Does abstract class extend Object?

Question

I have read about the difference between interfaces and abstract classes, but this one is confusing. Consider this interface and class.

interface I {     public int hashCode();     public boolean equals(Object obj); }  class B implements I {     // Works Fine } 

Here it works fine and i need not override interface methods because Object is a super class of B and those methods are implemented in it.

Now consider these

abstract class A {     public abstract int hashCode();     public abstract boolean equals(Object obj); }  class C extends A {     // Compile error because methods are not overridden } 

Why would this result in compile error? Does this mean Object is not a super class for abstract class? Or am i missing some point?

Answer 1

It results in a compile error because by definition abstract functions must be implemented downstream in the inheritance chain. You've created the requirement they must be implemented in a subclass of A.

Class C does not implement those methods, so compilation failure.

Object is a superclass of abstract classes... but it's not a subclass, and subclasses are responsible for implementing abstract functions.

In contrast, if a class implements an interface, the implementation can live anywhere in that class's inheritance hierarchy. It's less common to have those implementations lie in a superclass, because you'd generally declare the interface in the superclass.

There are use cases where you might not, like degenerate/poor design, or examples like this while poking around language features.

Answer 2

As already mentioned by others, class A overrides those methods in Object by declaring them again as abstract, so it forces subclasses to implement them.

As a clarification for your situation try defining A as follows:

abstract class A {     //public abstract int hashCode();     //public abstract boolean equals(Object obj); }  class C extends A {     // no compile error because no abstract methods have to be overridden } 

In this case both A and C inherit the implementation of those methods from Object and no compilation error occurs.