Looking at Android Programming Basics
Android applications are written in Java — not the full-blown Java that J2EE developers are used to, but a subset of Java that is sometimes known as the Dalvik virtual machine. This smaller subset of Java excludes classes that don't make sense for mobile devices. If you have any experience in Java, you should be right at home.
It may be a good idea to keep a Java reference book on hand, but if you have any trouble with a Java concept, you can always search Google to find out what you don't understand. Because Java is nothing new, you can find plenty of examples on the web that demonstrate how to do just about anything.
In Java source code, not all libraries are included (such as third-party libraries that assist in list processing or message queue management, and so on). Verify that the package is available to you. If it's not, an alternative that can work for your needs is probably bundled with Android.
Think of an activity as a container for your UI, like a screen. It contains your UI logic as well as the code that runs it. It's kind of like a form, for you Windows programmers out there. Android applications are made up of one or more activities. Your app must contain at least one activity, but an Android application can contain several. I discuss activities in more detail in Chapters 4 and 6.
Intents make up the core ...