Comparison of Java and Android API

This article compares the Java and Android API and virtual machines.

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.

Contents

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• 1 Android's Process Virtual machine
• 2 System properties
• 3 Class library
  • 3.1 java.lang package
  • 3.2 Graphics and Widget library
    • 3.2.1 Look and feel
    • 3.2.2 Layout manager
• 4 See also
• 5 References

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).

System properties

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

Class library

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.

java.lang package

By default, the default output stream System.out and System.err do not output anything,^[6] and developers are encouraged to use the Log class, which logs Strings on the LogCat tool.^[7] (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 thanJComponents. 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.

Layout manager

Contrary to Java where Layout managers can be applied to any container widget, Android layout behavior is encoded in the containers.