Question on Reference assignment for Interface references..

[Tanya Shetty]

 interface I1 { }
interface I2 { }
class C1 implements I1 { }
class C2 implements I2 { }
class C3 extends C1 implements I2 { }
C1 o1;
C2 o2;
C3 o3;   

source : Ethuware

Question : Interface I1 does not extend Interface I2, and also the vice versa is not true.. so when I give the compiler the following line of code,

I1 i1 = null; I2 i2 = (I2) i1; // it compiles fine.

Why does the compiler allow this assignment I2 i2 = (I2)i1; when there is clearly no inheritance between them ??

[kevinn lee]

compiler compiles any code unless it knows exactly that the particular casting is impossible.
In your case there can be a class which implements both the interfaces.Then both i1 and i2 can refer to an object of that class and the casting between I1 and I2 makes sense.

[Prasad Kharkar]

C1 IS-A I1
C2 IS-A I2
C3 IS-A C1 and IS-A I2 from the given data
If C1 can refer to object of c3 that passes IS-A for both C1 and I2
then C1 should also be able to refer to reference of I2 isn't it?

[Sandra Bachan]

I tried compiling the code

interface I1 { }
interface I2 { }
class C1 implements I1 { }
class C2 implements I2 { }
class C3 extends C1 implements I2 { }
C1 o1;
C2 o2;
C3 o3;  

and I receive the following errors:

C3.java:6: class, interface, or enum expected
C1 o1;
^
C3.java:7: class, interface, or enum expected
C2 o2;
^
C3.java:8: class, interface, or enum expected
C3 o3;
^
3 errors

Is this is typing error, or something else?

[Tanya Shetty]

Thanks! Got it!