of type C there is no problem.

    j = (J)i;

just assigns the same object to a new variable that is defined as J. And as the object is of type C which implements the interface J it is also of type J so there is no problem.

    i   = (I)null;

Just sets the variable i to null.

Java Interfaces Casting

5 Answers5