Rename year directories to allow natural ordering

year2/semester1/logseq-stuff/pages/Abstraction & Polymorphism.md

@@ -0,0 +1,38 @@
+- #[[CT2106 - Object-Oriented Programming]]
+- **Previous Topic:** [[Coding Up Inheritance]]
+- **Next Topic:** [[Interfaces]]
+- **Relevant Slides:** ![Lecture-13__2022.pdf](../assets/Lecture-13_2022_1666253165381_0.pdf) ![Lecture-14___2022.pdf](../assets/Lecture-14_2022_1666343488472_0.pdf)
+-
+- Why use an abstract class?
+	- You should use an abstract class in situations where you want to use inheritance but do not want another developer to create an object from the superclass.
+-
+- # Abstract Methods
+	- Abstract classes can also have **abstract methods**.
+	- Abstract methods are methods with no body.
+		- ```java
+		  public abstract void sing();
+		  ```
+		- In other words, they do nothing.
+	- Abstract methods provide the definition of a method that at least one of its subclasses must implement.
+-
+- # Concrete
+	- The adjective **concrete** is often used in OOP to denote a class or method that is **not abstract**.
+		- i.e., the class or method is fully implemented.
+-
+- # Reference Type
+	- An abstract class is often used as the type of a reference variable.
+		- ```java
+		  Animal animal = new Canary("bruh");
+		  ```
+-
+- # Polymorphism
+	- What is **polymorphism**? #card
+	  card-last-interval:: 3.45
+	  card-repeats:: 2
+	  card-ease-factor:: 2.46
+	  card-next-schedule:: 2022-11-18T06:09:31.768Z
+	  card-last-reviewed:: 2022-11-14T20:09:31.768Z
+	  card-last-score:: 5
+		- **Polymorphism** refers to how an object can be treated as belonging to several types as long as those types are **higher** than the object's type in the class hierarchy.
+	- In general, a variable of type $X$ can point to any object that has an "is-a" relationship to type $X$.
+		- e.g., a variable of type `Animal` can point to a `Bird`, `Frog`, or `Fish` object.