Java Object superclass

Question

I have a weird Java question:

As we know:

1. All Java classes extend java.lang.Object
2. All Java classes cannot extend itself

Then, java.lang.Object must extend java.lang.Object, which is itself, therefore, it should be impossible. How is Object implemented in Java?

Answer 1

Object is an exception to the first rule, and has no superclass. From JLS3 8.1.4:

The extends clause must not appear in the definition of the class Object, because it is the primordial class and has no direct superclass.

You can also try it out with reflection:

Object.class.getSuperclass(); // returns null

Answer 2

You'd be better off thinking of this as:

1. All java classes must implement the interface implied by the methods in java.lang.Object.
2. The concrete class java.lang.Object provides default implementations of these functions.
3. All other java classes are derived from the object java.lang.Object and may choose to use or override the default implementations of the methods.

The two main points are: all the classes must implement the implied interface and the Java language spec gives you (forces upon you?) default implementations for these methods for free.