Java is a general-purpose computer programming language that isconcurrent, 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 compiledto bytecode that can run on any Java virtual machine (JVM) regardless of computer architecture. Java is, as of 2014, 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 originally released by Sun under proprietary licences. 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).
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 appletswithin web pages, and Java quickly became popular. The Java 1.0 compiler was re-written in Java by Arthur van Hoff 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 J2MEfeatured APIs optimized for mobile applications. The desktop version was renamedJ2SE. In 2006, for marketing purposes, Sun renamed new J2 versions as Java EE,Java ME, and Java SE, respectively.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 Oakafter 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, said to be consumed in large quantities by the language's creators. Gosling designed Java with a C/C++-style syntax that system and application programmers would find familiar.
In 1997, Sun Microsystems approached the ISO/IEC JTC 1 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.
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 regard to Java was as an "evangelist". FollowingOracle 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 prevent Oracle 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 scientificsupercomputers. There are 930 million Java Runtime Environment downloads each year and over 3 billion mobile phones run Java. On April 2, 2010, James Gosling resigned from Oracle.
- VersionsThere were five primary goals in the creation of the Java language:
- It must be "simple, object-oriented 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, threaded, and dynamic"
- 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 (March 18, 2014)
Standardized libraries provide a generic way to access host-specific features such as graphics, threading, andnetworking.
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.
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 Microsoft Windows, Mac OS X, Linux 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 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 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.
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, such asHotSpot 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, amemory 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 parallel scavenge garbage collector. However, there are also several other garbage collectors that can be used to manage the heap. For 90% of applications in Java, the Concurrent Mark-Sweep garbage collector is sufficient. Oracle aims to replace CMS with the Garbage-first collector (G1).
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 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.
Unlike C++, Java does not support operator overloading or multiple inheritance for classes, though multiple inheritance is supported for interfaces. This simplifies the language and aids in preventing potential errors and anti-pattern design.
Java uses comments 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 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.
The traditional "Hello, world!" program can be written in Java as:
To compare this to other programming languages see the list of "Hello World!" program examples.
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 namedHelloWorldApp.class. 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 other than public access and any number of public inner classes. When the source file contains multiple classes, make one class 'public' 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.
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.
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 andEnterprise JavaBeans do not use or need a main() method. A Java program may contain multiple classes that havemain methods, which means that the VM needs to be explicitly told which class to launch from.
The main method must accept an array of String 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 (the args parameter 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.
Printing is part of a Java standard library: The System class defines a public static field called out. The outobject is an instance of the PrintStream class and provides many methods for printing data to standard out, including println(String) which also appends a new line to the passed string.
A more comprehensive example
- The import statement imports the JOptionPane class from the javax.swing package.
- The OddEven class declares a single private field of type int named userInput. Every instance of theOddEven class has its own copy of the userInput field. The private declaration means that no other class can access (read or write) the userInput field.
- OddEven() is a public constructor. Constructors have the same name as the enclosing class they are declared in, and unlike a method, have no return type. A constructor is used to initialize an object that is a newly created instance of the class.
- The calculate() method is declared without the static keyword. This means that the method is invoked using a specific instance of the OddEven class. (The reference used to invoke the method is passed as an undeclared parameter of type OddEven named this.) The method tests the expression userInput % 2 == 0using the if keyword to see if the remainder of dividing the userInput field belonging to the instance of the class by two is zero. If this expression is true, then it prints Even; if this expression is false it prints Odd. (Thecalculate method can be equivalently accessed as this.calculate and the userInput field can be equivalently accessed as this.userInput, which both explicitly use the undeclared this parameter.)
- OddEven number = new OddEven(); declares a local object reference variable in the main method namednumber. This variable can hold a reference to an object of type OddEven. The declaration initializes number by first creating an instance of the OddEven class, using the new keyword and the OddEven() constructor, and then assigning this instance to the variable.
- The statement number.showDialog(); calls the calculate method. The instance of OddEven object referenced by the number local variable is used to invoke the method and passed as the undeclared this parameter to thecalculate method.
- userInput = Integer.parseInt(JOptionPane.showInputDialog("Please Enter A Number")); is a statement that converts the type of String to the primitive data type int by using a utility function in the primitive wrapper class Integer.
AppletJava applets are programs that are embedded in other applications, typically in a Web page displayed in a web browser.
The import statements direct the Java compiler to include the javax.swing.JApplet andjava.awt.Graphics classes in the compilation. The import statement allows these classes to be referenced in the source code using the simple class name (i.e. JApplet) instead of the fully qualified class name (FQCN, i.e.javax.swing.JApplet).
The Hello class extends (subclasses) the JApplet (Java Applet) class; the JApplet class provides the framework for the host application to display and control the lifecycle of the applet. The JApplet class is a JComponent (Java Graphical Component) which provides the applet with the capability to display a graphical user interface (GUI) and respond to user events.
