This book is intended as a step-by-step guide to guide developers well versed in the arts of iOS into the realm of Android mobile application development.

The author assumes that the reader has never written Android applications before; at most, maybe, she or he has played with an Android device at some point, but nothing else. If you are already familiar with Android, you can skip directly to chapter 2, and start creating apps right away.

If you are not familiar with the Android developer tools, it is strongly recommended to read this book linearly, and to build the sample applications one after the other. This will help you build your skills step by step.

This book assumes that the reader is using a Mac – after all, the reader is supposed to be an iOS developer!

It also assumes working programming knowledge in Objective-C or Swift, and of the most common iOS frameworks, such as Foundation, UIKit, Core Location, Core Data and others.

Most importantly, it is also assumed that the reader already knows Java; if not, please be aware that you might have a hard time understanding the code snippets shown in this book. The Bibliography section at the end of the book provides a few useful titles for starting your exploration of Java. In particular I’d recommend reading [Bloch] own "Effective Java" to help you take your Java skills to the next level.

In terms of software requirements, this book assumes that the latest copy of Android Studio is installed in the development machine, as well as Homebrew.

The code bundled with the book has been prepared and tested with the latest version of Android Studio; make sure to download and install it in your system before starting.

All applications use the same baseline: API 16, also known as Jelly Bean 4.1. This version of Android was released in July 2012, and at the time of this writing, 97.3% of all Android devices in the wild run a version equal or older to Jelly Bean. This should hopefully give this book the widest possible reach.

Every time that the text of the book references some sample code, a "Follow Along" callout section will appear with the path of the project, which you can open on Android Studio to run the project directly on your device or the emulator:

Each application is as simple as possible, but not simpler. All the applications are working examples, tested at least in four environments:

Given the large variety of the Android market, it is possible that some bits and pieces of the source code will not work in some devices; I remember having trouble with some Android devices during my career, so I would not be surprised if some of you encounter difficulties. I will not be able to provide support for your particular device, but I guarantee that the code should work in the environments enumerated above.

This book is structured around code. The chapters are meant to be read with Android Studio open, in the order they have been written; I have reused bits and pieces of knowledge from previous chapters in many others, so you should be better served by reading them in order.

To help readers get up and running as fast as possible, every chapter features a section called "TL;DR" at its very beginning, including a handy summary of the most important similarities and differences between Android and iOS. You can use the tables in this section as a reference, and if you find them useful you can print a copy of the Appendix C, which contains all the TL;DR tables together in the same place.

The source code included in the book points directly to the applications available in the code folder, which contains all the sample applications bundled as part of this training.

As an introduction, these are the most important differences that distinguish the iOS developer experience from that of Android.

Android applications are primarily written in Java, a language that shares a lot of features and commonalities with Objective-C and Swift. After all, many Java engineers working for Sun in the nineties came from NeXT, the computer company founded by Steve Jobs that ultimately merged with Apple in 1996, bringing the Objective-C language to the Apple ecosystem.

Both Java and Objective-C share lots of commonalities, such as protocols (called "interfaces" in the realm of Java) or primitive boxing types such as NSNumber (represented by the family of Number subclasses in Java.) They also represents two opposite views in terms of compile-time checks; Objective-C delegates most method and type resolutions to runtime, while Java performs rather strict type checking. In this respect, Swift is much more close to Java than Objective-C will ever be.

Java has the following characteristics:

Of all the items above, the most important is the "Strongly and statically typed" one. This means that method calls in Java are bound at compile time, instead of being dispatched dynamically at runtime like in Objective-C. In spite of this, Java resolves polymorphic methods at runtime using a "vtable," just like in C++ or C#.

The table "Comparison of Java 1.7, Objective-C 2.0 and Swift 3" provides a comparison between these three languages.

When a user taps on the icon of an Android application a whole series of events happen in the device. Many of these events are very similar to those in iOS, and it turns out that, quite unsurprisingly, both operating systems use a very similar architecture, but with quite different class structures backing them.

Let us create a small project in Android Studio. In that project, add a subclass of the android.app.Application class, and register that class as the main application class in your AndroidManifest.xml file. Add two breakpoints in the source code, one in the Application.onCreate() method, and another in the MainActivity.onCreate() method.

The stack traces when hitting both breakpoints is shown below:

There are several interesting bits of information in the stack traces above. First of all, the android.os.Looper class, which as the name suggests provides the main run loop of the application. Most GUI toolkits include a similar construction, created at application runtime, holding an event queue and routing events from the operating system to the different activities and components of the application.

