What is the fragile base class problem?

Question

What is the fragile base class problem in java?

Answer 1

All what "Colin Pickard" has said is true , and here I want to add some of the best practices to be more protected when you are writing code that may cause this kind of issues in the Java language and especially if your are creating a framework or a library...

1. Make all your concrete classes final by default because you may don't want them to be inherited You found this kind of behavior as feature many languages as the Kotlin langage (if you need to extend it then it's ok remove the final keyword, so it can help the reader of your code).
2. For abstract classes, make all its implemented methods final to be not modified by its subClasses (even protected methods are a bad idea generally speaking, subClasses should'nt know so much about its subclasses)do not expose your method if they are not meant to be overriden...
3. Try to not use a relationShip [is a] instead try to use the [uses a] relationsheap between your classes diagram use interfaces to avoid the extends problems..
4. Remember that every extends can be replaced by implements and if you have to make a default implementation here's a code spinet:

public interface MyBehavior {
    void doAction();

    static class Implementation implements MyBehavior {
        public void doAction() {
            //do some stuff
        }
    }
}

//  instead of doing extends To a class that have the doAction method
//  we will make a [use a] relationShip between the Example class & the Implementation class
public class Example {
    private MyBehavior.Implementation helper = new MyBehavior.Implementation();

    public void doAction() {
        this.helper.doAction();
    }
}