The Hello class overrides the paintComponent(Graphics) method (additionally indicated with the annotation, supported as of JDK 1.5, Override) inherited from the Container superclass to provide the code to display the applet. The paintComponent() method is passed a Graphics object that contains the graphic context used to display the applet. The paintComponent() method calls the graphic context drawString(String, int, int)method to display the "Hello, world!" string at a pixel offset of from the upper-left corner in the applet's display.
An applet is placed in an HTML document using the <applet> HTML element. The applet tag has three attributes set: code="Hello" specifies the name of the JApplet class and width="200" height="200" sets the pixel width and height of the applet. Applets may also be embedded in HTML using either the object or embedelement, although support for these elements by web browsers is inconsistent. However, the applet tag is deprecated, so the object tag is preferred where supported.
The host application, typically a Web browser, instantiates the Hello applet and creates an AppletContext for the applet. Once the applet has initialized itself, it is added to the AWT display hierarchy. The paintComponent()method is called by the AWT event dispatching thread whenever the display needs the applet to draw itself.
ServletJava Servlet 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 (typically HTML pages) to requests (typically HTTP requests) from clients. A servlet can almost be thought of as an applet that runs on the server side—without a face.
The import statements direct the Java compiler to include all the public classes and interfaces from the java.ioand javax.servlet packages in the compilation. Packages make Java well suited for large scale applications.
The Hello class extends the GenericServlet class; the GenericServlet class provides the interface for the server to forward requests to the servlet and control the servlet's lifecycle.
The Hello class overrides the service(ServletRequest, ServletResponse) method defined by theServlet interface to provide the code for the service request handler. The service() method is passed: aServletRequest object that contains the request from the client and a ServletResponse object used to create the response returned to the client. The service() method declares that it throws the exceptionsServletException and IOException if a problem prevents it from responding to the request.
The setContentType(String) method in the response object is called to set the MIME content type of the returned data to "text/html". The getWriter() method in the response returns a PrintWriter object that is used to write the data that is sent to the client. The println(String) method is called to write the "Hello, world!" string to the response and then the close() method is called to close the print writer, which causes the data that has been written to the stream to be returned to the client.
JavaServer PagesJavaServer Pages (JSP) are server-side Java EE components that generate responses, typically HTML pages, toHTTP requests from clients. JSPs embed Java code in an HTML page by using the special delimiters <% 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 is a graphical user interface library 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, GTK+ and Motif are supplied by Sun. Applealso provides an Aqua look and feel for Mac OS X. 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 drawing routines of the underlying platforms.
This example Swing application creates a single window with "Hello, world!" inside:
The first import includes all the public classes and interfaces from the javax.swing package.
The Hello class extends the JFrame class; the JFrame class implements a window with a title bar and a close control.
The Hello class extends the JFrame class; the JFrame class implements a window with a title bar and a close control.
The Hello() constructor initializes the frame by first calling the superclass constructor, passing the parameter"hello", which is used as the window's title. It then calls the setDefaultCloseOperation(int) method inherited from JFrame to set the default operation when the close control on the title bar is selected toWindowConstants.EXIT_ON_CLOSE — this causes the JFrame to be disposed of when the frame is closed (as opposed to merely hidden), which allows the Java virtual machine to exit and the program to terminate. Next, aJLabel is created for the string "Hello, world!" and the add(Component) method inherited from theContainer superclass is called to add the label to the frame. The pack() method inherited from theWindow superclass is called to size the window and lay out its contents.
The main() method is called by the Java virtual machine when the program starts. It instantiates a new Helloframe and causes it to be displayed by calling the setVisible(boolean) method inherited from theComponent superclass with the boolean parameter true. Once the frame is displayed, exiting the main method does not cause the program to terminate because the AWT event dispatching thread remains active until all of the Swing top-level windows have been disposed.
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 many 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.
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 on unofficial software platforms
Java, the programming language, requires the presence of a software platform in order for compiled programs to be executed. A well-known unofficial Java-like software platform is the Android software platform, which allows the use of Java 6 and some Java 7 features, uses a different standard library (Apache Harmony reimplementation), different bytecode language and different virtual machine, and is designed for low-memory devices such as smartphones andtablet computers.
Google and Android, Inc. have chosen to use Java as a key pillar in the creation of the Android operating system, an open source mobile 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 does not use the Java virtual machine, instead using Java bytecode as an intermediate step and ultimately targeting Android's own Dalvik virtual machine.
Android also does not provide the full Java SE standard library, although the Android class library does include an independent implementation of a large subset of it. This 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 Haskell Alsup ruled on May 31, 2012, that APIs cannot be copyrighted, but this was reversed by the Ninth Circuit Court of Appeals in May 2014.
The Java Class Library is the standard library, developed to support application development in Java. It 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.[when?] The class library contains features such as:
The core libraries, which include:
- 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
- Internationalization and localization libraries
- 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
- 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.
DocumentationJavadoc 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 */.
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 calledpackages. Each package contains a set of related interfaces, classes and exceptions. Refer to the separate platforms for a description of the packages available.
Sun also provided an edition called PersonalJava that has been superseded by later, standards-based Java ME configuration-profile pairings.