Java Customs class instance can't cast to String. Why? -

this question has answer here:

why instance of can cast list can not cast string?

class a{} ... a = new a(); list list = (list)a;   //pass string s = (string)a;  //compile error

the compiler, according specification, takes declared type of a account when these checks casts.

so, write:

a = new a();

but compiler considers

a a; // = <something>;

so knows a cannot string, since class can have 1 superclass (no multiple inheritance), there can't subclass of class a string.

but interfaces isn't true. while know class a doesn't implement list, there class b defined this:

class b extends implements list {}

and since compiler considers declared type, must assumes possible assigned new b() a.

so, because subclass of a implement interface list, compiler can't assume cast list fails.

of course cast fail in practice though - @ runtime. not @ compile time.

Live one