Java (programming language)

"Java language" redirects here. For the natural language from the Indonesian island of Java, see Javanese language.

Not to be confused with JavaScript.

Java

File:Java logo and wordmark.svg
Paradigm	multi-paradigm: object-oriented, structured, imperative, generic, reflective, concurrent
Designed by	James Gosling and Sun Microsystems
Developer	Oracle Corporation
First appeared	1995 (1995)
Stable release	Java Standard Edition 7 Update 45 (1.7.45) / October 15, 2013 (2013-10-15)
Typing discipline	Static, strong, safe, nominative, manifest
Implementation language	C and C++
OS	Cross-platform (multi-platform)
License	GNU General Public License, Java Community Process
Filename extensions	.java , .class, .jar
Website	For Java Developers
Major implementations
OpenJDK, many others
Dialects
Generic Java, Pizza
Influenced by
Ada 83, C++, C#, Eiffel, Generic Java, Mesa, Modula-3, Oberon, Objective-C, UCSD Pascal, Smalltalk
Influenced
Ada 2005, BeanShell, C#, Clojure, D, ECMAScript, Groovy, J#, JavaScript, PHP, Python, Scala, Seed7, Vala, Kotlin
Java Programming at Wikibooks

Java is a computer programming language that is concurrent, class-based, object-oriented, and specifically designed to have as few implementation dependencies as possible. It is intended to let application developers "write once, run anywhere" (WORA), meaning that code that runs on one platform does not need to be recompiled to run on another. Java applications are typically compiled to bytecode (class file) that can run on any Java virtual machine (JVM) regardless of computer architecture. Java is, as of 2012, one of the most popular programming languages in use, particularly for client-server web applications, with a reported 9 million developers. Java was originally developed by James Gosling at Sun Microsystems (which has since merged into Oracle Corporation) and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++, but it has fewer low-level facilities than either of them.

The original and reference implementation Java compilers, virtual machines, and class libraries were developed by Sun from 1991 and first released in 1995. As of May 2007, in compliance with the specifications of the Java Community Process, Sun relicensed most of its Java technologies under the GNU General Public License. Others have also developed alternative implementations of these Sun technologies, such as the GNU Compiler for Java (bytecode compiler), GNU Classpath (standard libraries), and IcedTea-Web (browser plugin for applets).

History

See also: Java (software platform) § History

Duke, the Java mascot

James Gosling, Mike Sheridan, and Patrick Naughton initiated the Java language project in June 1991. Java was originally designed for interactive television, but it was too advanced for the digital cable television industry at the time. The language was initially called Oak after an oak tree that stood outside Gosling's office; it went by the name Green later, and was later renamed Java, from Java coffee, said to be consumed in large quantities by the language's creators. Gosling aimed to implement a virtual machine and a language that had a familiar C/C++ style of notation.

Sun Microsystems released the first public implementation as Java 1.0 in 1995. It promised "Write Once, Run Anywhere" (WORA), providing no-cost run-times on popular platforms. Fairly secure and featuring configurable security, it allowed network- and file-access restrictions. Major web browsers soon incorporated the ability to run Java applets within web pages, and Java quickly became popular. With the advent of Java 2 (released initially as J2SE 1.2 in December 1998 – 1999), new versions had multiple configurations built for different types of platforms. For example, J2EE targeted enterprise applications and the greatly stripped-down version J2ME for mobile applications (Mobile Java). J2SE designated the Standard Edition. In 2006, for marketing purposes, Sun renamed new J2 versions as Java EE, Java ME, and Java SE, respectively.

In 1997, Sun Microsystems approached the ISO/IEC JTC1 standards body and later the Ecma International to formalize Java, but it soon withdrew from the process. Java remains a de facto standard, controlled through the Java Community Process. At one time, Sun made most of its Java implementations available without charge, despite their proprietary software status. Sun generated revenue from Java through the selling of licenses for specialized products such as the Java Enterprise System. Sun distinguishes between its Software Development Kit (SDK) and Runtime Environment (JRE) (a subset of the SDK); the primary distinction involves the JRE's lack of the compiler, utility programs, and header files.

