About Runnable and Serializable

[Alain Narcisse]

Hello Can some one explain me why this code is compiling without errors ?
Thanks for your help .

import java.io.Serializable;

class B extends A{}
class C extends B{}

public class A {
public static void main(String[] args) {

A a = new B();
B b =new B() ;
C c = new C() ;

Runnable r1 = (Runnable)a ;
Serializable r2 = (Serializable)a ;
Serializable s2 = (Serializable)c ;
}

}

[Seema Kekre]

When we explicitly cast to an interface the compiler trusts us. You can cast it to any interface, try Set or Map instead of Runnable but not to a class (eg String). This compiles but will throw a run-time exception. If you remove the cast and just try

Runnable r1 = a ;

This will give you a compile time error. Experts could give better explanation.

[Paul Clapham]

It's not quite that the compiler trusts us. The actual reason is that something assigned to a variable of type A could actually be an object of some subclass of A which does implement Serializable, or Runnable, or any other interface. Since that might happen, the compiler can't call it an error, although it might fail at run time with a ClassCastException.

However if A is a final class, then the compiler knows that this situation can't ever happen -- there can't be any subclasses of A -- and in that situation it will flag the cast to an interface as an error. Try setting up an example like that and see what happens.

[Alain Narcisse]

Thanks for your answer and what about this ( also compile without errors ) .

A a = new B();
B b =new B() ;
C c = new C() ;

Runnable r1 = (Runnable)a ;
Serializable r2 = (Serializable)a ;
Serializable s2 = (Serializable)c ;

// ?
A a1 = (A) r1 ;

[Seema Kekre]

Thanks Paul for the awesome explanation. I tried the following code:

interface A1{}

class B1 implements A1{}
class C1{}
class tester{
	public static void main(String[] args){
		A1 a= new B1();
		B1 b = new B1();
		C1 c = (C1)a; //Compiles when casting a reference to an interface
		C1 e = (C1)b;  //Does not compile if casting a reference to a class
		String d= (String)a; //Does not compile for final class
	}
	
}

Since an implementation class of an interface can extend from a class, the compiler allows class casting as well. That will explain the A a1 = (A) r1 ;