At time of completing this book, Android 9 and Android Studio 3.2 had just been released. This book was written to accommodate these latest versions. One of the changes in the new releases is the way that Android supports devices running older versions of Android. It has just been significantly improved. Android uses a support library, which means that old devices (within reason) can make use of newer features.

The good news is that this book uses the new, improved version!

However, if you are a very early adopter (late 2018 and maybe into early 2019) of this book and you look very closely at the code generated by Android Studio, you will notice some slight differences with the code presented in the book. The differences occur in the import… statements at the top of the Java code files. The book presents code that looks a bit like this:

import androidx.appcompat.app.AppCompatActivity;

Whereas you might notice code in Android Studio 3.2 or earlier...

We are going to look at the resource files that have the code that defines our simple UI layout and the file that has our Java code. At this stage, we will not try to understand it all as we need to learn some more basics before it makes sense to do so. What we will see, however, is the basic content and structure of both files so we can reconcile their content with what we already know about Android resources and Java.

Examining the HelloWorldActivity.java file

Let's look at the Java code first. You can see this code by left-clicking on the HelloWorldActivity.java tab, as shown in the following screenshot:

As we are not looking at the intricate details of the code, an annotated screenshot is more useful than reproducing the actual code in text form. Regularly refer to the following screenshot while reading on with this section:

The first thing to note is that I have added a few empty lines amongst the code to space...

Here, we will add a couple of buttons to the screen, and we will then see a fast way to make them actually do something. We will add a button in two different ways: first, using the visual designer, and second, by adding to and editing XML code directly.

Adding a button via the visual designer

To get started adding our first button, switch back to the design view by clicking the Design tab underneath the XML code we have just been discussing, as shown next:

Notice that to the left-hand side of the layout, we have a window that is called the Palette, and this is shown next:

The palette is divided into two parts. The left-hand list has the categories of UI elements and allows you to select a category, while the right-hand side shows you all the available UI elements from the currently selected category.

Make sure that the Common category is selected as shown in the previous screenshot. Now, left-click and hold on the Button widget and then drag it onto the...

In programming it is always a clever idea to write messages known as code comments and sprinkle them liberally amongst your code. This is to remind us of what we were thinking at the time we wrote the code. To do this, you simply append a double forward slash and then type your comment, as follows:

// This is a comment and it could be useful

In addition, we can use comments to comment out a line of code. Suppose we have a line of code that we temporarily want to disable. Then we can do so by adding two forward slashes, as follows:

// The code below used to send a message
// Log.i("info","our message here");
// But now it doesn't do anything
// And I am getting ahead of where I should be

Let's look at two separate ways to send messages in...

In the introduction to this chapter and in the previous chapter, we talked a bit about using other people's code, specifically via the classes and their methods of the Android API. We saw that we could do some quite complex things with insignificant amounts of code (such as talking to satellites).

To get us started coding, we are going to use two different classes from the Android API that allow us to output messages. The first class, Log, allows us to output messages to the logcat window. The second class, Toast, is not a tasty breakfast treat, but it will produce a toast-shaped pop-up message for our app's user to see.

Here is the code we need to write to send a message to the logcat:

Log.i("info","our message here");

Exactly why this works will become clearer in Chapter 10, Object-Oriented Programming, but for now we just need to know that whatever we put between the two sets of quote marks will be output to the...

So, we now know the code that will output to logcat or the user's screen. But where do we put the code? To answer this question, we need to understand that the onCreate method in HelloWorldActivity.java executes as the app is preparing to be shown to the user. So, if we put our code at the end of this method, it will run just as the user sees it. Sounds good.

Let's quickly try this...