Comparison of Java and Android API

Last updated January 30, 2021

This article compares the application programming interfaces (APIs) and virtual machines (VMs) of the programming language Java and operating system Android.

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:^[1]

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 due to the way Dalvik loads classes, it is impossible to load library packages as jar 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.^[2]

System properties

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.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

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.^[3]

java.lang package

By default, the default output stream System.out and System.err do not output anything,^[4] and developers are encouraged to use the Log class, which logs Strings on the LogCat tool.^[5] This has changed at least from HoneyComb, and they now output to the log console also.

Graphics and widget library

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 View s rather than JComponent s. 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. However, a limited ability exists to set styles and themes for an application.^[6]

Layout manager

Contrary to Swing where layout managers can be applied to any container widget, Android layout behavior is encoded in the containers.^[7]

java.beans package

Android includes only a small subset of the java.beans package ( PropertyChangeEvent and related classes).

Related Research Articles

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A Java virtual machine (JVM) is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java bytecode. The JVM is detailed by a specification that formally describes what is required in a JVM implementation. Having a specification ensures interoperability of Java programs across different implementations so that program authors using the Java Development Kit (JDK) need not worry about idiosyncrasies of the underlying hardware platform.

Lua is a lightweight, high-level, multi-paradigm programming language designed primarily for embedded use in applications. Lua is cross-platform, since the interpreter of compiled bytecode is written in ANSI C, and Lua has a relatively simple C API to embed it into applications.

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Bytecode, also termed portable code or p-code, is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references that encode the result of compiler parsing and performing semantic analysis of things like type, scope, and nesting depths of program objects.

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Apache Harmony is a retired open source, free Java implementation, developed by the Apache Software Foundation. It was announced in early May 2005 and on October 25, 2006, the Board of Directors voted to make Apache Harmony a top-level project. The Harmony project achieved 99% completeness for J2SE 5.0, and 97% for Java SE 6. The Android operating system has historically been a major user of Harmony, although since Android Nougat it increasingly relies on OpenJDK libraries.

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Dalvik is a discontinued process virtual machine (VM) in Android operating system that executes applications written for Android. Dalvik was an integral part of the Android software stack in the Android versions 4.4 "KitKat" and earlier, which were commonly used on mobile devices such as mobile phones and tablet computers, and more in some devices such as smart TVs and wearables. Dalvik is open-source software, originally written by Dan Bornstein, who named it after the fishing village of Dalvík in Eyjafjörður, Iceland.

The Java Development Kit (JDK) is an implementation of either one of the Java Platform, Standard Edition, Java Platform, Enterprise Edition, or Java Platform, Micro Edition platforms released by Oracle Corporation in the form of a binary product aimed at Java developers on Solaris, Linux, macOS or Windows. The JDK includes a private JVM and a few other resources to finish the development of a Java application. Since the introduction of the Java platform, it has been by far the most widely used Software Development Kit (SDK).

Java bytecode is the instruction set of the Java virtual machine (JVM).

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Android Runtime (ART) is an application runtime environment used by the Android operating system. Replacing Dalvik, the process virtual machine originally used by Android, ART performs the translation of the application's bytecode into native instructions that are later executed by the device's runtime environment.

References

↑ Rose, John (2008-05-31). "with Android and Dalvik at Google I/O". Archived from the original on 2008-06-04. Retrieved 2008-06-08.
↑ Fred Chung (2011-07-28). "Custom Class Loading in Dalvik". Google. Archived from the original on 2014-02-18. Retrieved 2011-11-27.
↑ "Use Java 8 language features and APIs". Android Developers. Retrieved 2021-01-23.
↑ "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. Archived from the original on 2011-11-22. Retrieved 2011-11-27.
↑ "Applying Styles and Themes". Google. Archived from the original on 2014-03-28. Retrieved 2011-09-03.
↑ "Common Layout Objects". Google . Retrieved 2011-09-03.

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[1] Rose, John (2008-05-31). "with Android and Dalvik at Google I/O". Archived from the original on 2008-06-04. Retrieved 2008-06-08.

[2] Fred Chung (2011-07-28). "Custom Class Loading in Dalvik". Google. Archived from the original on 2014-02-18. Retrieved 2011-11-27.

[3] "Use Java 8 language features and APIs". Android Developers. Retrieved 2021-01-23.

[4] "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.

[5] "Reading and Writing Logs". Google. Archived from the original on 2011-11-22. Retrieved 2011-11-27.

[6] "Applying Styles and Themes". Google. Archived from the original on 2014-03-28. Retrieved 2011-09-03.

[7] "Common Layout Objects". Google . Retrieved 2011-09-03.

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