On November 13, 2006, Sun released much of Java as free and open source software, (FOSS), under the terms of the GNU General Public License (GPL). On May 8, 2007, Sun finished the process, making all of Java's core code available under free software/open-source distribution terms, aside from a small portion of code to which Sun did not hold the copyright.

Sun's vice-president Rich Green said that Sun's ideal role with regards to Java was as an "evangelist." Following Oracle Corporation's acquisition of Sun Microsystems in 2009–2010, Oracle has described itself as the "steward of Java technology with a relentless commitment to fostering a community of participation and transparency". This did not hold Oracle, however, from filing a lawsuit against Google shortly after that for using Java inside the Android SDK (see Google section below). Java software runs on everything from laptops to data centers, game consoles to scientific supercomputers. There are 930 million Java Runtime Environment downloads each year and 3 billion mobile phones run Java. On April 2, 2010, James Gosling resigned from Oracle.

Principles

James Gosling, the creator of Java

There were five primary goals in the creation of the Java language:

1. It should be "simple, object-oriented and familiar"
2. It should be "robust and secure"
3. It should be "architecture-neutral and portable"
4. It should execute with "high performance"
5. It should be "interpreted, threaded, and dynamic"

Versions

Main article: Java version history

Major release versions of Java, along with their release dates:

• JDK 1.0 (January 21, 1996)
• JDK 1.1 (February 19, 1997)
• J2SE 1.2 (December 8, 1998)
• J2SE 1.3 (May 8, 2000)
• J2SE 1.4 (February 6, 2002)
• J2SE 5.0 (September 30, 2004)
• Java SE 6 (December 11, 2006)
• Java SE 7 (July 28, 2011)
• Java SE 8 (October 27, 2013)

Practices

Java platform

Main articles: Java (software platform) and Java virtual machine

One characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any hardware/operating-system platform. This is achieved by compiling the Java language code to an intermediate representation called Java bytecode, instead of directly to platform-specific machine code. Java bytecode instructions are analogous to machine code, but they are intended to be interpreted by a virtual machine (VM) written specifically for the host hardware. End-users commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a Web browser for Java applets.

Standardized libraries provide a generic way to access host-specific features such as graphics, threading, and networking.

A major benefit of using bytecode is porting. However, the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would. Just-in-Time (JIT) compilers were introduced from an early stage that compile bytecodes to machine code during runtime.

Implementations

See also: Free Java implementations

Oracle Corporation is the current owner of the official implementation of the Java SE platform, following their acquisition of Sun Microsystems on January 27, 2010. This implementation is based on the original implementation of Java by Sun. The Oracle implementation is available for Mac OS X, Windows and Solaris. Because Java lacks any formal standardization recognized by Ecma International, ISO/IEC, ANSI, or other third-party standards organization, the Oracle implementation is the de facto standard.

The Oracle implementation is packaged into two different distributions: The Java Runtime Environment (JRE) which contains the parts of the Java SE platform required to run Java programs and is intended for end-users, and the Java Development Kit (JDK), which is intended for software developers and includes development tools such as the Java compiler, Javadoc, Jar, and a debugger.

OpenJDK is another notable Java SE implementation that is licensed under the GPL. The implementation started when Sun began releasing the Java source code under the GPL. As of Java SE 7, OpenJDK is the official Java reference implementation.

The goal of Java is to make all implementations of Java compatible. Historically, Sun's trademark license for usage of the Java brand insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support RMI or JNI and had added platform-specific features of their own. Sun sued in 1997, and in 2001 won a settlement of US$20 million, as well as a court order enforcing the terms of the license from Sun. As a result, Microsoft no longer ships Windows with Java.

