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Java is a high-level, class-based,
object-oriented Object-oriented programming (OOP) is a programming paradigm based on the concept of " objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming l ...
that is designed to have as few implementation dependencies as possible. It is a general-purpose programming language intended to let
programmer A computer programmer, sometimes referred to as a software developer, a software engineer, a programmer or a coder, is a person who creates computer programs — often for larger computer software. A programmer is someone who writes/creates ...
s ''write once, run anywhere'' ( WORA), meaning that compiled Java code can run on all platforms that support Java without the need to recompile. Java applications are typically compiled to
bytecode Bytecode (also called 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 (norma ...
that can run on any
Java virtual machine 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 describ ...
(JVM) regardless of the underlying
computer architecture In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, the ...
. The syntax of Java is similar to C and C++, but has fewer
low-level High-level and low-level, as technical terms, are used to classify, describe and point to specific goals of a systematic operation; and are applied in a wide range of contexts, such as, for instance, in domains as widely varied as computer scienc ...
facilities than either of them. The Java runtime provides dynamic capabilities (such as reflection and runtime code modification) that are typically not available in traditional compiled languages. , Java was one of the most popular programming languages in use according to
GitHub GitHub, Inc. () is an Internet hosting service for software development and version control using Git. It provides the distributed version control of Git plus access control, bug tracking, software feature requests, task management, co ...
, particularly for client–server
web application A web application (or web app) is application software that is accessed using a web browser. Web applications are delivered on the World Wide Web to users with an active network connection. History In earlier computing models like client-serve ...
s, with a reported 9 million developers. Java was originally developed by James Gosling at
Sun Microsystems Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computers, computer components, software, and information technology services and created the Java programming language, the Solaris operating system, ZFS, ...
. It was released in May 1995 as a core component of Sun Microsystems'
Java platform Java is a set of computer software and specifications developed by James Gosling at Sun Microsystems, which was later acquired by the Oracle Corporation, that provides a system for developing application software and deploying it in a cro ...
. The original and
reference implementation In the software development process, a reference implementation (or, less frequently, sample implementation or model implementation) is a program that implements all requirements from a corresponding specification. The reference implementation ...
Java
compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...
s, virtual machines, and class libraries were originally released by Sun under proprietary licenses. As of May 2007, in compliance with the specifications of the Java Community Process, Sun had relicensed most of its Java technologies under the GPL-2.0-only license.
Oracle An oracle is a person or agency considered to provide wise and insightful counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. As such, it is a form of divination. Description The wor ...
offers its own
HotSpot Hotspot, Hot Spot or Hot spot may refer to: Places * Hot Spot, Kentucky, a community in the United States Arts, entertainment, and media Fictional entities * Hot Spot (comics), a name for the DC Comics character Isaiah Crockett * Hot Spot (Tra ...
Java Virtual Machine, however the official
reference implementation In the software development process, a reference implementation (or, less frequently, sample implementation or model implementation) is a program that implements all requirements from a corresponding specification. The reference implementation ...
is the OpenJDK JVM which is free open-source software and used by most developers and is the default JVM for almost all Linux distributions. , Java 19 is the latest version, while Java 17, 11 and 8 are the current
long-term support Long-term support (LTS) is a product lifecycle management policy in which a stable release of computer software is maintained for a longer period of time than the standard edition. The term is typically reserved for open-source software, where it ...
(LTS) versions.


