classCastException

[jnrohit Jain]

When I cast a class into an unrelated class then it gives me a compile time error (incompatible types) and when I cast a class into an unrelated interface it gives me a runtime exception (classCastException). Why casting to interface not giving compile time error?

class MGCH7_Ink {}
interface MGCH7_Printable {}
class MGCH7_ColorInk
      extends MGCH7_Ink implements MGCH7_Printable {}
class MGCH7_BlackInk extends MGCH7_Ink {}

public class MGCH7_01 {
	public static void main(String[] args) {
		MGCH7_Printable printable = null;
		MGCH7_BlackInk blackInk = new MGCH7_BlackInk();
		printable = (MGCH7_Printable)blackInk; // Runtime error

		MGCH7_ColorInk colorInk = null;
		//colorInk = (MGCH7_ColorInk)blackInk; // Compile error
	}
}

[Campbell Ritchie]

Because it is possible to create a subtype of that black ink class which does implement that interface. The compiler cannot therefore say the cast can never succeed, so it has to permit it.
Remember that casts usually signify a bad inheritance structure, and also that the javac tool is programmed to believe that users know what they are doing with casts.