Android Java VM

• The Android operating system runs apps by launching a unique virtual machine (VM), based on the Java Virtual Machine (JVM) that converts compiled Java byte code into machine code, in which each individual app will run. This provides a number of security features to prevent apps from gaining unnecessary or dangerous permissions on the device. From a functional standpoint, it means that the easiest way for programmers to write apps for Android is to code them in the language these virtual machines process: Java.
  
  

SDK Tools

• In order for Java programmers to be able to build fully featured Android apps, they need the libraries of object and functions which Google provides. These allow programmers to code apps that use features of Android phones such as the camera, GPS, and gyroscope. Google provides these libraries, or software development kit (SDK), in two forms: specialized SDKs for a specific version of Android, and a base SDK which allows the programmer to download both version specific code libraries and other specialized Android development tools.
  
  

Android Device Emulator

• Just as other system emulation software allows users to run virtualized operating systems within their current system, Google provides an Android device emulator through its base SDK. This tool provides a fully featured Android system which programmers can use to test their apps. This allows programmers to save the time it takes to physically load apps onto a physical Android device. It also allows them to see if an app contains an error which would have adverse effects on an Android system without putting their own physical device at risk.
  
  

Android Debug Bridge

• The Android Debug Bridge (ADB) is a command line program that serves a number of uses for Android app developers. It allows programmers to load the apps they develop into both an Android device emulator running on their system, and onto physical Android devices. It also allows programmers to access various data about their app's status at various points during its execution to help the programmer diagnose and fix any bugs in their apps.