OOP and inheritance

We have seen how we can use other people's hard work by instantiating/creating objects from the classes of an API such as Android. But this whole OOP thing goes even further than that.

What if there is a class that has loads of useful functionality in it but not quite what we want? We can inherit from the class and then further refine or add to how it works and what it does.

You might be surprised to hear that we have done this already. In fact, we have done this with every single app we have created. I mentioned this near the start of the chapter. When we use the extends keyword we are inheriting from another class. Here is the code from the previous mini-app:

public class MainActivity extends...

Here, we are inheriting the Activity class along with all its functionality—or more specifically, all the functionality that the class designers want us to have access to. Here are some of the things we can do to classes we have extended.

We...