Comparison of Java and Android API
While most Android applications are written in Java, there are many differences between the Java API and the Android API, and Android does not use a Java Virtual Machine but another one called Dalvik.
Android's Process Virtual machine
There is no Java Virtual Machine in the Android platform. Java byte code is not executed. Instead Java classes are compiled into Dalvik executables and run on Dalvik, a specialized virtual machine (VM) designed specifically for Android. Unlike Java VMs, which are stack machines, the Dalvik VM is a register-based architecture.
Dalvik has some specific characteristics that differentiate it from other standard VMs:
- The VM was designed to use less space.
- The constant pool has been modified to use only 32-bit indexes to simplify the interpreter.
- Standard Java bytecode executes 8-bit stack instructions. Local variables must be copied to or from the operand stack by separate instructions. Dalvik instead uses its own 16-bit instruction set that works directly on local variables. The local variable is commonly picked by a 4-bit 'virtual register' field.
Because the bytecode loaded by the Dalvik virtual machine is not Java bytecode, and of the specific way Dalvik load classes, it is not possible to load Java libraries packages as jar files, and even a specific logic must be used to load Android libraries (specifically the content of the underlying dex file must be copied in the application private internal storage area, before being able to be loaded).
As it is the case for the Java SE class
System, the Android
System class allows the retrieval of system properties. However, some mandatory properties defined with the Java Virtual Machine have no meaning or a different meaning on Android. For example:
- "java.version" property returns 0 because it is not used on Android,
- "java.specification.version" invariably returns 0.9 independently of the version of Android used,
- "java.class.version" invariably returns 50 independently of the version of Android used,
- "user.dir" has a different meaning on Android,
- "user.home" and "user.name" properties do not exist on Android
Dalvik does not align to Java SE nor Java ME class library profiles (e.g., Java ME classes, AWT or Swing are not supported). Instead it uses its own library built on a subset of the Apache Harmony Java implementation.
By default, the default output stream
System.err do not output anything, and developers are encouraged to use the
Log class, which logs Strings on the LogCat tool. (this has changed at least from HoneyComb, and they now output to the log console as well)
Graphics and Widget library
Android does not use the Abstract Window Toolkit nor the Swing library. User Interface is built using View objects. Android uses a framework similar to Swing based around
Views rather than
JComponents. However, Android widgets are not JavaBeans: the Android application
Context must be provided to the widget at creation.
Look and feel
Android widget library does not support a Pluggable look and feel architecture; The Look and Feel of Android widgets must be embedded in the widgets themselves. There is, however, a limited capability to set styles and themes for an application.
- Rose, John (2008-05-31). "with Android and Dalvik at Google I/O". Retrieved 2008-06-08.
- Fred Chung (2011-07-28). "Custom Class Loading in Dalvik". Google. Retrieved 2011-11-27.
- "Google's Android SDK Bypasses Java ME in Favor of Java Lite and Apache Harmony". infoq.com. 2007-11-12. Retrieved 2009-05-31. "Instead of providing a full version of the Java SE or Java ME Google has diverged on two fronts. First, a limited subset of the core Java packages is provided. (...) By going this route Android is following in the footsteps of another Google project GWT which uses Java as its development language but does not support the full JDK"
- "Alternative to Point2D". Droid Tutorials. 2010-02-12. Retrieved 2010-02-17. "Given that AWT is not supported in Android API, the Point2D class, which is useful for writing 2D graphics, is missing either"
- "Package Index". Google. Retrieved 2011-11-27.
- "Android Debug Bridge". Google. Retrieved 2009-05-31. "By default, the Android system sends stdout and stderr (System.out and System.err) output to /dev/null."
- "Reading and Writing Logs". Google. Retrieved 2011-11-27.
- "Applying Styles and Themes". Google. Retrieved 2011-09-03.
- "Common Layout Objects". Google. Retrieved 2011-09-03.