September 12th, 2022 - OOP for GUI

In Java, Rectangle r1 creates a pointer to a variable of class Rectangle, but Rectangle r1 = new Rectangle() creates the real thing.
abstract class can’t be instantiated, it can only be inherited from.
We often store bounding boxes instead of x/y/width/height because it works for all shapes and simplifies calculations.
Variables of type Shape can hold Rectangle or Circle at runtime (and this is how you do inheritance).
```
Shape s1;
s1 = new Circle();
```
protected allows sub-classes to access a variable (method), while private does not.
abstract public void functionName() in the parent class givves a heads-up that subclasses provide that function.