Platform-independent Java is essential to Java EE, and an even more rigorous validation is required to certify an implementation. This environment enables portable server-side applications.

Performance

Main article: Java performance

Programs written in Java have a reputation for being slower and requiring more memory than those written in C++. However, Java programs' execution speed improved significantly with the introduction of Just-in-time compilation in 1997/1998 for Java 1.1, the addition of language features supporting better code analysis (such as inner classes, the StringBuilder class, optional assertions, etc.), and optimizations in the Java virtual machine itself, such as HotSpot becoming the default for Sun's JVM in 2000.

Some platforms offer direct hardware support for Java; there are microcontrollers that can run Java in hardware instead of a software Java virtual machine, and ARM based processors can have hardware support for executing Java bytecode through their Jazelle option.

Automatic memory management

Java uses an automatic garbage collector to manage memory in the object lifecycle. The programmer determines when objects are created, and the Java runtime is responsible for recovering the memory once objects are no longer in use. Once no references to an object remain, the unreachable memory becomes eligible to be freed automatically by the garbage collector. Something similar to a memory leak may still occur if a programmer's code holds a reference to an object that is no longer needed, typically when objects that are no longer needed are stored in containers that are still in use. If methods for a nonexistent object are called, a "null pointer exception" is thrown.

One of the ideas behind Java's automatic memory management model is that programmers can be spared the burden of having to perform manual memory management. In some languages, memory for the creation of objects is implicitly allocated on the stack, or explicitly allocated and deallocated from the heap. In the latter case the responsibility of managing memory resides with the programmer. If the program does not deallocate an object, a memory leak occurs. If the program attempts to access or deallocate memory that has already been deallocated, the result is undefined and difficult to predict, and the program is likely to become unstable and/or crash. This can be partially remedied by the use of smart pointers, but these add overhead and complexity. Note that garbage collection does not prevent "logical" memory leaks, i.e. those where the memory is still referenced but never used.

Garbage collection may happen at any time. Ideally, it will occur when a program is idle. It is guaranteed to be triggered if there is insufficient free memory on the heap to allocate a new object; this can cause a program to stall momentarily. Explicit memory management is not possible in Java.

Java does not support C/C++ style pointer arithmetic, where object addresses and unsigned integers (usually long integers) can be used interchangeably. This allows the garbage collector to relocate referenced objects and ensures type safety and security.

As in C++ and some other object-oriented languages, variables of Java's primitive data types are not objects. Values of primitive types are either stored directly in fields (for objects) or on the stack (for methods) rather than on the heap, as is commonly true for objects (but see escape analysis). This was a conscious decision by Java's designers for performance reasons. Because of this, Java was not considered to be a pure object-oriented programming language. However, as of Java 5.0, autoboxing enables programmers to proceed as if primitive types were instances of their wrapper class.

Java contains multiple types of garbage collectors. By default, HotSpot uses the concurrent mark sweep collector, also known as the CMS garbage collector or CMS. However, there are also several other garbage collectors that can be used to manage the heap. For 90% of applications in Java, the CMS garbage collector is sufficient. Oracle aims to replace CMS with the Garbage-first collector (G1).

Syntax

Main article: Java syntax

The syntax of Java is largely derived from C++. Unlike C++, which combines the syntax for structured, generic, and object-oriented programming, Java was built almost exclusively as an object-oriented language. All code is written inside a class, and everything is an object, with the exception of the primitive data types (i.e. integers, floating-point numbers, boolean values, and characters), which are not classes for performance reasons.

Unlike C++, Java does not support operator overloading or multiple inheritance for classes. This simplifies the language and aids in preventing potential errors and anti-pattern design.

Java uses similar commenting methods to C++. There are three different styles of comments: a single line style marked with two slashes (//), a multiple line style opened with /* and closed with */, and the Javadoc commenting style opened with /** and closed with */. The Javadoc style of commenting allows the user to run the Javadoc executable to compile documentation for the program.