History

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. Later the project went by the name ''Green'' and was finally renamed ''Java'', from
Java coffee Indonesia was the fourth-largest producer of coffee in the world in 2014.http://www.ico.org/prices/po-production.pdf Coffee cultivation in Indonesia began in the late 1600s and early 1700s, in the early Dutch colonial period, and has played an ...
, a type of coffee from
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...
. Gosling designed Java with a C/ C++-style syntax that system and application programmers would find familiar. Sun Microsystems released the first public implementation as Java 1.0 in 1996. It promised write once, run anywhere (WORA) functionality, providing no-cost run-times on popular platforms. Fairly secure and featuring configurable security, it allowed network- and file-access restrictions. Major
web browser A web browser is application software for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's screen. Browsers are used on ...
s soon incorporated the ability to run
Java applet Java applets were small applications written in the Java programming language, or another programming language that compiles to Java bytecode, and delivered to users in the form of Java bytecode. The user launched the Java applet from a ...
s within web pages, and Java quickly became popular. The Java 1.0 compiler was re-written in Java by
Arthur van Hoff Arthur van Hoff (born 16 February 1963) is a Dutch computer scientist and businessman. Biography After studying computer science at the University of Strathclyde and Hogere Informatica Opleiding, Van Hoff joined Sun Microsystems as an engineer ...
to comply strictly with the Java 1.0 language specification. 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. J2EE included technologies and APIs for enterprise applications typically run in server environments, while J2ME featured APIs optimized for mobile applications. The desktop version was renamed J2SE. In 2006, for marketing purposes, Sun renamed new J2 versions as '' Java EE'', ''
Java ME Java Platform, Micro Edition or Java ME is a computing platform for development and deployment of portable code for embedded and mobile devices (micro-controllers, sensors, gateways, mobile phones, personal digital assistants, TV set-top ...
'', and '' Java SE'', respectively. In 1997, Sun Microsystems approached the
ISO/IEC JTC 1 ISO/IEC JTC 1, entitled "Information technology", is a joint technical committee (JTC) of the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). Its purpose is to develop, maintain and pr ...
standards body and later the
Ecma International Ecma International () is a nonprofit standards organization for information and communication systems. It acquired its current name in 1994, when the European Computer Manufacturers Association (ECMA) changed its name to reflect the organizatio ...
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 Proprietary software is computer software, software that is deemed within the free and open-source software to be non-free because its creator, publisher, or other rightsholder or rightsholder partner exercises a legal monopoly afforded by modern ...
status. Sun generated revenue from Java through the selling of licenses for specialized products such as the Java Enterprise System. On November 13, 2006, Sun released much of its Java virtual machine (JVM) as
free and open-source software Free and open-source software (FOSS) is a term used to refer to groups of software consisting of both free software and open-source software where anyone is freely licensed to use, copy, study, and change the software in any way, and the source ...
(FOSS), under the terms of the GPL-2.0-only license. On May 8, 2007, Sun finished the process, making all of its JVM's core code available under
free software Free software or libre software is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any adapted versions. Free software is a matter of liberty, ...
/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 regard to Java was as an ''evangelist''. Following
Oracle Corporation Oracle Corporation is an American multinational computer technology corporation headquartered in Austin, Texas. In 2020, Oracle was the third-largest software company in the world by revenue and market capitalization. The company sells da ...
's acquisition of Sun Microsystems in 2009–10, 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 prevent Oracle from filing a lawsuit against Google shortly after that for using Java inside the Android SDK (see the ''
Android Android may refer to: Science and technology * Android (robot), a humanoid robot or synthetic organism designed to imitate a human * Android (operating system), Google's mobile operating system ** Bugdroid, a Google mascot sometimes referred to ...
'' section). On April 2, 2010, James Gosling resigned from
Oracle An oracle is a person or agency considered to provide wise and insightful counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. As such, it is a form of divination. Description The wor ...
. In January 2016, Oracle announced that Java run-time environments based on JDK 9 will discontinue the browser plugin. Java software runs on everything from laptops to data centers, game consoles to scientific supercomputers.
Oracle An oracle is a person or agency considered to provide wise and insightful counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. As such, it is a form of divination. Description The wor ...
(and others) highly recommend uninstalling outdated and unsupported versions of Java, due to unresolved security issues in older versions.


Principles

There were five primary goals in the creation of the Java language:
# It must be simple,
object-oriented Object-oriented programming (OOP) is a programming paradigm based on the concept of " objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...
, and familiar. # It must be robust and secure. # It must be architecture-neutral and portable. # It must execute with high performance. # It must be interpreted,
thread Thread may refer to: Objects * Thread (yarn), a kind of thin yarn used for sewing ** Thread (unit of measurement), a cotton yarn measure * Screw thread, a helical ridge on a cylindrical fastener Arts and entertainment * ''Thread'' (film), 2016 ...
ed, and dynamic.


Versions

, Java 8, 11 and 17 are supported as
Long-Term Support Long-term support (LTS) is a product lifecycle management policy in which a stable release of computer software is maintained for a longer period of time than the standard edition. The term is typically reserved for open-source software, where it ...
(LTS) versions. Oracle released the last zero-cost public update for the
legacy In law, a legacy is something held and transferred to someone as their inheritance, as by will and testament. Personal effects, family property, marriage property or collective property gained by will of real property. Legacy or legacies may refer ...
version
Java 8 The Java language has undergone several changes since JDK 1.0 as well as numerous additions of classes and packages to the standard library. Since J2SE 1.4, the evolution of the Java language has been governed by the Java Community ...
LTS in January 2019 for commercial use, although it will otherwise still support Java 8 with public updates for personal use indefinitely. Other vendors have begun to offer zero-cost builds of OpenJDK 18 and 8, 11 and 17 that are still receiving security and other upgrades. Major release versions of Java, along with their release dates:


Editions

Sun has defined and supports four editions of Java targeting different application environments and segmented many of its
APIs Apis or APIS may refer to: * Apis (deity), an ancient Egyptian god * Apis (Greek mythology), several different figures in Greek mythology * Apis (city), an ancient seaport town on the northern coast of Africa **Kom el-Hisn, a different Egyptian ci ...
so that they belong to one of the platforms. The platforms are: * Java Card for smart-cards. *
Java Platform, Micro Edition Java Platform, Micro Edition or Java ME is a computing platform for development and deployment of portable code for embedded and mobile devices (micro-controllers, sensors, gateways, mobile phones, personal digital assistants, TV set-top ...
(Java ME) – targeting environments with limited resources. *
Java Platform, Standard Edition Java Platform, Standard Edition (Java SE) is a computing platform for development and deployment of portable code for desktop and server environments. Java SE was formerly known as Java 2 Platform, Standard Edition (J2SE). The platform uses J ...
(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, subpackages and exceptions. Sun also provided an edition called Personal Java that has been superseded by later, standards-based Java ME configuration-profile pairings.


