Android Programming for Beginners - Second Edition

The Android API is code that makes it easy to do exceptional things. A simple analogy could be drawn with a machine, perhaps a car. When you press on the accelerator, a whole bunch of things happen under the hood. We don't need to understand about combustion or fuel pumps because some smart engineer has made an interface for us; in this case a mechanical interface—the accelerator pedal.

For example, the following line of Java code probably looks a little intimidating at this stage in the book, but it serves as a good example of how the Android API helps us:

locationManager.getLastKnownLocation(LocationManager.GPS_PROVIDER)

Once you learn that this single line of code searches for available satellites in space, then communicates with them in their orbits around the Earth, and then retrieves your precise latitude and longitude on the surface of the planet, it becomes easy to begin to glimpse the power and depth of the Android API in conjunction with the DVM.

For sure, that code does look a little challenging, even mind boggling at this stage of the book, but imagine trying to talk to a satellite some other way!

The Android API has a whole bunch of Java code that has already been written for us to use as we like.

The question we must ask, and the one this book tries to answer, is as follows:

How do we use all this code to do cool stuff? Or, to frame the question to fit the earlier analogy: How do we find and manipulate the pedals, steering wheel, and sunroof of the Android API?

The answer to this question is the Java programming language and the fact that Java was designed to help programmers handle complexity. Let's talk a bit about Java and object-oriented programming (OOP).

Java is a programming language that has been around a lot longer than Android. It is an object-oriented language. This means it uses the concept of reusable programming objects. If this sounds like technical jargon, another analogy will help. Java enables us and others (like the Android development team) to write Java code that can be structured based on real-world things, and here is the important part: it can be reused.

So, using the car analogy, we could ask the following question: if a manufacturer makes more than one car in a day, do they redesign every part for each and every car?

The answer, of course, is no. They get highly skilled engineers to develop exactly the right components, honed, refined, and improved over years. Then that same component is reused again and again, as well as being occasionally improved.

If you are going to be fussy about my analogy, then you can point out that each of the car's components still has to be built from the raw materials using real-life engineers or robots.

This is true. What the software engineers do when they write their code is build a blueprint for an object. We then create an object from their blueprint using Java code, and once we have that object we can configure it, use it, combine it with other objects, and more besides.

Furthermore, as well as this, we can design blueprints ourselves and make objects from them as well. The compiler then transforms (manufactures) our bespoke creation into DEX code. Hey presto! We have an Android app.

In Java, a blueprint is called a class. When a class is transformed into a real working "thing", we call it an object or an instance of the class.

So, some smart software engineer up at Android HQ writes a desperately complex Java program that can talk to satellites. He then considers how he can make this code useful to all the Android programmers who want to make amazing apps that use users' locations to do cool things. One of the things he will do is make features such as getting the device's location in the world into a simple one-line task.

So, the single line of code we saw previously sets in action many more lines of code that we don't see and don't need to see. This is an example of using somebody else's code to make our code infinitely simpler.

Welcome to the world of object-oriented programming (OOP). I will constantly refer to OOP in every chapter and there is the big reveal in Chapter 10, Object Oriented Programming.

To get things done on Android, we write Java code of our own, which also uses the Java code of the Android API. This is then compiled into DEX code and run by the DVM, which in turn has connections to an underlying operating system called Linux that handles the complex and extremely diverse range of hardware that are the different Android devices.

The manufacturers of the Android devices and of the individual hardware components obviously know this too, and they write advanced software called drivers that ensure that their hardware (for example, CPU, GPU, GPS receivers, memory chips, and hardware interfaces) can run on the underlying Linux operating system.

The DEX code (along with some other resources) is placed in a bundle of files called an Android application PacKage (APK), and this is what the DVM needs to run our app:

The next question is where exactly does all this Java coding and compiling into DEX code, along with APK packaging, take place? Let's look at the development environment we will be using.

A development environment is a term that refers to having everything you need to develop, set up, and be ready to go in one place. We need two things to get started.

Android Studio is an integrated development environment (IDE) that will take care of all the complexities of compiling our code and linking with the JDK and the Android API. Once we have installed Android Studio, we can do everything we need inside this single application and put to the back of our minds many of the complexities we have been discussing.

So we had better get our hands on Android Studio.

Using your preferred file manager software, perhaps Windows Explorer, create a folder called AndroidProjects. Make it at the root of the same drive where you installed Android Studio. So, if you installed Android Studio at C:/Program Files/Android, then create your new folder at C:/AndroidProjects.

