A Quick Tour - The Java Programming Language - page 40

1.11.2. The Object Class
Classes that do not explicitly extend any other class implicitly extend the
Object class. All objects are polymorphically of class Object , so Object is
the general-purpose type for references that can refer to objects of any
class:
Object oref = new Pixel();
oref = "Some String";
In this example, oref is correctly assigned references to Pixel and String
objects even though those classes have no relationship except that both
have Object as a superclass. The Object class also defines several im-
portant methods that you'll learn about in Chapter 3 .
1.11.3. Type Casting
The following code fragment seems quite reasonable (if not particularly
useful) but results in a compile-time error:
String name = "Petronius";
Object obj = name;
name = obj; // INVALID: won't compile
We declare and initialize a String reference which we then assign to a
general-purpose Object reference, and then we try to assign the refer-
ence to a String back to the String reference. Why doesn't this work?
The problem is that while a String is always an Object , an Object is not
necessarily a String , and even though you can see that in this case it
really is a String , the compiler is not so clever. To help the compiler you
have to tell it that the object referred to by obj is actually a String and
so can be assigned to name :