I put my question on comment section in the code. I have already passed the verification. But I still don't understand why it displays 0 in protected modifier. 
public class Solution {
    public static void main(String[] args) {
        new B(6);
    }

    public static class A {
        private int f1 = 7;

        public A(int f1) {
            this.f1 = f1;
            initialize(); //The reason why it displays 0 rather than 6 is because this method calls B.initialize() rather than A.initialize(), right? But why?
        }                   //Can any one suggest Codegym lessons that can explain why this is happening?

        protected void initialize() {
            System.out.println(f1);
        }
    }

    public static class B extends A {
        protected int f1 = 3;

        public B(int f1) {
            super(f1);
            this.f1 += f1;
            initialize();
        }

        protected void initialize() {
            System.out.println(f1);
        }
    }
}