Execution system


Java JVM and bytecode

One design goal of Java is
portability Portability may refer to: *Portability (social security), the portability of social security benefits * Porting, the ability of a computer program to be ported from one system to another in computer science ** Software portability, the portability ...
, which means that programs written for the Java platform must run similarly on any combination of hardware and operating system with adequate run time support. This is achieved by compiling the Java language code to an intermediate representation called
Java bytecode In computing, Java bytecode is the bytecode-structured instruction set of the Java virtual machine (JVM), a virtual machine that enables a computer to run programs written in the Java programming language and several other programming languages, ...
, instead of directly to architecture-specific
machine code In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a ver ...
. Java bytecode instructions are analogous to machine code, but they are intended to be executed by a
virtual machine In computing, a virtual machine (VM) is the virtualization/ emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized har ...
(VM) written specifically for the host hardware.
End-user In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ultimately use a product. The end user stands in contrast to users who support or maintain the product, such as sysops, system administrato ...
s commonly use a
Java Runtime Environment Java is a set of computer software and specifications developed by James Gosling at Sun Microsystems, which was later acquired by the Oracle Corporation, that provides a system for developing application software and deploying it in a c ...
(JRE) installed on their device for standalone Java applications or a web browser for
Java applet Java applets were small applications written in the Java programming language, or another programming language that compiles to Java bytecode, and delivered to users in the form of Java bytecode. The user launched the Java applet from a ...
s. Standard libraries provide a generic way to access host-specific features such as graphics, threading, and
networking Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...
. The use of universal bytecode makes porting simple. However, the overhead of
interpreting Interpreting is a translational activity in which one produces a first and final target-language output on the basis of a one-time exposure to an expression in a source language. The most common two modes of interpreting are simultaneous interp ...
bytecode into machine instructions made interpreted programs almost always run more slowly than native
executable In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instructions", as opposed to a data fil ...
s. Just-in-time (JIT) compilers that compile byte-codes to machine code during runtime were introduced from an early stage. Java's Hotspot compiler is actually two compilers in one; and with GraalVM (included in e.g. Java 11, but removed as of Java 16) allowing tiered compilation. Java itself is platform-independent and is adapted to the particular platform it is to run on by a
Java virtual machine 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 describ ...
(JVM) for it, which translates the
Java bytecode In computing, Java bytecode is the bytecode-structured instruction set of the Java virtual machine (JVM), a virtual machine that enables a computer to run programs written in the Java programming language and several other programming languages, ...
into the platform's machine language.


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 computing, just-in-time (JIT) compilation (also dynamic translation or run-time compilations) is a way of executing computer code that involves compiler, compilation during execution of a program (at run time (program lifecycle phase), run tim ...
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, such as
HotSpot Hotspot, Hot Spot or Hot spot may refer to: Places * Hot Spot, Kentucky, a community in the United States Arts, entertainment, and media Fictional entities * Hot Spot (comics), a name for the DC Comics character Isaiah Crockett * Hot Spot (Tra ...
becoming Sun's default JVM in 2000. With Java 1.5, the performance was improved with the addition of the java.util.concurrent package, including lock-free implementations of the ConcurrentMaps and other multi-core collections, and it was improved further with Java 1.6.


Non-JVM

Some platforms offer direct hardware support for Java; there are micro controllers that can run Java bytecode in hardware instead of a software Java virtual machine, and some ARM-based processors could have hardware support for executing Java bytecode through their Jazelle option, though support has mostly been dropped in current implementations of ARM.


