Java is a programming language A programming language is a machine-readable artificial language designed to express computations that can be performed by a machine, particularly a computer. Programming languages can be used to create programs that specify the behavior of a machine, to express algorithms precisely, or as a mode of human communication originally developed by James Gosling at Sun Microsystems Sun Microsystems, Inc. is a multinational vendor of computers, computer components, computer software, and information technology services, founded on February 24, 1982. The company is headquartered in Santa Clara, California (part of Silicon Valley), on the former west campus of the Agnews Developmental Center and released in 1995 as a core component of Sun Microsystems' Java platform Java refers to a number of computer software products and specifications from Sun Microsystems that together provide a system for developing application software and deploying it in a cross-platform environment. Java is used in a wide variety of computing platforms from embedded devices and mobile phones on the low end, to enterprise servers and. The language derives much of its syntax In computer science, the syntax of a programming language is the set of rules that define the combinations of symbols that are considered to be syntactically correct programs in that language. The syntax of a language defines its surface form. Text-based programming languages are based on sequences of characters, while visual programming languages from C C is one of the most popular programming languages. It is widely used on many different software platforms, and there are few computer architectures for which a C compiler does not exist. C has greatly influenced many other popular programming languages, most notably C++, which originally began as an extension to C and C++ but has a simpler object model and fewer low-level facilities. Java applications are typically compiled A compiler is a computer program that transforms source code written in a computer language (the source language) into another computer language (the target language, often having a binary form known as object code). The most common reason for wanting to transform source code is to create an executable program to bytecode Java bytecode is the form of instructions that the Java virtual machine executes. Each bytecode opcode is one byte in length, although some require parameters, resulting in some multi-byte instructions. Not all of the possible 256 opcodes are used. In fact, Sun Microsystems, the original creators of the Java programming language, the Java virtual (class file In the Java programming language, source files are compiled into class files which have a .class extension. Since Java is a platform-independent language, source code is compiled into an output file known as bytecode, which it stores in a .class file. If a source file has more than one class, each class is compiled into a separate .class file) that can run on any Java virtual machine A Java Virtual Machine is a set of computer software programs and data structures that use a virtual machine model for the execution of other computer programs and scripts. The model used by a JVM accepts a form of computer intermediate language commonly referred to as Java bytecode. This language conceptually represents the instruction set of a (JVM) regardless of computer architecture Computer architecture in computer engineering is the conceptual design and fundamental operational structure of a computer system. It is a blueprint and functional description of requirements and design implementations for the various parts of a computer, focusing largely on the way by which the central processing unit performs internally and.

The original and reference implementation In computing, a reference implementation is a software example of a specification. These are intended to help others implement their own version of the specification or find problems during the creation of a specification. The following example clarifies this specification: Java compilers A compiler is a computer program that transforms source code written in a computer language (the source language) into another computer language (the target language, often having a binary form known as object code). The most common reason for wanting to transform source code is to create an executable program, virtual machines, and class libraries In computer science, a library is a collection of subroutines or classes used to develop software. Libraries contain code and data that provide services to independent programs. This allows the sharing and changing of code and data in a modular fashion. Some executables are both standalone programs and libraries, but most libraries are not were developed by Sun from 1995. As of May 2007, in compliance with the specifications of the Java Community Process The Java Community Process or JCP, established in 1998, is a formalized process that allows interested parties to get involved in the definition of future versions and features of the Java platform, Sun made available most of their Java technologies as free software Free software or software libre is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with minimal restrictions only to ensure that further recipients can also do these things and that manufacturers of consumer-facing hardware under the GNU General Public License The GNU General Public License is a widely used free software license, originally written by Richard Stallman for the GNU project. The GPL is the most popular and well-known example of the type of strong copyleft license that requires derived works to be available under the same copyleft. Under this philosophy, the GPL grants the recipients of a. Others have also developed alternative implementations of these Sun technologies, such as the GNU Compiler for Java and GNU Classpath GNU Classpath is a project aiming to create a free software implementation of the standard class library for the Java programming language. Despite the massive size of the library to be created, the majority of the task is already done, including Swing, CORBA, and other major parts. The Classpath developers have implemented almost all of the.

