What is the fragile base class problem?

Question

What is the fragile base class problem in java?

Answer 1

A fragile base class is a common problem with inheritance, which applies to Java and any other language which supports inheritance.

In a nutshell, the base class is the class you are inheriting from, and it is often called fragile because changes to this class can have unexpected results in the classes that inherit from it.

There are few methods of mitigating this; but no straightforward method to entirely avoid it while still using inheritance. You can prevent other classes inheriting from a class by labelling the class declaration as final in Java.

A best practice to avoid the worst of these problems is to label all classes as final unless you are specifically intending to inherit from them. For those to intend to inherit from, design them as if you were designing an API: hide all the implementation details; be strict about what you emit and careful about what you accept, and document the expected behaviour of the class in detail.

Answer 2

A base class is called fragile when changes made to it break a derived class.

class Base{     protected int x;     protected void m(){        x++;     }      protected void n(){       x++;      // <- defect        m();      }  }   class Sub extends Base{         protected void m(){             n();         }     } 

Answer 3

It is widely described in below article By Allen Holub on JavaWorld

Why extends is evil. Improve your code by replacing concrete base classes with interfaces