JDelage JDelage - 1 year ago 49
Java Question

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

I'm reading this:


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

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

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



Answer Source

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.