instanceof

[neel sri]

A question from one of chisholm's mock

 
class Color {}
class Red extends Color {}
class Blue extends Color {}
class A {
  public static void main (String[] args) {
    Color color1 = new Red(); Red color2 = new Red();
    boolean b1 = color1 instanceof Color;//line1 
    boolean b2 = color1 instanceof Blue;// line2 
    boolean b3 = color2 instanceof Blue; // line3 fails at compile time
    System.out.print(b1+","+b2+","+b3);
}}
 

That's understandable.. say I change the code by declaring an interface
"interface iface {}" and replacing the line3 with the following
"boolean b3 = color2 instanceof iface;". Output is true,false,false.
Why is compiler not complaining? Does this mean "instanceof" behaves differently for interfaces?

Thanks,
Neel.

[Nicholas Cheung]

Interfaces and classes are behaved the same for the instanceof operation.

Your case is that, during the compilation time, in line 3, the compiler already knows color2 (as you declared it as Red) is NOT an instance of BLUE (as BLUE does not extend from RED), hence, you get the compilation error.

For line 2, as color1 is declared as COLOR only, the compiler does NOT know whether it is RED or BLUE, and thus, it left to the runtime engine (JVM) to determine this.

Same case for the interface case.

Nick

[neel sri]

Thanks!! Nick..
I did understand why compiler is throwing an error for this
boolean b3 = color2 instanceof Blue;
But my question is why whould it not throw an error
boolean b3 = color2 instanceof iface; ( Assuming that iface is an interface declared in the same file).

For that matter any interface compiles fine in the place of Blue in line "boolean b3 = color2 instanceof Blue;" but not any class other than "Red" and "Color".

boolean b3 = color2 instanceof Collection; also compiles fine!! why??

Hope I'm clear this time.

[Steven Bell]

The trick here is the compiler knows the reference type of the Object, in this case Red. The compiler knows that this object is an Object of type Red or a subclass of red. It is impossible for any Object of type Red to also be of type Blue, but it is possible for a subclass of Red to implement the interface iface (or any other interface) so the compiler is unable to rule out that possiblility.

[neel sri]

Steven, thank you!!

[Leandro Melo]

That's an interesting post.