Automatic memory management

Java uses an automatic garbage collector to manage memory in the
object lifecycle In object-oriented programming (OOP), the object lifetime (or life cycle) of an object (computing), object is the time between an object's Instance (computer science)#Object-oriented_programming, creation and its destruction. Rules for object lif ...
. 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 In computer science, a memory leak is a type of resource leak that occurs when a computer program incorrectly manages memory allocations in a way that memory which is no longer needed is not released. A memory leak may also happen when an object ...
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 non-existent object are called, a
null pointer In computing, a null pointer or null reference is a value saved for indicating that the pointer or reference does not refer to a valid object. Programs routinely use null pointers to represent conditions such as the end of a list of unknown lengt ...
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 Stack may refer to: Places * Stack Island, an island game reserve in Bass Strait, south-eastern Australia, in Tasmania’s Hunter Island Group * Blue Stack Mountains, in Co. Donegal, Ireland People * Stack (surname) (including a list of people ...
or explicitly allocated and deallocated from the
heap Heap or HEAP may refer to: Computing and mathematics * Heap (data structure), a data structure commonly used to implement a priority queue * Heap (mathematics), a generalization of a group * Heap (programming) (or free store), an area of memory f ...
. In the latter case, the responsibility of managing memory resides with the programmer. If the program does not deallocate an object, a
memory leak In computer science, a memory leak is a type of resource leak that occurs when a computer program incorrectly manages memory allocations in a way that memory which is no longer needed is not released. A memory leak may also happen when an object ...
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 or crash. This can be partially remedied by the use of
smart pointer In computer science, a smart pointer is an abstract data type that simulates a pointer while providing added features, such as automatic memory management or bounds checking. Such features are intended to reduce bugs caused by the misuse of poin ...
s, but these add overhead and complexity. Note that garbage collection does not prevent
logical memory In computing, a logical address is the address at which an item ( memory cell, storage element, network host) appears to reside from the perspective of an executing application program. A logical address may be different from the physical ad ...
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 can be arithmetically manipulated (e.g. by adding or subtracting an offset). 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 type In computer science, primitive data types are a set of basic data types from which all other data types are constructed. Specifically it often refers to the limited set of data representations in use by a particular processor, which all compiled ...
s are either stored directly in fields (for objects) or on the
stack Stack may refer to: Places * Stack Island, an island game reserve in Bass Strait, south-eastern Australia, in Tasmania’s Hunter Island Group * Blue Stack Mountains, in Co. Donegal, Ireland People * Stack (surname) (including a list of people ...
(for methods) rather than on the heap, as is commonly true for non-primitive data types (but see escape analysis). This was a conscious decision by Java's designers for performance reasons. Java contains multiple types of garbage collectors. Since Java 9, HotSpot uses the Garbage First Garbage Collector (G1GC) as the default. However, there are also several other garbage collectors that can be used to manage the heap. For most applications in Java, G1GC is sufficient. Previously, th
Parallel Garbage Collector
was used in Java 8. Having solved the memory management problem does not relieve the programmer of the burden of handling properly other kinds of resources, like network or database connections, file handles, etc., especially in the presence of exceptions.


Syntax

The syntax of Java is largely influenced by C++ and 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 classes, and every data item is an object, with the exception of the primitive data types, (i.e. integers, floating-point numbers, boolean values, and characters), which are not objects for performance reasons. Java reuses some popular aspects of C++ (such as the method). Unlike C++, Java does not support operator overloading or
multiple inheritance Multiple inheritance is a feature of some object-oriented computer programming languages in which an object or class can inherit features from more than one parent object or parent class. It is distinct from single inheritance, where an object o ...
for classes, though multiple inheritance is supported for
interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...
s. Java uses
comments Comment may refer to: * Comment (linguistics) or rheme, that which is said about the topic (theme) of a sentence * Bernard Comment (born 1960), Swiss writer and publisher Computing * Comment (computer programming), explanatory text or informa ...
similar to those of 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 Javadoc (originally cased JavaDoc) is a documentation generator created by Sun Microsystems for the Java language (now owned by Oracle Corporation) for generating API documentation in HTML format from Java source code. The HTML format is used ...
commenting style opened with /** and closed with */. The Javadoc style of commenting allows the user to run the Javadoc executable to create documentation for the program and can be read by some
integrated development environment An integrated development environment (IDE) is a software application that provides comprehensive facilities to computer programmers for software development. An IDE normally consists of at least a source code editor, build automation tools a ...
s (IDEs) such as
Eclipse An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ce ...
to allow developers to access documentation within the IDE.


