https://www.youtube.com/watch?v=X98Tl5ANQs0 interface Calculable{ double PI = 3.14; int sum(int v1, int v2); } interface Printable{ void print(); } class RealCal implements Calculable, Printable{ public int sum(int v1, int v2) { return v1+v2; } public void print() { System.out.println("This is RealCal!!"); } } class AdvancedPrint implements Printable{ public void print() { System.out.println("Th..