If you click on the name of a class in Android Studio while holding down the Cmd key, the IDE will open the corresponding class file; if you do not have the Android source code available in your local workstation the IDE will simply decompile the code from the local SDK and show a stub implementation of the corresponding class, with most of its methods.

By doing this repeatedly, from both the MainActivity and the Application subclass you created previously, you are going to arrive to the android.content.Context class, which is arguably the most important class in the Android SDK. The Context class includes many different methods, ranging from file management to database creation to inter-process communication, and it also holds a reference to the underlying Looper class.

The diagram "The android.content.Context class" shows a simplified class hierarchy of the Activity and Application classes and its relationship with the Looper class.

Another important element in the stack trace above is Zygote; this piece of software, named after a fertilized living cell, is the application launcher of the Android operating system. To ensure fast application startup, Zygote loads a copy of the ART virtual machine and when a new application launches, forks this VM to sandbox the process in it. If you want to know more about Zygote, this answer in Stack Overflow provides an excellent summary.

Android Studio is a free IDE provided by Google to develop Android applications. It replaced the venerable Eclipse Android Developer Tools, historically the first official IDE for Android software development for many years. It was announced for the first time in May 2013 at the Google I/O conference. The first stable release was in December 2014. It is available for Windows, macOS and Linux, and is now considered the official IDE for Android development.

Android Studio is powered by IntelliJ IDEA, a popular IDE for Java development for the past 15 years. It has a solid reputation, and is particularly appreciated by its advanced support for refactoring, code generation and project navigation features.

Android Studio is available from the Android Studio website. The current version at the time of this writing is 2.2.2; version 2.2 was a major milestone released September 19th, 2016. Android Studio is, by far, the most important piece in the daily workflow of an Android Developer, and includes many different features targeted to simplify the development of Android apps, which like all software development can be quite a complex endeavour sometimes.

Once downloaded and launched, Android Studio will launch a configuration wizard, as shown in Android Studio Setup Wizard – Step 1.

Most developers will choose the standard settings, as shown in image Android Studio Setup Wizard – Step 2.

Finally, Android Studio will automatically download all the elements required for it to work properly, as shown in image Android Studio Setup Wizard – Step 3.

Once Android Studio is ready to go, it will display some tips and tricks every day – something you can easily dismiss if you want, as shown in Android Studio Tips.

At the beginning of this chapter I mentioned that Java is the only programming language supported by Google for Android development. The ecosystem, however, includes many other possibilities; at the end of this book, the appendix "Third Party Android Developer Tools" contains a list of other environments and programming languages available to create Android applications. As you will see there, there are lots of different possibilities out there.

In this section we are going to explore one of those options: Kotlin. This programming language was created by the same engineers that created IntelliJ IDEA and Android Studio, and it is uncannily similar to Swift in many ways. Kotlin might be an interesting choice for Swift developers looking to write Android applications using a safe, fast and modern programming language.

Using Kotlin in your Android project requires adding a small library used at runtime for interoperability with Java; this library increases the size of the final application in around one megabyte.

We are going to create a small application using Kotlin, to show how easy it is to integrate and how similar it is to Swift.

Kotlin is available as a plugin for Android Studio. You can manage plugins in Android Studio by opening the Preferences ▸ Plugins menu, as shown in figure "Browsing plugin repositories"

Once the Kotlin plugin is installed, you should restart Android Studio for it to be loaded in memory. After relaunching, create a simple Android application and open the MainActivity class. Select the Code ▸ Convert Java File to Kotlin File… menu entry and watch Android Studio transform the standard Java code of your project into Kotlin.

The listing "Simple activity in Kotlin" shows the resulting code in a very simple application, with a lambda set as the event callback for button clicks.

Kotlin not only has a syntax that looks a lot like that of Swift, it shares many features with Apple’s new language:

Kotlin compiles its code to native Android bytecode, which means that setting the small overhead of the runtime library aside, an application created with Kotlin behaves and is distributed exactly like one created with Java.

As the name implies, the SDK Manager allows the developer to install, manage and uninstall different versions of the Android Software Development Kit (SDK) in the local workstation. Figure Android SDK Manager shows the default state after launching Android Studio and installing the latest available version of Android at the time of this writing, 7.1.1 (also known as Nougat.)