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Java (programming language)

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Java
File:Java logo.gif
Paradigmobject-oriented
Designed bySun Microsystems
First appeared1990s
Typing disciplinestrong, static
Websitewww.oracle.com/java/
Major implementations
numerous
Influenced by
Objective-C, C++, Smalltalk
Influenced
C#, J#, VJ#, Python, JavaScript

Java is an object-oriented programming language developed by James Gosling and colleagues at Sun Microsystems in the early 1990s. Unlike conventional languages which are generally designed to be compiled to native code, Java is compiled to a bytecode which is then run (generally using JIT compilation) by a Java virtual machine.

The language itself borrows much syntax from C and C++ but has a much simpler object model and does away with low level tools like programmer-manipulable pointers.

Java is only distantly related to JavaScript, though they have similar names and share a C-like syntax.


Version history

This history focuses on the programming language, for a more complete history of Java see: Java (Sun).

As with other parts of the Java platform, the Java language has evolved over the years while largely maintaining backwards compatibility. 

void displayWidgets (Iterable<Widget> widgets) {
    for (Widget w : widgets) {
        w.display();
    }
}

This example iterates over the Iterable object widgets, assigning each of its items in turn to the variable w, and then calling the Widget method display() for each item. (Specified by JSR 201.)

  • Java SE 6 — Codename Mustang. As of 2006 this is currently in development under JSR 270. A beta version was released on February 15, 2006[press release] and is available at http://java.sun.com/javase/6/. Another beta is expected summer 2006 with the final release in autumn 2006. New builds including enhancements and bug fixes are released approximately weekly. As of this version, Sun replaced the name "J2SE" with Java SE and dropped the ".0" from the version number.[2]
  • Java SE 7 — Codename Dolphin. As of 2006, this is in the early planning stages. Development is expected to begin in spring 2006, with release estimated in 2008.[3]

In addition to the language changes, much more dramatic changes have been made to the Java class library over the years, which has grown from a few hundred classes in JDK 1.0 to over three thousand in J2SE 5.0. Entire new APIs, such as Swing and Java2D, have been introduced, and many of the original JDK 1.0 classes and methods have been deprecated.

Philosophy

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

  1. It should use the object-oriented programming methodology.
  2. It should allow the same program to be executed on multiple operating systems.
  3. It should contain built-in support for using computer networks.
  4. It should be designed to execute code from remote sources securely.
  5. It should be easy to use and borrow the good parts of older object-oriented languages like C++.

To achieve the goals of networking support and remote code execution, Java programmers sometimes find it necessary to use extensions such as CORBA, Internet Communications Engine, or OSGi.

Object orientation

The first characteristic, object orientation ("OO"), refers to a method of programming and language design. Although there are many interpretations of OO, one primary distinguishing idea is to design software so that the various types of data it manipulates are combined together with their relevant operations. Thus, data and code are combined into entities called objects. An object can be thought of as a self-contained bundle of behavior (code) and state (data). The principle is to separate the things that change from the things that stay the same; often, a change to some data structure requires a corresponding change to the code that operates on that data, or vice versa. This separation into coherent objects provides a more stable foundation for a software system's design. The intent is to make large software projects easier to manage, thus improving quality and reducing the number of failed projects.

Another primary goal of OO programming is to develop more generic objects so that software can become more reusable between projects. A generic "customer" object, for example, should in theory have roughly the same basic set of behaviors between different software projects, especially when these projects overlap on some fundamental level as they often do in large organizations. In this sense, software objects can hopefully be seen more as pluggable components, helping the software industry build projects largely from existing and well tested pieces, thus leading to a massive reduction in development times. Software reusability has met with mixed practical results, with two main difficulties: the design of truly generic objects is poorly-understood, and a methodology for broad communication of reuse opportunities is lacking. Some open source communities want to help ease the reuse problem, by providing authors with ways to disseminate information about generally reusable objects and object libraries.

Platform independence

File:SwingSet.png
The look and feel of Java Swing GUIs is independent of the platform on which they are running

The second characteristic, platform independence, means that programs written in the Java language must run similarly on diverse hardware. One should be able to write a program once and run it anywhere.

This is achieved by most Java compilers by compiling the Java language code "halfway" to bytecode (specifically Java bytecode)—simplified machine instructions specific to the Java platform. The code is then run on a virtual machine (VM), a program written in native code on the host hardware that interprets and executes generic Java bytecode. Further, standardized libraries are provided to allow access to features of the host machines (such as graphics, threading and networking) in unified ways. Note that, although there's an explicit compiling stage, at some point, the Java bytecode is interpreted or converted to native machine instructions by the JIT compiler.

There are also implementations of Java compilers that compile to native object code, such as GCJ, removing the intermediate bytecode stage, but the output of these compilers can only be run on a single architecture.

Sun's license for Java insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support the RMI and JNI interfaces and had added platform-specific features of their own. Sun sued and won both damages (some $20 million) and a court order enforcing the terms of the license from Sun. In response, Microsoft no longer ships Java with Windows, and in recent versions of Windows, Internet Explorer cannot support Java applets without a third-party plugin. However, Sun and others have made available Java run-time systems at no cost for those and other versions of Windows.