Contents

History

Duke, the Java mascot See also: Java (Sun) history Java refers to a number of computer software products and specifications from Sun Microsystems that together provide a system for developing application software and deploying it in a cross-platform environment. Java is used in a wide variety of computing platforms from embedded devices and mobile phones on the low end, to enterprise servers and and Java version history

James Gosling initiated the Java language project in June 1991 for use in one of his many set-top box A set-top box or set-top unit (STU) is a device that connects to a television and an external source of signal, turning the signal into content which is then displayed on the television screen projects.[6] The language, initially called Oak after an oak tree An oak is a tree or shrub in the genus Quercus , of which about 400 species exist. "Oak" may also apear in the names of species in related genera, notably Lithocarpus. The genus is native to the northern hemisphere, and includes deciduous and evergreen species extending from cold latitudes to tropical Asia and the Americas that stood outside Gosling's office, also went by the name Green and ended up later renamed as Java, from a list of random words.[7] Gosling aimed to implement a virtual machine In computer science, a virtual machine is a software implementation of a machine (computer) that executes programs like a real machine and a language that had a familiar C/C++ style of notation.[8]

Sun released the first public implementation as Java 1.0 in 1995. It promised "Write Once, Run Anywhere" (WORA), providing no-cost run-times on popular platforms. Fairly secure and featuring configurable security, it allowed network- and file-access restrictions. Major web browsers soon incorporated the ability to run Java applets An applet is a software component that runs in the context of another program, for example a web browser. An applet usually performs a very narrow function that has no independent use. Hence, it is an application -let within web pages, and Java quickly became popular. With the advent of Java 2 (released initially as J2SE 1.2 in December 1998), new versions had multiple configurations built for different types of platforms. For example, J2EE targeted enterprise applications and the greatly stripped-down version J2ME for mobile applications. J2SE designated the Standard Edition. In 2006, for marketing purposes, Sun renamed new J2 versions as Java EE Java Platform, Enterprise Edition or Java EE is a widely used platform for server programming in the Java programming language. The Java EE Platform differs from the Java Standard Edition Platform in that it adds libraries which provide functionality to deploy fault-tolerant, distributed, multi-tier Java software, based largely on modular, Java ME In computing, the Java Platform, Micro Edition or Java ME is a specification of a subset of the Java platform aimed at providing a certified collection of Java APIs for the development of software for tiny, small and resource-constrained devices. Target devices range from industrial control and automotive devices to cell phones and set-top boxes, and Java SE, respectively.

In 1997, Sun Microsystems approached the ISO/IEC JTC1 standards body The International Organization for Standardization , widely known as ISO (pronounced /ˈaɪsoʊ/), is an international-standard-setting body composed of representatives from various national standards organizations. Founded on 23 February 1947, the organization promulgates worldwide proprietary industrial and commercial standards. It is and later the Ecma International Ecma International is an international, private (membership-based) non-profit standards organization for information and communication systems. It acquired its name in 1994, when the European Computer Manufacturers Association (ECMA) changed its name to reflect the organization's international reach. As a consequence, the name is no longer to formalize Java, but it soon withdrew from the process.[9] Java remains a de facto De facto is a Latin expression that means "concerning fact". In law, it is meant to mean "in practice but not necessarily ordained by law" or "in practice or actuality, but without being officially established". It is commonly used in contrast to de jure when referring to matters of law, governance, or technique (such standard, controlled through the Java Community Process The Java Community Process or JCP, established in 1998, is a formalized process that allows interested parties to get involved in the definition of future versions and features of the Java platform.[10] At one time, Sun made most of its Java implementations available without charge, despite their proprietary software Proprietary software is computer software which is the legal property of one party. The terms of use for other parties is defined by contracts or licensing agreements. These terms may include various privileges to share, alter, dissemble, and use the software and its code status. Sun generated revenue from Java through the selling of licenses for specialized products such as the Java Enterprise System. Sun distinguishes between its Software Development Kit (SDK) A software development kit is typically a set of development tools that allows a software engineer to create applications for a certain software package, software framework, hardware platform, computer system, video game console, operating system, or similar platform and Runtime Environment (JRE) (a subset of the SDK); the primary distinction involves the JRE's lack of the compiler, utility programs, and header files.

