While most Android applications are written in Java-like language, there are some differences between the Java API and the Android API, and Android does not run Java bytecode by a traditional Java virtual machine (JVM), but instead by a Dalvik virtual machine in older versions of Android, and an Android Runtime (ART) in newer versions, that compile the same code that Dalvik runs to Executable and Linkable Format (ELF) executables containing machine code.
Java bytecode in Java Archive (JAR) files is not executed by Android devices. Instead, Java classes are compiled into a proprietary bytecode format and run on Dalvik (or compiled version thereof with newer ART), a specialized virtual machine (VM) designed for Android. Unlike Java VMs, which are stack machines (stack-based architecture), the Dalvik VM is a register machine (register-based architecture).
Dalvik has some traits that differentiate it from other standard VMs:
Because the bytecode loaded by the Dalvik virtual machine is not Java bytecode and due to the way Dalvik loads classes, it is impossible to load library packages as
JJAR files. A different procedure must be used to load Android libraries, in which the content of the underlying
dex file must be copied in the application private internal storage area before it is loaded.
As is the case for the Java SE class
System, the Android
System class allows retrieving system properties. However, some mandatory properties defined with the Java virtual machine have no meaning or a different meaning on Android. For example:
java.versionproperty returns 0 because it is not used on Android.
java.specification.versioninvariably returns 0.9 independently of the version of Android used.
java.class.versioninvariably returns 50 independently of the version of Android used.
user.dirhas a different meaning on Android.
user.nameproperties do not exist on Android.
Current versions of Android use the latest Java language and its libraries (but not full graphical user interface (GUI) frameworks), not the Apache Harmony Java implementation, that older versions used. Java 8 source code that works in latest version of Android, can be made to work in older versions of Android.
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 also.
Android does not use the Abstract Window Toolkit nor the Swing library. User interfaces are built using View objects. Android uses a framework similar to Swing, based on
Views rather than
JComponents. However, Android widgets are not JavaBeans: the Android application
Context must be provided to the widget at creation.
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. However, a limited ability exists to set styles and themes for an application.
Android includes only a small subset of the
java.beans package (
PropertyChangeEvent and related classes).
By default, the Android system sends stdout and stderr (System.out and System.err) output to /dev/null.
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