The first implementations of the language used an interpreted virtual machine to achieve portability. These implementations produced programs that ran more slowly than programs written in C or C++, so the language suffered a reputation for poor performance. More recent JVM implementations produce programs that run significantly faster than before, using multiple techniques.

The first technique is to simply compile directly into native code like a more traditional compiler, skipping bytecodes entirely. This achieves good performance, but at the expense of portability. Another technique, known as just-in-time compilation (JIT), translates the Java bytecodes into native code at the time that the program is run. More sophisticated VMs use dynamic recompilation, in which the VM can analyze the behavior of the running program and selectively recompile and optimise critical 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. JIT compilation and dynamic recompilation allow Java programs to take advantage of the speed of native code without losing portability.

Portability is a technically difficult goal to achieve, and Java's success at that goal has been mixed. Although it is indeed possible to write programs for the Java platform that behave consistently across many host platforms, the large number of available platforms with small errors or inconsistencies led some to parody Sun's "Write once, run anywhere" slogan as "Write once, debug everywhere".

Platform-independent Java is however very successful with server-side applications, such as web services, servlets, and Enterprise Java Beans, as well as with Embedded systems based on OSGi, using Embedded Java environments.

Automatic garbage collection

One possible argument against languages such as C++ is that programmers should be spared the burden of having to perform manual memory management. In C++, the programmer must allocate memory to create any object stored on the heap, and deallocate memory to delete any such objects. If a programmer forgets to deallocate memory or writes code that fails to do so in a timely fashion, a memory leak can occur: the program will consume a potentially arbitrarily large amount of memory. In addition, if a region of memory is deallocated twice, the program can become unstable and may crash.

In Java, this potential problem is avoided by automatic garbage collection. The programmer determines when objects are created and the Java runtime is responsible for managing the objects' lifecycle. The program or other objects can reference an object by holding a reference to it (which, from a low-level point of view, is its address on the heap). When no references to an object remain, the Java garbage collector automatically deletes the unreachable object, freeing memory and preventing a memory leak. Memory leaks may still occur if a programmer's code holds a reference to an object that is no longer needed—in other words, they can still occur but at higher conceptual levels. On the whole, Java's automatic garbage collection makes creation and deletion of objects in Java simpler, potentially safer, and often faster than in C++.

Garbage collection in Java is virtually invisible to the developer. That is, developers may have no notion of when garbage collection will take place as it may not necessarily correlate with any actions being explicitly performed by the code they write.

Note that memory is only one of many resources which must be managed.

Syntax

Main article: Java syntax

The syntax of Java is largely derived from C++. But unlike C++, which combines the syntax for structured, generic, and object-oriented programming, Java was built from the ground up to be fully object-oriented. Everything in Java is an object (with a few exceptions), and everything in Java is written inside a class.

Hello world

For an explanation of the tradition of programming "Hello World", see Hello world program.

Stand-alone application

// Hello.java
public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, world!"); 
    } 
}

The above example merits a bit of explanation.

  • Everything in Java is written inside a class, including stand-alone programs.
  • Source files are by convention named the same as the class they contain, appending the mandatory suffix .java. A class which is declared public is required to follow this convention. (In this case, the class is Hello, therefore the source must be stored in a file called Hello.java).
  • 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.
  • Programs to be executed as stand-alone must have a main() method.
  • The keyword void indicates that the main method does not return anything.
  • 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.
  • The keyword static indicates that the method is a class method, associated with the class rather than object instances. Main methods must be static.
  • 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. Main methods must also be public.
  • The printing facility is part of the 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 the method println(String) for displaying data to the screen (standard out).
  • Standalone programs are run by giving the Java runtime the name of the class whose main method is to be invoked. For example, at a Unix command line java -cp . Hello will start the above program (compiled into Hello.class) from the current directory. The name of the class whose main method is to be invoked can also be specified in the MANIFEST of a Java archive (Jar) file.

Applet

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

// Hello.java
import java.applet.Applet;
import java.awt.Graphics;

public class Hello extends Applet {
    public void paint(Graphics gc) {
        gc.drawString("Hello, world!", 65, 95);
    }    
}

<!-- Hello.html -->
<html>
  <head>
    <title>Hello World Applet</title>
  </head>
  <body>
    <applet code="Hello" width="200" height="200">
    </applet>
  </body>
</html>

The import statements direct the Java compiler to include the java.applet.Applet 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. Applet) instead of the fully-qualified class name (i.e. java.applet.Applet).

The Hello class 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) 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 paint(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 paint() method calls the graphic context drawString(String, int, int) method to display the "Hello, world!" string at a pixel offset of (65, 95) 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 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 object or embed element, although support for these elements by Web browsers is inconsistent.[4][5])

Servlet

Java servlets are server-side Java EE components that generate responses to requests from clients. 

// 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");
        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.

Swing application

Swing is the advanced graphical user interface library for the Java SE platform. 