On 13 November 2006, Sun released much of Java as free and open source software Free and open source software, also F/OSS, FOSS, or FLOSS is software which is liberally licensed to grant the right of users to study, change, and improve its design through the availability of its source code. This approach has gained both momentum and acceptance as the potential benefits have been increasingly recognized by both individuals and under the terms of the GNU General Public License The GNU General Public License is a widely used free software license, originally written by Richard Stallman for the GNU project. The GPL is the most popular and well-known example of the type of strong copyleft license that requires derived works to be available under the same copyleft. Under this philosophy, the GPL grants the recipients of a (GPL). On 8 May 2007 Sun finished the process, making all of Java's core code available under free software Free software or software libre is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with minimal restrictions only to ensure that further recipients can also do these things and that manufacturers of consumer-facing hardware / open-source distribution terms, aside from a small portion of code to which Sun did not hold the copyright.[11]

Philosophy

Primary goals

There were five primary goals in the creation of the Java language:[12]

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

Java Platform

Main articles: Java Platform Java refers to a number of computer software products and specifications from Sun Microsystems that together provide a system for developing application software and deploying it in a cross-platform environment. Java is used in a wide variety of computing platforms from embedded devices and mobile phones on the low end, to enterprise servers and and Java Runtime Environment

One characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any supported hardware/operating-system platform. One should be able to write a program once, compile it once, and run it anywhere.

This is achieved by compiling the Java language code, not to machine code but to Java bytecode Java bytecode is the form of instructions that the Java virtual machine executes. Each bytecode opcode is one byte in length, although some require parameters, resulting in some multi-byte instructions. Not all of the possible 256 opcodes are used. In fact, Sun Microsystems, the original creators of the Java programming language, the Java virtual – instructions analogous to machine code but intended to be interpreted In computer science, an interpreter normally means a computer program that executes, i.e. performs, instructions written in a programming language by a virtual machine In computer science, a virtual machine is a software implementation of a machine (computer) that executes programs like a real machine (VM) written specifically for the host hardware. End-users Economics and commerce define an end-user as the person who uses a product. The end-user or consumer may differ from the person who purchases the product. For instance, a zookeeper, the customer, might purchase elephant food for an end-user: the elephant commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a Web browser for Java applets An applet is a software component that runs in the context of another program, for example a web browser. An applet usually performs a very narrow function that has no independent use. Hence, it is an application -let.

Standardized libraries provide a generic way to access host specific features such as graphics, threading In computer science, a thread of execution results from a fork of a computer program into two or more concurrently running tasks. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process. Multiple threads can exist within the same process and share resources and networking A computer network is a group of interconnected computers. Networks may be classified according to a wide variety of characteristics. This article provides a general overview of some types and categories and also presents the basic components of a network. In some JVM versions, bytecode can be compiled to native code, either before or during program execution, resulting in faster execution.

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, and Java suffered a reputation for poor performance. This gap has been narrowed by a number of optimization techniques introduced in the more recent JVM implementations.

One such technique, known as just-in-time (JIT) compilation In computing, just-in-time compilation , also known as dynamic translation, is a technique for improving the runtime performance of a computer program. JIT builds upon two earlier ideas in run-time environments: bytecode compilation and dynamic compilation. It converts code at runtime prior to executing it natively, for example bytecode into, translates Java bytecode into native code the first time that code is executed, then caches it. This results in a program that starts and executes faster than pure interpreted code can, at the cost of introducing occasional compilation overhead during execution. More sophisticated VMs also use dynamic recompilation, in which the VM analyzes the behavior of the running program and selectively recompiles and optimizes parts of the program. Dynamic recompilation can achieve optimizations superior to static compilation because the dynamic compiler can base optimizations on knowledge about the runtime environment and the set of loaded classes, and can identify hot spots - parts of the program, often inner loops, that take up the most execution time. JIT compilation and dynamic recompilation allow Java programs to approach the speed of native code without losing portability.

Another technique, commonly known as static compilation, or ahead-of-time (AOT) compilation An ahead-of-time compiler is a compiler that implements ahead-of-time compilation. This refers to the act of compiling an intermediate language, such as Java bytecode, .NET Common Intermediate Language (CIL), or IBM System/38 or IBM System i "Technology Independent Machine Interface" code, into a system-dependent binary, is to compile directly into native code like a more traditional compiler. Static Java compilers translate the Java source or bytecode to native object code In computer science, an object file is an organised collection of named objects, and typically these objects are sequences of computer instructions in a machine code format, which may be directly executed by a computer's CPU. Object files are typically produced by a compiler as a result of processing a source code file. Object files contain. This achieves good performance compared to interpretation, at the expense of portability; the output of these compilers can only be run on a single architecture Computer architecture in computer engineering is the conceptual design and fundamental operational structure of a computer system. It is a blueprint and functional description of requirements and design implementations for the various parts of a computer, focusing largely on the way by which the central processing unit performs internally and. AOT could give Java something close to native performance, yet it is still not portable since there are no compiler directives, and all the pointers are indirect with no way to micro manage garbage collection.

