What is the fragile base class problem?

Question

What is the fragile base class problem in java?

Answer 1

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();
        }
    }