Unit 9: Inheritance

The Big Takeaway Of This Unit

While methods and instance variables of a superclass are carried over to a subclass, constructors are not. That means that we have to write our constructors again! Luckily, we don't have to start over from scratch. We have a special keyword that can help us. This is the super keyword. We will learn how to use it with constructors in this topic and then with methods in the next topic.

Now that we know how to make subclasses and their constructors, it is time to write their methods. Most methods that are inherited from the superclass do not need to be rewritten. However, some methods will have to be rewritten because their implementations will be different. This can be done by overloading, where the parameters for the methods are different but the methods achieve the same thing.

Other times, we are overriding or overloading a method, and then we want to borrow the superclass’s implementation of that method as a substep. Like constructors, we can also use the super keyword. When using the super keyword, we are calling the superclass's implementation of the method. This allows us to not have to duplicate code twice across multiple classes, which will save you both time and space!

Now that we are done with making our subclass, let's look at inheritance from a user standpoint. Inheritance can be thought as an upside down tree with the "root" on the top and the "branches" on the bottom. The root is the superclass while the branches are the subclasses of this superclass. A visual representation of this tree is called a type diagram or hierarchy tree. Here is the hierarchy tree that we will look at for the next two topics:

Now that we have established a little bit about polymorphism in the last topic, let's talk some more about it. Every object has a static type and a dynamic type. Let's use the hierarchy tree from the last topic:

Now that we have learned about inheritance, what even allows our classes and objects that we have created to work the way they do? Where do the general characteristics of all objects come from? The answer lies in the Object class. The Object class is the superclass of all other classes as well as arrays and other data types.