Java's performance has improved substantially since the early versions, and performance of JIT compilers relative to native compilers has in some tests been shown to be quite similar.[13][14] The performance of the compilers does not necessarily indicate the performance of the compiled code; only careful testing can reveal the true performance issues in any system.

One of the unique advantages of the concept of a runtime engine is that even the most serious errors (exceptions) in a Java program should not 'crash' the system under any circumstances, provided the JVM itself is properly implemented. Moreover, in runtime engine environments such as Java there exist tools that attach to the runtime engine and every time that an exception of interest occurs they record debugging information that existed in memory at the time the exception was thrown (stack and heap values). These Automated Exception Handling tools provide 'root-cause' information for exceptions in Java programs that run in production, testing or development environments. Such precise debugging is much more difficult to implement without the run-time support that the JVM offers.

Implementations

Sun Microsystems officially licenses the Java Standard Edition platform for Microsoft Windows Microsoft Windows is a series of software operating systems and graphical user interfaces produced by Microsoft. Microsoft first introduced an operating environment named Windows in November 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal, Linux Linux is a generic term referring to Unix-like computer operating systems based on the Linux kernel. Their development is one of the most prominent examples of free and open source software collaboration; typically all the underlying source code can be used, freely modified, and redistributed by anyone under the terms of the GNU GPL and other free, Mac OS X Mac OS X is a line of computer operating systems developed, marketed, and sold by Apple Inc., and since 2002 has been included with all new Macintosh computer systems. It is the successor to Mac OS 9, the final release of the "classic" Mac OS, which had been Apple's primary operating system since 1984. Mac OS X, whose "X", and Solaris Solaris is a UNIX-based operating system introduced by Sun Microsystems in 1992 as the successor to SunOS. Through a network of third-party vendors and licensees[15], alternative Java environments are available for these and other platforms.

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 a United States-based multinational computer technology corporation that develops, manufactures, licenses, and supports a wide range of software products for computing devices. Headquartered in Redmond, Washington, USA, its most profitable products are the Microsoft Windows operating system and the Microsoft Office suite 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 $20 million as well as a court order enforcing the terms of the license from Sun.[16] As a result, Microsoft no longer ships Java with Windows, and in recent versions of Windows, Internet Explorer cannot support Java applets without a third-party plugin. Sun, and others, have made available free Java run-time systems for those and other versions of Windows.

Platform-independent Java is essential to the Java EE strategy, and an even more rigorous validation is required to certify an implementation. This environment enables portable server-side applications, such as Web services, servlets, and Enterprise JavaBeans, as well as with embedded systems based on OSGi, using Embedded Java environments. Through the new GlassFish project, Sun is working to create a fully functional, unified open-source implementation of the Java EE technologies.

Sun also distributes a superset of the JRE called the Java Development Kit (commonly known as the JDK), which includes development tools such as the Java compiler, Javadoc, Jar and debugger.

Automatic memory management

See also: Garbage collection (computer science)

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 object 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. [17][18]

One of the ideas behind Java's automatic memory management model is that programmers 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. Either way the responsibility of managing memory resides with the programmer. If the program does not deallocate an object, a memory leak occurs. If the program attempts to access or deallocate memory that has already been deallocated, the result is undefined and difficult to predict, and the program is likely to become unstable and/or crash. This can be partially remedied by the use of smart pointers, but these add overhead and complexity.

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. Where performance or response time is important, explicit memory management and object pools are often used.

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 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 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 are instances of their wrapper classes.

Syntax

Main article: Java syntax

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

Java suppresses several features (such as operator overloading and multiple inheritance) for classes in order to simplify the language and to prevent possible errors and anti-pattern design.

Java uses the same commenting methods as C++. There are two different styles of comment: a single line style marked with two forward slashes, and a multiple line style opened with a forward slash asterisk (/*) and closed with an asterisk forward slash (*/).