Hello world example

The traditional
Hello world program ''Hello'' is a salutation or greeting in the English language. It is first attested in writing from 1826. Early uses ''Hello'', with that spelling, was used in publications in the U.S. as early as the 18 October 1826 edition of the '' Norwich ...
can be written in Java as: public class HelloWorldApp All source files must be named after the public class they contain, appending the suffix .java, for example, HelloWorldApp.java. It must first be compiled into bytecode, using a Java compiler, producing a file with the .class suffix (HelloWorldApp.class, in this case). Only then can it be executed or launched. The Java source file may only contain one public class, but it can contain multiple classes with a non-public access modifier and any number of public inner classes. When the source file contains multiple classes, it is necessary to make one class (introduced by the class keyword) public (preceded by the public keyword) and name the source file with that public class name. A class that is not declared public may be stored in any .java file. The compiler will generate a class file for each class defined in the source file. The name of the class file is the name of the class, with ''.class'' appended. For class file generation, anonymous classes are treated as if their name were the concatenation of the name of their enclosing class, a ''$'', and an integer. The keyword public denotes that a method can be called from code in other classes, or that a class may be used by classes outside the class hierarchy. The class hierarchy is related to the name of the directory in which the .java file is located. This is called an access level modifier. Other access level modifiers include the keywords private (a method that can only be accessed in the same class) and protected (which allows code from the same package to access). If a piece of code attempts to access private methods or protected methods, the JVM will throw a SecurityException The keyword static in front of a method indicates a static method, which is associated only with the class and not with any specific instance of that class. Only static methods can be invoked without a reference to an object. Static methods cannot access any class members that are not also static. Methods that are not designated static are instance methods and require a specific instance of a class to operate. The keyword void indicates that the main method does not return any value to the caller. If a Java program is to exit with an error code, it must call System.exit() explicitly. The method name main is not a keyword in the Java language. It is simply the name of the method the Java launcher calls to pass control to the program. Java classes that run in managed environments such as applets and Enterprise JavaBeans do not use or need a main() method. A Java program may contain multiple classes that have main methods, which means that the VM needs to be explicitly told which class to launch from. The main method must accept an
array An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...
of objects. By convention, it is referenced as args although any other legal identifier name can be used. Since Java 5, the main method can also use variable arguments, in the form of public static void main(String... args), allowing the main method to be invoked with an arbitrary number of String arguments. The effect of this alternate declaration is semantically identical (to the args parameter which is still an array of String objects), but it allows an alternative syntax for creating and passing the array. The Java launcher launches Java by loading a given class (specified on the command line or as an attribute in a JAR) and starting its public static void main(String[]) method. Stand-alone programs must declare this method explicitly. The String[] args parameter is an array of String objects containing any arguments passed to the class. The parameters to main are often passed by means of a
command line A command-line interpreter or command-line processor uses a command-line interface (CLI) to receive commands from a user in the form of lines of text. This provides a means of setting parameters for the environment, invoking executables and pro ...
. Printing is part of a Java standard library: The class defines a public static field called . The out object is an instance of the class and provides many methods for printing data to
standard out In computer programming, standard streams are interconnected input and output communication channels between a computer program and its environment when it begins execution. The three input/output (I/O) connections are called standard input (stdin ...
, including which also appends a new line to the passed
string String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects Arts, entertainment, and media Films * ''Strings'' (1991 film), a Canadian anim ...
. The string "Hello World!" is automatically converted to a String object by the compiler.


Example with methods

// 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. */ package fibsandlies; import java.util.Map; import java.util.HashMap; /** * This is an example of a Javadoc comment; Javadoc can compile documentation * from this text. Javadoc comments must immediately precede the class, method, * or field being documented. * @author Wikipedia Volunteers */ public class FibCalculator extends Fibonacci implements Calculator


Special classes


Applet

Java applets were programs that were embedded in other applications, typically in a Web page displayed in a web browser. The Java applet API is now deprecated since Java 9 in 2017.


Servlet

Java servlet A Jakarta Servlet (formerly Java Servlet) is a Java software component that extends the capabilities of a server. Although servlets can respond to many types of requests, they most commonly implement web containers for hosting web applicatio ...
technology provides Web developers with a simple, consistent mechanism for extending the functionality of a Web server and for accessing existing business systems. Servlets are server-side Java EE components that generate responses to requests from clients. Most of the time, this means generating
HTML The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaScri ...
pages in response to
HTTP The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, ...
requests, although there are a number of other standard servlet classes available, for example for
WebSocket WebSocket is a computer communications protocol, providing full-duplex communication channels over a single Transmission Control Protocol, TCP connection. The WebSocket protocol was standardized by the Internet Engineering Task Force, IETF as in ...
communication. The Java servlet API has to some extent been superseded (but still used under the hood) by two standard Java technologies for web services: * the Java API for RESTful Web Services (JAX-RS 2.0) useful for AJAX, JSON and REST services, and * the Java API for XML Web Services (JAX-WS) useful for
SOAP Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are used ...
Web Services. Typical implementations of these APIs on Application Servers or Servlet Containers use a standard servlet for handling all interactions with the HTTP requests and responses that delegate to the web service methods for the actual business logic.


JavaServer Pages

JavaServer Pages ( JSP) are server-side Java EE components that generate responses, typically
HTML The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaScri ...
pages, to
HTTP The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, ...
requests from clients. JSPs embed Java code in an HTML page by using the special
delimiter A delimiter is a sequence of one or more characters for specifying the boundary between separate, independent regions in plain text, mathematical expressions or other data streams. An example of a delimiter is the comma character, which acts as ...
s <% and %>. A JSP is compiled to a Java ''servlet'', a Java application in its own right, the first time it is accessed. After that, the generated servlet creates the response.


