JDelage JDelage - 4 months ago 9
Java Question

Why does `instanceof` error rather than return `false` when used for 2 incompatible classes?

I'm reading this:

http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.20.2

They say:


Consider the example program:


class Point { int x, y; }
class Element { int atomicNumber; }
class Test {
public static void main(String[] args) {
Point p = new Point();
Element e = new Element();
if (e instanceof Point) { // compile-time error
System.out.println("I get your point!");
p = (Point)e; // compile-time error
}
}
}



The
instanceof
expression is incorrect because no instance of
Element
or any of its possible subclasses (none are shown here) could possibly be an instance of any subclass of
Point
.


Why does this result in an error, rather than simply in
instanceof
returning false?

Thanks,

JDelage

Answer

instanceof check is a runtime check. The compiler is able to discover that this condition is incorrect at compile time (much earlier), so it tells you that it is wrong. Always remember, that failing fast is a good practice, it will save you a lot of time and nerves.