Java-Testing Inheritance

class TestInheritance {
 public static void main(String[] args) {
  SomeClass sc = new SomeClass();
  Shape s = sc.getShape(Integer.parseInt(args[0]));
  s.draw();
  if (s instanceof Square) {
   Square sq = (Square) s;
   System.out.println("It's an instance of Square.");
   sq.someFunction();
  } else if (s instanceof Circle) {
   System.out.println("It's an instance of Circle.");
  } else if (s instanceof Triangle) {
   System.out.println("It's an instance of Triangle.");
  }
 }
}

class SomeClass {
 Shape getShape(int option)  {
  Shape s = null;
  switch (option) {
  case 1:
   s = new Square();
   break;
  case 2:
   s = new Circle();
   break;
  case 3:
   s = new Triangle();
  }
  return s;
 }
}

class Shape {
 void draw() {
  System.out.println("This is an unknown shape.");
 }
}
class Square extends Shape {
 void draw() {
  System.out.println("This is a square.");
 }
 void someFunction() {
  super.draw();
 }
}
class Circle extends Shape {
 void draw() {
  System.out.println("This is a circle.");
 }
}
class Triangle extends Shape {
 void draw() {
  System.out.println("This is a triangle.");
 }
}

---Case Study---
java TestInheritance
ArrayIndexOutOfBoundException
-> args[0] is not initialized but is referred

java TestInheritance 0
NullPointerException
-> Shape s = null; -initially
No case in the switch is matched and hence null is returned.
s.draw() {.draw()} causes the NullPointerException

java TestInheritance 1
This is a square.
It’s an instance of Square.
This is an unknown shape.
-> case 1 in the switch is matched and an object of Square is returned from getShape(). It is assigned to s [up-casting: super-class object = sub-class object. sub-class object is wrapped inside a super-class object]. It is down-casted [unwrapped it from the cover of super-class that hinders the visibility of sub-class specific fields and methods] to type Square after proper checking using instanceof before actually using it.

java TestInheritance 2
This is a circle.
It’s an instance of Circle.
-> case 2 in the switch is matched and an object of Circle is returned from getShape(). It is assigned to s [up-casting: super-class object = sub-class object. sub-class object is wrapped inside a super-class object]. It should be down-casted [unwrapped it from the cover of super-class that hinders the visibility of sub-class specific fields and methods] to type Circle after proper checking using instanceof before actually using it.


java TestInheritance 3
This is a triangle.
It’s an instance of Triangle.
-> case 3 in the switch is matched and an object of Triangle is returned from getShape(). It is assigned to s [up-casting: super-class object = sub-class object. sub-class object is wrapped inside a super-class object]. It should be down-casted [unwrapped it from the cover of super-class that hinders the visibility of sub-class specific fields and methods] to type Triangle after proper checking using instanceof before actually using it.

java TestInheritance square
NumberFormatException
-> “square” [String type] cannot be parsed into int using Integer.parseInt(String);