What is the reason for java Fragile base class problem

I would like to know the reason for fragile base class problem in java.

Here is the code and i cant find the reason for triggering child class method even if i have given instruction to trigger a method in parent class

in public class Array

public class Array {
    private ArrayList<Object> a = new ArrayList<Object>();

    public void add(Object element)
    {
        a.add(element);
    }

    public void addAll(Object elements[])
    {
        for (int i = 0; i < elements.length; ++i)
            add(elements[i]);
    }
}

in ArrayCount class... this class is responsible for store the count of the list

public class ArrayCount extends Array {
    public int count = 0;

    @Override
    public void add(Object element)
    {
        super.add(element);
        ++count;
    }

    @Override
    public void addAll(Object elements[])
    {
        super.addAll(elements);
        count += elements.length;
    }
}

main class as follows

public class Main {

    public static void main(String[] args) {

        ArrayCount ac = new ArrayCount();
        Object [] objArray = new Object[7];
        objArray[0] = "xxx";
        objArray[1] = "xxx";
        objArray[2] = "xxx";
        objArray[3] = "xxx";
        objArray[4] = "xxx";
        objArray[5] = "xxx";
        objArray[6] = "xxx";

        ac.addAll(objArray);
        System.out.println("count is : " + ac.count);
    }
}