Swing application

Swing Swing or swinging may refer to: Apparatus * Swing (seat), a hanging seat that swings back and forth * Pendulum, an object that swings * Russian swing, a swing-like circus apparatus * Sex swing, a type of harness for sexual intercourse * Swing rid ...
is a graphical user interface
library A library is a collection of materials, books or media that are accessible for use and not just for display purposes. A library provides physical (hard copies) or digital access (soft copies) materials, and may be a physical location or a vi ...
for the Java SE platform. It is possible to specify a different look and feel through the pluggable look and feel system of Swing. Clones of
Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...
,
GTK+ GTK (formerly GIMP ToolKit and GTK+) is a free and open-source cross-platform widget toolkit for creating graphical user interfaces (GUIs). It is licensed under the terms of the GNU Lesser General Public License, allowing both free and prop ...
, and
Motif Motif may refer to: General concepts * Motif (chess composition), an element of a move in the consideration of its purpose * Motif (folkloristics), a recurring element that creates recognizable patterns in folklore and folk-art traditions * Moti ...
are supplied by Sun.
Apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus '' Malus''. The tree originated in Central Asia, where its wild ances ...
also provides an Aqua look and feel for
macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac (computer), Mac computers. Within the market of ...
. Where prior implementations of these looks and feels may have been considered lacking, Swing in Java SE 6 addresses this problem by using more native
GUI widget A graphical widget (also graphical control element or control) in a graphical user interface is an element of interaction, such as a button or a scroll bar. Controls are software components that a computer user interacts with through di ...
drawing routines of the underlying platforms.


JavaFX application

JavaFX is a software platform for creating and delivering desktop applications, as well as
rich web applications A rich web application (originally called a rich Internet application, or RIA or installable Internet application) is a web application that has many of the characteristics of desktop application software. The concept is closely related to a sin ...
that can run across a wide variety of devices. JavaFX is intended to replace
Swing Swing or swinging may refer to: Apparatus * Swing (seat), a hanging seat that swings back and forth * Pendulum, an object that swings * Russian swing, a swing-like circus apparatus * Sex swing, a type of harness for sexual intercourse * Swing rid ...
as the standard GUI library for Java SE, but since JDK 11 JavaFX has not been in the core JDK and instead in a separate module. JavaFX has support for
desktop computer A desktop computer (often abbreviated desktop) is a personal computer designed for regular use at a single location on or near a desk due to its size and power requirements. The most common configuration has a case that houses the power supply ...
s and
web browser A web browser is application software for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's screen. Browsers are used on ...
s on Microsoft Windows,
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...
, and
macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac (computer), Mac computers. Within the market of ...
. JavaFX does not have support for native OS look and feels.


Generics

In 2004,
generics Generic or generics may refer to: In business * Generic term, a common name used for a range or class of similar things not protected by trademark * Generic brand, a brand for a product that does not have an associated brand or trademark, other ...
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 many container classes, each containing almost identical code. In addition to enabling more efficient code, certain runtime exceptions are prevented from occurring, by issuing compile-time errors. If Java prevented all runtime type errors (ClassCastExceptions) from occurring, it would be
type safe In computer science, type safety and type soundness are the extent to which a programming language discourages or prevents type errors. Type safety is sometimes alternatively considered to be a property of facilities of a computer language; that ...
. In 2016, the type system of Java was proven unsound in that it is possible to use generics to construct classes and methods that allow assignment of an instance one class to a variable of another unrelated class. Such code is accepted by the compiler, but fails at run time with a class cast exception.


Criticism

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 Hotspot, Hot Spot or Hot spot may refer to: Places * Hot Spot, Kentucky, a community in the United States Arts, entertainment, and media Fictional entities * Hot Spot (comics), a name for the DC Comics character Isaiah Crockett * Hot Spot (Tra ...
.


Class libraries

The
Java Class Library The Java Class Library (JCL) is a set of Library (computer science)#Dynamic linking, dynamically loadable libraries that List of JVM languages, Java Virtual Machine (JVM) languages can call at Run time (program lifecycle phase), run time. Because ...
is the standard library, developed to support application development in Java. It is controlled by
Oracle An oracle is a person or agency considered to provide wise and insightful counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. As such, it is a form of divination. Description The wor ...
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 during the 2010s. The class library contains features such as: * The core libraries, which include: ** I
NIO
*

