Spring源码分析【0】-框架的基础：继承和接口调用链

Spring源码大量的使用继承和接口调用，现举个例子，不搞清楚这个无法看代码。

public class A extends B{

    public void f1() {
        System.out.println("f1 in a");
    }
}

public abstract class B extends C {

    protected abstract void f1();

    protected String ff() {
        f1();
        return "ff in b";
    }
}

public abstract class C  extends D{
    public void f2() {
        super.f2();
        System.out.println("f2 in c");
    }
}

public abstract class D implements I{

    public void f2() {
        System.out.println("f2 in d");
        f3();
    }

    public void f3() {
        System.out.println("f3 in d");
        System.out.println(ff());
    }

    protected abstract String ff();

}