question about ==

[Andres Gonzalez]

When using == to compare reference variables, the compiler will verify that the classes are the same or in the same inheritance hierarchy, otherwise it'll throw a compiler error

can you guys provide example(s) of this statement? specially when the compiler might throw a compilation error...

[Alton Hernandez]

class D1{}
class D2{}
class D extends D1 {
  public static void main(String [] args) {
     D  d  = new D();
     D1 d1 = new D1();
     D2 d2 = new D2();
     if (d == d1){}       // ok
     if (d == d2){}       // compilation error
  }

[Miki Muzsi]

Andres, is exactly the same as with the instanceof operator or casting. If in the above example you try:
if (d instanceof D2) {} // compiler error!
or:
d = (D2) d2; // compiler error!
They must be in the same inheritence hierarchy.
Miki