(NOTE: ne

since Java 11) ** Reflection **
Concurrency Concurrent means happening at the same time. Concurrency, concurrent, or concurrence may refer to: Law * Concurrence, in jurisprudence, the need to prove both ''actus reus'' and ''mens rea'' * Concurring opinion (also called a "concurrence"), a ...
**
Generics Generic or generics may refer to: In business * Generic term, a common name used for a range or class of similar things not protected by trademark * Generic brand, a brand for a product that does not have an associated brand or trademark, other ...
** Scripting/Compiler **
Functional programming In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions that ...
(Lambda, Streaming) ** Collection libraries that implement
data structure In computer science, a data structure is a data organization, management, and storage format that is usually chosen for Efficiency, efficient Data access, access to data. More precisely, a data structure is a collection of data values, the rel ...
s such as
lists A ''list'' is any set of items in a row. List or lists may also refer to: People * List (surname) Organizations * List College, an undergraduate division of the Jewish Theological Seminary of America * SC Germania List, German rugby union ...
, dictionaries,
trees In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...
, sets, queues and double-ended queue, or stacks **
XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. ...
Processing (Parsing, Transforming, Validating) libraries **
Security" \n\n\nsecurity.txt is a proposed standard for websites' security information that is meant to allow security researchers to easily report security vulnerabilities. The standard prescribes a text file called \"security.txt\" in the well known locat ...
**
Internationalization and localization In computing, internationalization and localization ( American) or internationalisation and localisation (British English), often abbreviated i18n and L10n, are means of adapting computer software to different languages, regional peculiarities an ...
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 RMI may refer to: Science and technology * Radio-magnetic indicator, an instrument used in aircraft navigation * Repetitive motion injury, an injury to the musculoskeletal and nervous systems * Richtmyer–Meshkov instability, an instability occu ...
and CORBA for distributed application development ** JMX for managing and monitoring applications *
User interface In the industrial design field of human–computer interaction, a user interface (UI) is the space where interactions between humans and machines occur. The goal of this interaction is to allow effective operation and control of the machine f ...
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 Swing or swinging may refer to: Apparatus * Swing (seat), a hanging seat that swings back and forth * Pendulum, an object that swings * Russian swing, a swing-like circus apparatus * Sex swing, a type of harness for sexual intercourse * Swing rid ...
libraries, which are built on AWT but provide (non-native) implementations of the AWT widgetry ** APIs for audio capture, processing, and playback ** JavaFX * 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 In computing, Java Web Start (also known as JavaWS, javaws or JAWS) is a deprecated framework developed by Sun Microsystems (now Oracle) that allows users to start application software for the Java Platform directly from the Internet using a web ...
, which allows Java applications to be efficiently distributed to
end user In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ultimately use a product. The end user stands in contrast to users who support or maintain the product, such as sysops, system administrat ...
s across the Internet * Licensing and documentation


Documentation

Javadoc is a comprehensive documentation system, created by
Sun Microsystems Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computers, computer components, software, and information technology services and created the Java programming language, the Solaris operating system, ZFS, ...
. It provides developers with an organized system for documenting their code. Javadoc comments have an extra asterisk at the beginning, i.e. the delimiters are /** and */, whereas the normal multi-line comments in Java are delimited by /* and */, and single-line comments start with //.


Implementations

Oracle Corporation Oracle Corporation is an American multinational computer technology corporation headquartered in Austin, Texas. In 2020, Oracle was the third-largest software company in the world by revenue and market capitalization. The company sells da ...
is the current owner of the official implementation of the Java SE platform, following their acquisition of
Sun Microsystems Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computers, computer components, software, and information technology services and created the Java programming language, the Solaris operating system, ZFS, ...
on January 27, 2010. This implementation is based on the original implementation of Java by Sun. The Oracle implementation is available for Microsoft Windows (still works for XP, while only later versions are currently officially supported),
macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac (computer), Mac computers. Within the market of ...
,
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...
, and Solaris. Because Java lacks any formal standardization recognized by
Ecma International Ecma International () is a nonprofit standards organization for information and communication systems. It acquired its current name in 1994, when the European Computer Manufacturers Association (ECMA) changed its name to reflect the organizatio ...
, ISO/IEC, ANSI, or other third-party standards organizations, the Oracle implementation is the
de facto standard A ''de facto'' standard is a custom or convention that has achieved a dominant position by public acceptance or market forces (for example, by early entrance to the market). is a Latin phrase (literally "in fact"), here meaning "in practice but ...
. 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 The Java Development Kit (JDK) is a distribution of Java Technology by Oracle Corporation. It implements the Java Language Specification (JLS) and the Java Virtual Machine Specification (JVMS) and provides the Standard Edition (SE) of the Java ...
(JDK), which is intended for software developers and includes development tools such as the Java compiler,
Javadoc Javadoc (originally cased JavaDoc) is a documentation generator created by Sun Microsystems for the Java language (now owned by Oracle Corporation) for generating API documentation in HTML format from Java source code. The HTML format is used ...
, Jar, and a
debugger A debugger or debugging tool is a computer program used to test and debug other programs (the "target" program). The main use of a debugger is to run the target program under controlled conditions that permit the programmer to track its execut ...
. Oracle has also released GraalVM, a high performance Java dynamic compiler and interpreter. OpenJDK is another notable Java SE implementation that is licensed under the GNU 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 Microsoft Corporation is an American multinational corporation, multinational technology company, technology corporation producing Software, computer software, consumer electronics, personal computers, and related services headquartered at th ...
after Sun claimed that the Microsoft implementation did not support
RMI RMI may refer to: Science and technology * Radio-magnetic indicator, an instrument used in aircraft navigation * Repetitive motion injury, an injury to the musculoskeletal and nervous systems * Richtmyer–Meshkov instability, an instability occu ...
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 Java with
Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...
. 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.