Example:

//This is an example of a single line comment using two forward slashes
/* This is an example of a multiple line comment using the forward slash
and asterisk. This type of comment can be used to hold a lot of information
but it is very important to remember to close the comment. */

Examples

Hello world

The traditional Hello world program can be written in Java as:

/*
* Outputs "Hello, world!" and then exits
*/
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world!");
}
}

By convention, source files are named after the public class they contain, appending the suffix .java, for example, HelloWorld.java. It must first be compiled into bytecode, using a Java compiler, producing a file named HelloWorld.class. Only then can it be executed, or 'launched'. The java source file may only contain one public class but can contain multiple classes with less than public access and any number of public inner classes.

A class that is declared private 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 was 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.

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 method variables that are not 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 and Enterprise Java Beans 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 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 allows an alternate 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 out object 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.

The string "Hello, world!" is automatically converted to a String object by the compiler.

A more comprehensive example

// OddEven.java
import javax.swing.JOptionPane;
public class OddEven {
// "input" is the number that the user gives to the computer
private int input; // a whole number("int" means integer)
/*
* This is the constructor method. It gets called when an object of the OddEven type
* is being created.
*/
public OddEven() {
//Code not shown
}
// This is the main method. It gets called when this class is run through a Java interpreter.
public static void main(String[] args) {
/*
* This line of code creates a new instance of this class called "number" (also known as an
* Object) and initializes it by calling the constructor. The next line of code calls
* the "showDialog()" method, which brings up a prompt to ask you for a number
*/
OddEven number = new OddEven();
number.showDialog();
}
public void showDialog() {
/*
* "try" makes sure nothing goes wrong. If something does,
* the interpreter skips to "catch" to see what it should do.
*/
try {
/*
* The code below brings up a JOptionPane, which is a dialog box
* The String returned by the "showInputDialog()" method is converted into
* an integer, making the program treat it as a number instead of a word.
* After that, this method calls a second method, calculate() that will
* display either "Even" or "Odd."
*/
input = new Integer(JOptionPane.showInputDialog("Please Enter A Number"));
calculate();
} catch (NumberFormatException e) {
/*
* Getting in the catch block means that there was a problem with the format of
* the number. Probably some letters were typed in instead of a number.
*/
System.err.println("ERROR: Invalid input. Please type in a numerical value.");
}
}
/*
* When this gets called, it sends a message to the interpreter.
* The interpreter usually shows it on the command prompt (For Windows users)
* or the terminal (For Linux users).(Assuming it's open)
*/
private void calculate() {
if (input % 2 == 0) {
System.out.println("Even");
} else {
System.out.println("Odd");
}
}
}

Special classes

Applet

Main article: Java applet

Java applets are programs that are embedded in other applications, typically in a Web page displayed in a Web browser.

// Hello.java
import javax.swing.JApplet;
import java.awt.Graphics;
public class Hello extends JApplet {
public void paintComponent(Graphics g) {
g.drawString("Hello, world!", 65, 95);
}
}

The import statements direct the Java compiler to include the javax.swing.JApplet and java.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 (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 inherited from the Container superclass to provide the code to display the applet. The paint() 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 (65, 95) from the upper-left corner in the applet's display.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<!-- Hello.html -->
<html>
<head>
<title>Hello World Applet</title>
</head>
<body>
<applet code="Hello" width="200" height="200">
</applet>
</body>
</html>

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 embed element[19], although support for these elements by Web browsers is inconsistent.[20] 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 paint method is called by the AWT event dispatching thread whenever the display needs the applet to draw itself.

Servlet

Main article: Java Servlet

Java 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.

// Hello.java
import java.io.*;
import javax.servlet.*;
public class Hello extends GenericServlet {
public void service(ServletRequest request, ServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
final PrintWriter pw = response.getWriter();
pw.println("Hello, world!");
pw.close();
}
}

The import statements direct the Java compiler to include all of the public classes and interfaces from the java.io and javax.servlet packages in the compilation.

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 the Servlet interface to provide the code for the service request handler. The service() method is passed a ServletRequest 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 exceptions ServletException 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 Page

Main article: JavaServer Pages

JavaServer Pages (JSPs) are server-side Java EE components that generate responses, typically HTML pages, to HTTP 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 application

Main article: Swing (Java)

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. Apple also 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 widget drawing routines of the underlying platforms.

