We have already seen that each time we create a new Android project we also create a new Java package, as a kind of container for the code we will write.
We have also learned about and played around with classes. We have imported and taken direct advantage of classes from the Android API such as Log and Toast. We have also used the AppCompatActivity class, but in a different manner to that of Log and Toast. You might remember the first line of code in all our projects so far, after the import statements, used the extends keyword:
public class MyActivity extends AppCompatActivity {When we extend a class as opposed to just importing it, we are kind of making it our own. In fact, if you take another look at the line of code you can see that we are making a new class, with a new name, MyActivity, but basing it on the AppCompatActivity class from the Android API.
Note
AppCompatActivity is a slightly modified version of Activity. It basically provides extra features for older versions of Android that would otherwise not be present. Everything we have discussed about Activity is equally true for AppCompatActivity. We will see some more variations on the Activity class as we progress. It is entirely possible that you have a different class in place of AppCompatActivity, dependent upon changes that have taken place since this was written. Updates of Android Studio will often change the default Activity class that it uses when it creates a new project. As long as the name ends in …Activity it doesn't matter because everything we have discussed and will discuss is equally true. I will usually just refer to this class simply as Activity.
In summary, we can import classes to use them, we can extend classes to use them and, very importantly, we will eventually make our own classes.
Our own classes and those written by others are the building blocks of our code, and the methods within the classes wrap the functional code, the code that actually does the work.
We can write methods within the classes that we extend, as we did with topClick and bottomClick in Chapter 2, Java – First Contact. Furthermore, we overrode methods that are already part of classes written by others, such as onCreate, onPause, and so on.
However, the only code that we put in these methods was a few calls using Toast and Log. We aren't going to code the next killer app with just that.