Classes in Object-oriented programming
In-class discussion
- A class is a kind of thing - they don't actually exist until you make an instance
- Classes can also be variations of other classes
- Inheritance - I'm a cat, but also an animal
- Do everything the superclass does, but maybe with a few changes and some extra features
- Child class has the parent class inside like a nesting doll
- In programming, a class makes a new type.
- You know types from other variables: they are things like int and string
- You can't ever just have an int - you always have a particular int like 7.
- The same thing applies to classes, when you create one, you have an object, whose type is the class it came from
- An object or instance of a class can act like itself or any of its ancestor classes, because it has all of them embedded into itself
- This lets you do things like have an array of Pet, where each item is actually a Dog,Cat,Bird, or other pet object. None of them are actually instances of just Pet, but all of them know how to do Pet stuff!
Typescript syntax
The class keyword is used to create a new class in typescript. It'll look like this:
class Cat extends Animal {
breed: string
constructor(kingdom:string,phylum:string,class_:string,order:string,family:string,genus:string,species:string,breed:string) {
super(kingdom,phylum,class_,order,family,genus,species);
this.breed = breed;
}
meow():string {
return "MRowl"
}
}
Remember Kids Playing Chess On Freeways Get Squashed? (Kingdom, phylum, class, order, family, genus, species...)
Anyway, there are a few things in this class to notice.
extendstells the class that aCatis a kind ofAnimal. This means that the cat gets to have all of the features of the Animal class, and it can override them or add new ones as you wish.constructoris a special function that gets called whenever you make a new cat, which looks likelet my_cat = new Cat("animal",...)(extra args left as an exercise for the reader...)- Member functions or Methods, like
meowin the example, don't need thefunctionkeyword. Just put the name of the function, its arg list, and the return type. - Member variables, like
breed, are declared inside the class but not inside a function. They don't need avarorletkeyword. These become values that you can use anywhere inside the class' code. They're part of the cat, so any cat function can see them.
Practice
-
Design a class hierarchy for pets
- All pets are animals
- Some pets are mammals, some reptiles, some avian
- What things can each particular type of pet do that others may not?
- What can all animals do?
- What can all birds do?
-
Design a class hierarchy for vehicles
- Cars, trucks, trains, bicycles, planes, spaceships, cargo ships
- What is the same, and what is different?
- How much stuff can you fit into the super class for each category?