This example Swing application creates a single window with "Hello, world!" inside:

// Hello.java (Java SE 5)
import java.awt.BorderLayout;
import javax.swing.*;
public class Hello extends JFrame {
public Hello() {
super("hello");
setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
setLayout(new BorderLayout());
add(new JLabel("Hello, world!"));
pack();
}
public static void main(String[] args) {
new Hello().setVisible(true);
}
}

The first import statement directs the Java compiler to include the BorderLayout class from the java.awt package in the compilation; the second import includes all of 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() 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 to WindowConstants.EXIT_ON_CLOSE — this causes the JFrame to be disposed of when the frame is closed (as opposed to merely hidden), which allows the JVM to exit and the program to terminate. Next, the layout of the frame is set to a BorderLayout; this tells Swing how to arrange the components that will be added to the frame. A JLabel is created for the string "Hello, world!" and the add(Component) method inherited from the Container superclass is called to add the label to the frame. The pack() method inherited from the Window superclass is called to size the window and lay out its contents, in the manner indicated by the BorderLayout.

The main() method is called by the JVM when the program starts. It instantiates a new Hello frame and causes it to be displayed by calling the setVisible(boolean) method inherited from the Component 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.

Generics

Main article: Generics in Java

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

Class libraries

Documentation

Main article: Javadoc

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

Examples

The following is an example of java code commented with simple Javadoc-style comments:

/**
* A program that does useful things.
*/
public class Program {
/**
* A main method.
* @param args The arguments
*/
public static void main(String[] args) {
//do stuff
}
}

Editions

See also: Free Java implementations#Class library
Java editions
Java Card
Micro Edition (ME)
Standard Edition (SE)
Enterprise Edition (EE)
PersonalJava (discontinued)

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

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

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

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

Criticism

Main article: Criticism of Java

See also

Notes

  1. ^ James Gosling cites UCSD Pascal as a key influence on the design of the Java virtual machine.
  2. ^ Java 5.0 added several new language features (the enhanced for loop, autoboxing, varargs and annotations), after they were introduced in the similar (and competing) C# language [1] [2]
  3. ^ "The Java Language Environment". May 1996. http://java.sun.com/docs/white/langenv/Intro.doc1.html#943.
  4. ^ "The Java Language Specification, 2nd Edition". http://java.sun.com/docs/books/jls/second_edition/html/intro.doc.html#237601.
  5. ^ http://www.computerworld.com.au/index.php/id;1422447371;pp;3;fp;4194304;fpid;1
  6. ^ Jon Byous, Java technology: The early years. Sun Developer Network, no date [ca. 1998]. Retrieved April 22, 2005.
  7. ^ http://blogs.sun.com/jonathan/entry/better_is_always_different.
  8. ^ Heinz Kabutz, Once Upon an Oak. Artima, Retrieved April 29, 2007.
  9. ^ Java Study Group; Why Java Was - Not - Standardized Twice; What is ECMA—and why Microsoft cares
  10. ^ Java Community Process website
  11. ^ open.itworld.com - JAVAONE: Sun - The bulk of Java is open sourced
  12. ^ 1.2 Design Goals of the JavaTM Programming Language
  13. ^ Performance of Java versus C++, J.P.Lewis and Ulrich Neumann, Computer Graphics and Immersive Technology Lab, University of Southern California
  14. ^ FreeTTS - A Performance Case Study, Willie Walker, Paul Lamere, Philip Kwok
  15. ^ Java SE - Licensees
  16. ^ James Niccolai (January 23, 2001). "Sun, Microsoft settle Java lawsuit". JavaWorld (IDG). http://www.javaworld.com/javaworld/jw-01-2001/jw-0124-iw-mssuncourt.html. Retrieved on 2008-07-09.
  17. ^ NullPointerException
  18. ^ Exceptions in Java
  19. ^ Using the applet Tag (The Java Tutorials > Deployment > Applets)
  20. ^ Deploying Applets in a Mixed-Browser Environment (The Java Tutorials > Deployment > Applets)

References

External links

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Is the Java programming language software available for free? ?
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A. Try the development kit with the code tool. Link below, look for NetBeans bundle about halfway down the page. The first problem with C++ is it doesn't come with buttons. The 2nd problem with C++ is you must tune your software efforts per CPU and OS.
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