java - The 'instanceof' operator behaves differently for interfaces and classes -

i know regarding following behavior of instanceof operator in java.

interface c {}  class b {}  public class {     public static void main(string args[]) {         b obj = new b();         system.out.println(obj instanceof a);      //gives compiler error         system.out.println(obj instanceof c);      //gives false output     } } 

why so? there no relation between interface c , class b, gives false whereas in case of obj instanceof a gives compiler error?

because java has no multiple class inheritance it's absolutely known during compilation obj object of type b cannot subtype of a. on other hand possibly can subtype of interface c, example in case:

interface c {}  class b {}  class d extends b implements c {}  public class {     public static void main(string args[]) {         b obj = new d();         system.out.println(obj instanceof c);      //compiles , gives true output       } } 

so looking @ obj instanceof c expression compiler cannot tell in advance whether true or false, looking @ obj instanceof a knows false, meaningless , helps prevent error. if still want have meaningless check in program, can add explicit casting object:

system.out.println(((object)obj) instanceof a);      //compiles fine