Java is a programming language originally developed by Sun Microsystems 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 has a simpler object model and fewer low-level facilities. Java applications are typically compiled to bytecode that can run on any Java virtual machine (JVM) regardless of computer architecture.
The original and reference implementation Java compilers, virtual machines, and class libraries were developed by Sun from 1995. As of May 2007, in compliance with the specifications of the Java Community Process, Sun made available most of their Java technologies as free software under the GNU General Public License. Others have also developed alternative implementations of these Sun technologies, such as the GNU Compiler for Java and GNU Classpath.
James Gosling initiated the Java language project in June 1991 for use in one of his many set-top box projects. The language, initially called Oak after an oak tree 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. Gosling aimed to implement a virtual machine and a language that had a familiar C/C++ style of notation.
Sun released the first public implementation as Java 1.0 in 1996. 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 secure Java applets within web pages, and Java quickly became popular. With the advent of Java 2 (released initially as J2SE 1.2 in December 1998), 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 ME, and Java SE, respectively.
In 1997, Sun Microsystems approached the ISO/IEC JTC1 standards body and later the Ecma International to formalize Java, but it soon withdrew from the process. Java remains a de facto standard, controlled through the Java Community Process. At one time, Sun made most of its Java implementations available without charge, despite their proprietary software status. Sun generated revenue from Java through the selling of licenses for specialized products such as the Java Enterprise System. Sun distinguishes between its Software Development Kit (SDK) and Runtime Environment (JRE) (a subset of the SDK); the primary distinction involves the JRE's lack of the compiler, utility programs, and many necessary header files.
On 13 November 2006, Sun released much of Java as free and open source software under the terms of the GNU General Public License (GPL). On 8 May 2007 Sun finished the process, making all of Java's core code free and open-source, aside from a small portion of code to which Sun did not hold the copyright.
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 – instructions analogous to machine code but intended to be interpreted by a virtual machine (VM) written specifically for the host hardware. End-users commonly use a JRE installed on their own machine, or in a Web browser.
Standardized libraries provide a generic way to access host specific features such as graphics, threading and networking. 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 optimisation techniques introduced in the more recent JVM implementations.
One such technique, known as (just-in-time compilation)JIT, 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, 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. 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. AOT could give Java something like 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. 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 errors (exceptions) should not 'crash' the system. 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.
Sun Microsystems officially licenses the Java Standard Edition platform for Microsoft Windows, Linux, and Solaris. Through a network of third-party vendors and licensees, 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 from 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 $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, 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 Enterprise Edition 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.
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.
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 the program may become unstable and/or may crash. This can be partially remedied by the use of smart pointers, but it adds overhead and complexity.
Garbage collection is allowed to happen at any time. Ideally, it will occur when a program is idle. It is guaranteed to occur if there is insufficient free memory on the heap to allocate a new object, which 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 integers (usually long integers) can be used interchangeably. This is to allow the garbage collector to relocate referenced objects, and to ensure 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 is the common case 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 write as if primitive types are their wrapper classes, with their object-oriented counterparts representing classes of their own, and freely interchange between them for improved flexibility.
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.
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, which can only be accessed from other classes within the file.
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.
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.
static in front of a method indicates a static method, which 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.
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()
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 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 a 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 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 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, including which also appends a new line to the passed string.
The string "Hello world!" is automagically converted to a String object by the compiler.
OddEvenclass declares a single
privatefield of type
input. Every instance of the
OddEvenclass has its own copy of the
inputfield. The private declaration means that no other class can access (read or write) the
publicconstructor. 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 dialog returns a
Stringthat is converted to an
intby the method.
calculate()method is declared without the
statickeyword. This means that the method is invoked using a specific instance of the
OddEvenclass. (The reference used to invoke the method is passed as an undeclared parameter of type
this.) The method tests the expression
input % 2 == 0using the
ifkeyword to see if the remainder of dividing the
inputfield 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. (The
inputfield can be equivalently accessed as
this.input, which explicitly uses the undeclared
OddEven number = new OddEven();declares a local object reference variable in the
number. This variable can hold a reference to an object of type
OddEven. The declaration initializes
numberby first creating an instance of the
OddEvenclass, using the
newkeyword and the
OddEven()constructor, and then assigning this instance to the variable.
number.showDialog();calls the calculate method. The instance of
OddEvenobject referenced by the
numberlocal variable is used to invoke the method and passed as the undeclared
thisparameter to the
Java applets are programs that are embedded in other applications, typically in a Web page displayed in a Web browser.
import statements direct the Java compiler to include the and classes in the compilation. The import statement allows these classes to be referenced in the source code using the simple class name (i.e.
Applet) instead of the fully qualified class name (i.e.
extends (subclasses) the
Applet class; the
Applet class provides the framework for the host application to display and control the lifecycle of the applet. The
Applet class is an Abstract Windowing Toolkit (AWT) , which provides the applet with the capability to display a graphical user interface (GUI) and respond to user events.
Hello class overrides the method inherited from the 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
paint() method calls the graphic context method to display the "Hello, world!" string at a pixel offset of (
65, 95) from the upper-left corner in the applet's display.
An applet is placed in an HTML document using the
HTML element. The
applet tag has three attributes set:
code="Hello" specifies the name of the
Applet 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
embed element, 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 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.
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  requests) from clients. A servlet can almost be thought of as an applet that runs on the server side—without a face.
extends the class; the
GenericServlet class provides the interface for the server to forward requests to the servlet and control the servlet's lifecycle.
Hello class overrides the method defined by the interface to provide the code for the service request handler. The
service() method is passed a object that contains the request from the client and a object used to create the response returned to the client. The
service() method declares that it
throws the exceptions and if a problem prevents it from responding to the request.
The method in the response object is called to set the MIME content type of the returned data to "text/html". The method in the response returns a object that is used to write the data that is sent to the client. The method is called to write the "Hello, world!" string to the response and then the 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 Pages (JSPs) are server-side Java EE components that generate responses, typically HTML pages, to  requests from clients. JSPs embed Java code in an HTML page by using the special delimiters
%>. 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. Apple also provides an Aqua look and feel for Mac OS X. Though prior implementations of these looks and feels 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:
import statement directs the Java compiler to include the class from the package in the compilation; the second
import includes all of the public classes and interfaces from the package.
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 method inherited from
JFrame to set the default operation when the close control on the title bar is selected to — 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 is created for the string "Hello, world!" and the method inherited from the superclass is called to add the label to the frame. The method inherited from the superclass is called to size the window and lay out its contents, in the manner indicated by 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 method inherited from the superclass with the boolean parameter
true. Note that 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.
Prior to 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.
Sun has defined three platforms 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.
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