Use outside the Java platform

The Java programming language requires the presence of a software platform in order for compiled programs to be executed. Oracle supplies the
Java platform Java is a set of computer software and specifications developed by James Gosling at Sun Microsystems, which was later acquired by the Oracle Corporation, that provides a system for developing application software and deploying it in a cro ...
for use with Java. The Android SDK is an alternative software platform, used primarily for developing Android applications with its own GUI system.


Android

The Java language is a key pillar in
Android Android may refer to: Science and technology * Android (robot), a humanoid robot or synthetic organism designed to imitate a human * Android (operating system), Google's mobile operating system ** Bugdroid, a Google mascot sometimes referred to ...
, an
open source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized sof ...
mobile operating system A mobile operating system is an operating system for mobile phones, tablet computer, tablets, smartwatches, smartglasses, or other non-laptop personal computing, personal mobile computing devices. While computers such as typical laptops are "mobi ...
. Although Android, built on the
Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ...
, is written largely in C, the Android SDK uses the Java language as the basis for Android applications but does not use any of its standard GUI, SE, ME or other established Java standards. The bytecode language supported by the Android SDK is incompatible with Java bytecode and runs on its own virtual machine, optimized for low-memory devices such as
smartphone A smartphone is a portable computer device that combines mobile telephone and computing functions into one unit. They are distinguished from feature phones by their stronger hardware capabilities and extensive mobile operating systems, whic ...
s and
tablet computer A tablet computer, commonly shortened to tablet, is a mobile device, typically with a mobile operating system and touchscreen display processing circuitry, and a rechargeable battery in a single, thin and flat package. Tablets, being comput ...
s. Depending on the Android version, the bytecode is either interpreted by the Dalvik virtual machine or compiled into native code by the Android Runtime. Android does not provide the full Java SE standard library, although the Android SDK does include an independent implementation of a large subset of it. It supports Java 6 and some Java 7 features, offering an implementation compatible with the standard library (
Apache Harmony Apache Harmony is a retired Open-source software, open source, free Java implementations, 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 ...
).


Controversy

The use of Java-related technology in Android led to a legal dispute between Oracle and Google. 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. District Judge William Alsup ruled on May 31, 2012, that APIs cannot be copyrighted, but this was reversed by the United States Court of Appeals for the Federal Circuit in May 2014. On May 26, 2016, the district court decided in favor of Google, ruling the copyright infringement of the Java API in Android constitutes fair use. In March 2018, this ruling was overturned by the Appeals Court, which sent down the case of determining the damages to federal court in San Francisco. Google filed a petition for
writ of certiorari In law, ''certiorari'' is a court process to seek judicial review of a decision of a lower court or government agency. ''Certiorari'' comes from the name of an English prerogative writ, issued by a superior court to direct that the record of ...
with the
Supreme Court of the United States The Supreme Court of the United States (SCOTUS) is the highest court in the federal judiciary of the United States. It has ultimate appellate jurisdiction over all U.S. federal court cases, and over state court cases that involve a point ...
in January 2019 to challenge the two rulings that were made by the Appeals Court in Oracle's favor. On April 5, 2021, the Court ruled 6-2 in Google's favor, that its use of Java APIs should be considered
fair use Fair use is a doctrine in United States law that permits limited use of copyrighted material without having to first acquire permission from the copyright holder. Fair use is one of the limitations to copyright intended to balance the intere ...
. However, the court refused to rule on the copyrightability of APIs, choosing instead to determine their ruling by considering Java's API copyrightable "purely for argument’s sake."


See also

* C# * C++ * Dalvik, used in old Android versions, replaced by non-JIT Android Runtime *
Java Heterogeneous Distributed Computing Java Heterogeneous Distributed Computing refers to a programmable Java distributed system which was developed at the National University of Ireland in Maynooth. It allows researchers to access the spare clock cycles of a large number of semi-idle ...
* List of Java virtual machines *
List of Java APIs There are two types of Java (programming language), Java programming language Application programming interface, application programming interfaces (APIs): * The official core Java API, contained in the Android (Google), SE (OpenJDK and Oracle), ...
* List of JVM languages


Comparison of Java with other languages

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


References


Works cited

* * *


External links


Official Website
* * * *
Java Weekly
{{Authority control Articles with example Java code C programming language family Class-based programming languages Computer-related introductions in 1995 Concurrent programming languages
Programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming l ...
Programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming l ...
JVM programming languages Multi-paradigm programming languages Object-oriented programming languages Programming languages created in 1995 Programming languages Statically typed programming languages Sun Microsystems