Example:

// This is an example of a single line comment using two slashes

/* This is an example of a multiple line comment using the slash and asterisk.
 This type of comment can be used to hold a lot of information or deactivate
 code, but it is very important to remember to close the comment. */

/**
 * This is an example of a Javadoc comment; Javadoc can compile documentation
 * from this text.
 */

/** Finally, an example of a method written in Java, wrapped in a class. */
package fibsandlies;
import java.util.HashMap;

public class FibCalculator extends Fibonacci implements Calculator {
    private static HashMap<Integer, Integer> memoized = new HashMap<Integer, Integer>();
    static {
        memoized.put(1, 1);
        memoized.put(2, 1);
    }

    /** Given a non-negative number FIBINDEX, returns
     *  the Nth Fibonacci number, where N equals FIBINDEX.
     *  @param fibIndex The index of the Fibonacci number
     *  @return The Fibonacci number itself
     */
    @Override
    public static int fibonacci(int fibIndex) {
        if (memoized.containsKey(fibIndex)) {
            return memoized.get(fibIndex);
        } else {
            int answer = fibonacci(fibIndex - 1) + fibonacci(fibIndex - 2);
            memoized.put(fibIndex, answer);
            return answer;
        }
    }
}

Generics

Main article: Generics in Java

In 2004, generics were added to the Java language, as part of J2SE 5.0. Prior to the introduction of generics, each variable declaration had to be of a specific type. For container classes, for example, this is a problem because there is no easy way to create a container that accepts only specific types of objects. Either the container operates on all subtypes of a class or interface, usually Object, or a different container class has to be created for each contained class. Generics allow compile-time type checking without having to create a large number of container classes, each containing almost identical code. In addition to enabling more efficient code, certain runtime exceptions are converted to compile-time errors, a characteristic known as type safety.

Criticism

Main article: Criticism of Java

Criticisms directed at Java include the implementation of generics, speed, the handling of unsigned numbers, the implementation of floating-point arithmetic, and a history of security vulnerabilities in the primary Java VM implementation HotSpot.

Use by external companies

Android makes extensive use of Java-related technology

Google

See also: Oracle v. Google

Google and Android, Inc. have chosen to use Java as a key pillar in the creation of the Android operating system, an open-source smartphone operating system. Although the Android operating system, built on the Linux kernel, was written largely in C, the Android SDK uses the Java language as the basis for Android applications. However, Android uses Java only for its syntax, but not its class library. Instead of instantiating classes from the standard Java Class Library, Android provides its own class library which apps are expected to use. Android apps are compiled through Java bytecode as an intermediate step into Dalvik Executables targeting Android's own Dalvik virtual machine. Some of the classes in the Dalvik class library resembled their Java counterparts, which became a major point of contention between Sun/Oracle and Google/Android.

On May 7, 2012, a San Francisco jury found that if APIs could be copyrighted, then Google had infringed Oracle's copyrights by the use of Java in Android devices. Oracle's stance in this case had raised questions about the legal status of the use of Java on Android. However, U.S. District Judge William Haskell Alsup ruled on May 31, 2012, that APIs cannot be copyrighted.

Class libraries

• The Java Class Library are the compiled bytecodes of source code developed by the JRE implementor to support application development in Java. Examples of these libraries are:
  • The core libraries, which include:
    • Many low level, GUI, Integration, Deployment and Tools Java Platform Standard Edition 7 Documentation
    • Collection libraries that implement data structures such as lists, dictionaries, trees, sets, queues and double-ended queue, or stacks
    • XML Processing (Parsing, Transforming, Validating) libraries
    • Security
    • Internationalization and localization libraries
  • The integration libraries, which allow the application writer to communicate with external systems. These libraries include:
    • The Java Database Connectivity (JDBC) API for database access
    • Java Naming and Directory Interface (JNDI) for lookup and discovery
    • RMI and CORBA for distributed application development
    • JMX for managing and monitoring applications
  • User interface libraries, which include:
    • The (heavyweight, or native) Abstract Window Toolkit (AWT), which provides GUI components, the means for laying out those components and the means for handling events from those components
    • The (lightweight) Swing libraries, which are built on AWT but provide (non-native) implementations of the AWT widgetry
    • APIs for audio capture, processing, and playback
• A platform dependent implementation of the Java virtual machine that is the means by which the bytecodes of the Java libraries and third party applications are executed
• Plugins, which enable applets to be run in Web browsers
• Java Web Start, which allows Java applications to be efficiently distributed to end-users across the Internet
• Licensing and documentation.

