polymorphism completed

README.md

@@ -17,7 +17,7 @@ Lessons:
 - [encapsulation](./lessons/Encapsulation.md)
 - [inheritance](./lessons/Inheritance.md)
 - [properties](./lessons/Properties.md) -
-- [polymorphism](./lessons/Polymorphism.md) -
+- [polymorphism](./lessons/Polymorphism.md)
 
 Work in progress:
 

java/Polymorphism.md

@@ -0,0 +1,5 @@
+# Polymorphism
+
+Relevant files
+
+- [Program.java](./inheritance/Program.java)

java/inheritance/Program.java

@@ -1,9 +1,14 @@
 public class Program {
   public static void main(String[] args) {
-    Dog d = new Dog("buster");
-    Cat c = new Cat("mittens");
+    // because cat and dog are subclasses of animal, they can be stored in an array of animals
+    Animal[] animals = { 
+      new Dog("buster"), 
+      new Cat("mittens") 
+    };
 
-    d.make_sound();
-    c.make_sound();
+    for (Animal a : animals) {
+      // when calling the make_sound function, the implementation of the actual class gets called (in this case cat or dog)
+      a.make_sound();
+    }
   }
 }

java/inheritance/README.md

@@ -2,7 +2,6 @@
 
 Relevant files:
 
-- [Program.java](./Program.java)
 - [Animal.java](./Animal.java)
 - [Cat.java](./Cat.java)
 - [Dog.java](./Dog.java)

lessons/Polymorphism.md

@@ -0,0 +1,19 @@
+# Polymorphism
+
+Polymorphism is the ability to treat subclasses like their base classes, for
+example passing cat, which extends animal to a function that expects an animal
+as parameter. or storing objects of different types that all extend a common
+class in an array of that common type.
+
+You lose acess to the properties of the subclass, but you can still access the
+base class like usual. When the subclass overrides behaviour of the base class,
+the overridden functionality in the subclass is still called.
+
+You can regain access to properties of the subclass with casting, but you have
+to make sure that the object is actually an instance of that class or you might
+run into runtime errors.
+
+- [Java](../java/Polymorphism.md)
+- [Rust](../rust/Inheritance.md)
+- [C++](../cpp/Polymorphism.md)
+- [JavaScript (using TypeScript)](../typescript/Polymorphism.md)

typescript/Polymorphism.md

@@ -0,0 +1,5 @@
+# Polymorphism
+
+Relevant files
+
+- [index.ts](./inheritance/index.ts)

typescript/inheritance/README.md

@@ -2,7 +2,6 @@
 
 Relevant files:
 
-- [index.ts](./index.ts)
 - [animal.ts](./animal.ts)
 - [cat.ts](./cat.ts)
 - [dog.ts](./dog.ts)

typescript/inheritance/index.ts

@@ -1,8 +1,14 @@
 import { Dog } from "./dog";
 import { Cat } from "./cat";
+import { Animal } from "./animal";
 
-const d = new Dog('buster');
-const c = new Cat('mittens');
+// because cat and dog are subclasses of animal, they can be stored in an array of animals
+const animals: Animal[] = [
+  new Dog('buster'),
+  new Cat('mittens')
+]
 
-d.make_sound();
-c.make_sound();
+for (const a of animals) {
+  // when calling the make_sound function, the implementation of the actual class gets called (in this case cat or dog)
+  a.make_sound();
+}