Each Android application runs in its own process inside a virtual machine called Dalvik. As we have seen, programs are typically written in Java and then compiled to bytecode. From the bytecode (.class files) they are afterwards transformed into DEX format, commonly using a special tool provided by the Android SDK called
dx. This DEX format is more optimized and designed to have a smaller memory footprint in comparison with normal Java .class files, since mobile devices lack the computational capabilities of desktops. This is achieved through compression and merging/optimization of the multiple .class files.
Note
It is not completely accurate that the coding has to be strictly done in Java. Android allows using native code in our applications, too. Therefore, existing code that we were using before can be reused here. Also, in the computer vision area, there is a lot of code that has been reused from the OpenCV framework. This is achieved through the Native Development Kit (NDK), which is explored in Chapter 9, Native Coding in Android and Chapter 10, Performance Tips.
The Dalvik Virtual Machine also includes some Java Virtual Machine (JVM) features, such as garbage collection (GC). There has been a lot of criticism through the GC because of its non-generational nature; it's famous for driving developers crazy. However, since Android 2.3, an improved concurrent garbage collector makes some of the development easier.
Each application running on Dalvik has at least a total of 16 MB of available heap memory. This can be a real limitation for some applications, since we will likely need to deal with large amounts of image and audio resources. However, newer devices such as tablets or high-end devices have a higher heap limit to allow the usage of high-resolution graphics. We expect this situation to improve in the near future due to the fast evolution of mobile hardware.