Vysvetlenie poskytuje dokumentacia k Borland JBuilder4. Toto sa tam pise o
operatore instanceof:

instanceof 

15.19.2 Type Comparison Operator instanceof 

The type of a RelationalExpression operand of the instanceof operator must
be a reference type or the null type; otherwise, a compile-time error
occurs. The ReferenceType mentioned after the instanceof operator must
denote a reference type; otherwise, a compile-time error occurs. At run
time, the result of the instanceof operator is true if the value of the
RelationalExpression is not null and the reference could be cast to the
ReferenceType without raising a ClassCastException. Otherwise the result is
false. If a cast of the RelationalExpression to the ReferenceType would be
rejected as a compile-time error, then the instanceof relational expression
likewise produces a compile-time error. In such a situation, the result of
the instanceof expression could never be true.

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
  }
 }
}

This example results in two compile-time errors. The cast (Point)e 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. The instanceof expression is incorrect for exactly the same reason.
If, on the other hand, the class Point were a subclass of Element (an
admittedly strange notion in this example): 

class Point extends Element { int x, y; }

then the cast would be possible, though it would require a run-time check,
and the instanceof expression would then be sensible and valid. The cast
(Point)e would never raise an exception because it would not be executed if
the value of e could not correctly be cast to type Point. 

Zdravim
Ing. Peter Krútel