Android Programming with Kotlin for Beginners: Build Android apps starting from zero programming experience with the new Kotlin programming language

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 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 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, 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 glimpse the power and depth of the Android API.

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 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, most importantly, the sunroof of the Android API?

The answer to this question is the Kotlin programming language, and the fact that Kotlin was designed to help programmers handle complexity, avoid mistakes, and make fast progress. Let's look deeper into Kotlin and object-oriented programming (OOP).

Kotlin is an object-oriented language. This means that it uses the concept of reusable programming objects. If this sounds like technical jargon, another analogy will help. Kotlin enables us and others (like the Android API development team) to write 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 every car that comes off the production line?

The answer, of course, is no. They get highly skilled engineers to develop exactly the right components, honed, refined, and improved over years. Then, those same components are 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 must still be built from the raw materials using real-life engineers or robots. This is true.

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

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

In Kotlin, 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 engineers up at Android HQ write a desperately complex program that can talk to satellites, and as a result of this, in a single line of code, we can get our location on the surface of the planet. Nice.

Most of the Android API is written in another language (Java), but this doesn't matter to us as we have full access to the functionality (coded in Java) while using the more succinct Kotlin. Android Studio and the Kotlin compiler handle the complexities behind the scenes.

The software engineers have considered how they can make this code useful to all Android programmers who want to make amazing apps that use users' locations to do cool things. One of the things they 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 OOP. I will constantly refer to OOP in every chapter, and there will be a big reveal in Chapter 10, Object-Oriented Programming.

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

The manufacturers of 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) are placed in a bundle of files called an Android application package (APK), and this is what the device needs to run our app.

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

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

There is an entire range of tools needed to develop for Android, and we also need the Android API, of course. This whole suite of requirements is collectively known as the SDK. Fortunately, downloading and installing a single application will give us these things all bundled together. The application is called Android Studio.

Android Studio is an integrated development environment (IDE) that will take care of all the complexities of compiling our code and linking with 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 now really close to building our first app.

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

As mentioned earlier in this chapter, our app will include resources, such as images, sound, and user interface layouts, that are kept in separate files from the Kotlin 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 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 read from text-based files by Android.

Android (or any computer) 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; 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, when 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.xml extension.

You do not need to memorize this, as we will constantly be returning to this concept in 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 very soon, in the section titled Our first Android app. Packages are often separated into sub-packages so that 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 usually be contained in its own file with the same name as the class.

In Kotlin, we further break up our classes into sections that perform the different actions of our class. We call these action-oriented sections functions. It is the functions of the class that we will use to access the functionality provided within all the millions of lines of Android 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 function from within the class gives us precisely the result we are after.

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 Kotlin, we will only be using selected parts from this API. Note also that each class has its own distinct data. Typically, if you want access to the data in a class, you need to have an object of that class.

By the end of this chapter, we will have imported multiple packages, as well as some classes from them. By the end of Chapter 2, Kotlin, XML, and the UI Designer, we will have even written our very own functions.

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

You may get another message like this:

If the preceding message appears, click Install Build tools…. and then click Finish.

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

You're done. Here is what the app looks like so far in the emulator. Remember that you might (probably do) have a different emulator, which is 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 we can 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 Kotlin and add our own Kotlin code to the Hello World project.