Or, if you installed Android Studio at D:/Program Files/Android, then create your new folder at D:/AndroidProjects.

Android Studio and the supporting tools that we need are installed and ready to go. We are really close now to building our first app.

Now, let's look a little bit at the composition of an Android app.

Our app will include resources such as images, sound, and user interface layouts that are kept in separate files from the Java code. We will slowly introduce ourselves to them over the course of the book.

They will also include files that have the textual content of our app. It is convention to refer to the text in our app through separate files because it makes them easy to change, and makes it easy to create apps that work for multiple different languages and geographical regions.

Furthermore, the actual User Interface (UI) layout of our apps, despite the option to implement them with a visual designer, are actually read from text-based files by Android.

Android (or any computer) of course cannot read and recognize text in the same way that a human can. Therefore, we must present our resources in a highly organized and predefined manner. To do so, we will use Extensible Markup Language (XML). XML is a huge topic, but fortunately, its whole purpose is to be both human-and machine-readable. We do not need to learn this language; we just need to note (and then conform to) a few rules. Furthermore, most of the time we interact with XML, we will do so through a neat visual editor provided by Android Studio. We can tell when we are dealing with an XML resource because the filename will end with the extension .xml.

You do not need to memorize this as we will constantly be returning to this concept throughout the book.

Whenever we create a new Android app, we will choose a unique name known as a package. We will see how we do this in the section "Our first Android app". Packages are often separated into sub-packages, so they can be grouped together with other similar packages. We can simply think of these as folders and sub-folders, which is almost exactly what they are.

We can think of all the packages that the Android API makes available to us as code from a code library. Some common Android packages that we will use include the following:

As you can see, they are arranged and named to make what is in them as obvious as possible.

Earlier, we learned that the reusable code blueprints that we can transform into objects are called classes. Classes are contained in these packages. We will see in our very first app how we can easily import other people's packages, along with specific classes from those packages for use in our projects. A class will almost always be contained in its own file with the same name as the class, and it will also have the .java file extension.

In Java (and therefore Android), we further break up our classes into sections that perform the different actions of our class. We call these action-oriented sections methods. It is most often the methods of the class that we will use to access the functionality provided within all those millions of lines of code.

We do not need to read the code. We just need to know which class has what we need, which package it is in, and which methods from within the class give us precisely the result we are after.

The following diagram shows a representation of the Android API. We can think of the structure of the code we will write ourselves in the same way, although we will usually have just one package per app.

Of course, because of the object-oriented nature of Java, we will only be using selected parts from this API. Notice also that each class has its own distinct data. Typically, if you want access to the data in that class, you need to have an object of that class:

You do not need to memorize this as we will constantly be returning to this concept throughout the book.

By the end of this chapter, we will have imported multiple packages, as well as dozens of classes from them, and we will have used many of their methods as well. By the end of Chapter 2, First Contact: Java, XML and the UI Designer we will have even written our very own methods.

If you do, click Install missing platform(s) and sync project, accept the license agreement, and then click Next, followed by Finish.

If you get another message like this:

Click Install Build tools…. and then click Finish.

Follow these simple steps to run the game on the default Android emulator:

You're done. Here is what the app looks like so far in the emulator. Remember that you might (and probably do) have a different emulator—that's fine:

Clearly, we have more work to do before we move to Silicon Valley and look for financial backing, but it is a good start.

We need to test and debug our apps often throughout development to check for any errors, crashes, or anything else unintended.

It is also important to make sure it looks good and runs correctly on every device type/size that you want to target. Obviously, we do not own one of each of the many thousands of Android devices. This is where emulators come in.

Emulators, however, are sometimes a bit slow and cumbersome, although they have improved a lot recently. If we want to get a genuine feel for the experience our user will get then you can't beat deploying to a real device. So, we will want to use both real devices and emulators while developing our apps.

If you want to try out your app on a tablet, you're going to need a different emulator.

Now we can look at how to get our app onto a real device.

The first thing to do is to visit your device manufacturer's website and obtain and install any drivers that are needed for your device and operating system.

The next few steps will set up the Android device for debugging. Note that different manufacturers structure the menu options slightly differently to others. But the following sequence is probably very close, if not exact, for enabling debugging on most devices:

We are now ready to learn some Java and add our own Java code to the Hello World project.