Documentation

Main article: Javadoc

Javadoc is a comprehensive documentation system, created by Sun Microsystems, used by many Java developers. It provides developers with an organized system for documenting their code. Javadoc comments have an extra asterisk at the beginning, i.e. the tags are /** and */, whereas the normal multi-line comments in Java are set off with the tags /* and */.

Editions

See also: Free Java implementations § Class library

Sun has defined and supports four editions of Java targeting different application environments and segmented many of its APIs so that they belong to one of the platforms. The platforms are:

• Java Card for smartcards.
• Java Platform, Micro Edition (Java ME) — targeting environments with limited resources.
• Java Platform, Standard Edition (Java SE) — targeting workstation environments.
• Java Platform, Enterprise Edition (Java EE) — targeting large distributed enterprise or Internet environments.

The classes in the Java APIs are organized into separate groups called packages. Each package contains a set of related interfaces, classes and exceptions. Refer to the separate platforms for a description of the packages available.

The set of APIs is controlled by Sun Microsystems in cooperation with others through the Java Community Process program. Companies or individuals participating in this process can influence the design and development of the APIs. This process has been a subject of controversy.

Sun also provided an edition called PersonalJava that has been superseded by later, standards-based Java ME configuration-profile pairings.

See also

• Dalvik
• JavaOne
• Javapedia
• List of Java virtual machines
• List of Java APIs
• List of JVM languages
• Graal, a project aiming to implement a high performance Java dynamic compiler and interpreter

Comparison of Java with other languages

• Comparison of programming languages
• Comparison of Java and C++
• Comparison of Java and C#

References

• Gosling, James, A brief history of the Green project. Java.net, no date [ca. Q1/1998]. Retrieved April 29, 2007.
• Gosling, James, A brief history of the Green project. anonymous-insider.net, no date [ca. Q1/1998]. Retrieved September 4, 2013.
• Gosling, James; Joy, Bill; Steele, Guy L., Jr.; Bracha, Gilad (2005). The Java Language Specification (3rd ed.). Addison-Wesley. ISBN 0-321-24678-0.{{cite book}}: CS1 maint: multiple names: authors list (link)
• Lindholm, Tim; Yellin, Frank (1999). The Java Virtual Machine Specification (2nd ed.). Addison-Wesley. ISBN 0-201-43294-3.

External links

• java.com - Java for end-users
• Oracle's Developer Resources for Java Technology
• Java SE 24 API Javadocs
• Oracle's Beginner's tutorial for Java SE Programming
• A Brief History of the Green Project (dead link)
• Michael O'Connell: Java: The Inside Story, SunWord (1995)
• One of the developers' blog
• Patrick Naughton: Java Was Strongly Influenced by Objective-C
• David Bank: The Java Saga, Wired Issue 3.12 (1995)
• Patrick Naughton: The Long Strange Trip to Java (1996)
• Open University (UK): M254 Java Everywhere (free open content documents)
• is-research GmbH: List of programming languages for a Java Virtual Machine
• How Java's Floating-Point Hurts Everyone Everywhere, by W. Kahan and Joseph D. Darcy, University of California, Berkeley
• Shahrooz Feizabadi: A history of Java in: Marc Abrams, ed., World Wide Web – Beyond the Basics, Prentice Hall (1998)