// Hello.java
import javax.swing.*;

public class Hello extends JFrame {
    Hello() {
        setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
        add(new JLabel("Hello, world!"));
        pack();
    }

    public static void main(String[] args) {
        new Hello().setVisible(true);
    }
}

The import statement directs the Java compiler to include all of the public classes and interfaces from the javax.swing package in the compilation.

The Hello class extends the JFrame class; the JFrame class implements a window with a title bar with a close control.

The Hello() constructor initializes the frame by first calling the setDefaultCloseOperation(int) method inherited from JFrame to set the default operation when the close control on the title bar is selected to WindowConstants.DISPOSE_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 a new 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 layout its contents.

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. 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.

Criticism

Java was intended to serve as a novel way to manage software complexity. Most consider Java technology to deliver reasonably well on this promise. However, Java is not without flaws, and it does not universally accommodate all programming styles, environments, or requirements.

General

  • Java failed to deliver industry standard arithmetic capabilities. The IEEE 754 Standard for Binary Floating-Point Arithmetic had appeared in 1985 and has remained an industry standard since then. While Java's floating point arithmetic is largely based on the standard, certain features are not supported. Details about this kind of criticism can be found below in the external links section.
  • Java's semi-proprietary nature, supposed inflexibility to change, and growing entrenchment in the corporate sector, have caused some to refer to Java as "the new COBOL". Many consider this to be a somewhat hyperbolic assertion, although it does allude to some legitimate concerns with Java's prospects for the future. A more appropriate comparison might be ALGOL 60, the Java of its day back in the mainframe era, which lost its way after it over-inflated into ALGOL 68.
  • Java's garbage collection manages only memory resources, and the time at which finalizers are called cannot be controlled by the programmer. Therefore objects that reserve external resources must still be disposed of manually (sometimes using the "finally" mechanism), or kept until the program terminates.

Language

  • Java can be considered a less pure object-oriented programming language than for instance Ruby or Smalltalk because it makes certain compromises (such as the fact that not all values are objects) for performance reasons. The distinction between reference types and primitive types may be viewed as artificial.
  • Conversely, C++ programmers can become confused with Java because in Java primitives are always automatic variables and objects always reside on the heap, whereas C++ programmers are explicitly given the choice in both cases by means of operator new.
  • Java code is often more verbose than code written in other languages. This is due in part to its frequent type declarations and manual casts. It is also due to the lack of operator overloading (see below) and the relatively simple syntax. However, J2SE 5.0 introduced Generics which addresses the issue of manual casts, as well as new syntax for foreach-loops which also reduces verbosity.
  • Unlike C++, Java does not support user-definable operator overloading; however, this is a deliberate design decision, seen by many as a plus because of the well-known readability and maintenance problems experienced with operator overloading.
  • Java is predominantly a single-paradigm language. Historically, it has not been very accommodating of paradigms other than object-oriented programming. As of J2SE 5.0, the procedural paradigm is somewhat better supported in Java with the addition of the ability to import static methods and fields so that they can be referenced without prepending the class name.
  • Java's support of text matching and manipulation is not as strong as languages such as Perl, Ruby, or PHP, although regular expressions were introduced in J2SE 1.4.

Look and feel

The look and feel of GUI applications written in Java using the Swing platform is often different from native applications. While programmers can choose to use the AWT toolkit that displays native widgets (and thus look like the operating platform), the AWT toolkit is unable to meet advanced GUI programming needs by wrapping around advanced widgets and not sacrificing portability across the various supported platforms, each of which have vastly different APIs especially for higher-level widgets. The Swing toolkit, written completely in Java, avoids this problem by reimplementing widgets using only the most basic drawing mechanisms that are guaranteed available on all platforms. The drawback is that extra effort is required to resemble the operating platform. While this is possible (using the GTK+ and Windows Look-and-Feel), most users do not know how to change the default Metal Look-And-Feel to one that resembles their native platform, and as a result they are stuck with Java applications that look and feel different from their native applications. Of note however, Apple Computer's own optimized version of the Java Runtime, which is included within the Mac OS X distribution, by default implements its "Aqua" look-and-feel, giving Swing applications on the Macintosh a similar appearance to native software.

Performance

It is misleading to make any generalization about the performance of Java programs, because runtime performance is affected much more by the quality of the compiler or JVM than by any intrinsic properties of the language itself. Java bytecode can either be interpreted at run time by a virtual machine, or can be compiled at load time or runtime into machine code which runs directly on the computer's hardware. Interpretation is slower than native execution, and compilation at load time or runtime has an initial performance penalty for the compilation.

There are a few language requirements which incur an unavoidable time penalty, although these features are not unique to Java. Among these are array bounds checking, run-time type checking, and virtual function indirection.

The use of a garbage collector to automatically delete objects adds overhead compared to manual deallocation can have a positive or negative impact, or no discernable impact at all, on performance depending upon the garbage collector implementation and the characteristics of the application's use of objects. With the modern generational garbage collectors used in many JVMs, many applications actually experience greater performance because of faster allocation and deallocation algorithms.

Relative performance of JIT compilers as compared to native compilers can be quite close, and is often a subject of debate. The JIT compilation stage may be time consuming, which is inconvenient for applications that are short-lived and/or contain large amounts of code. Once compiled to native code, however, the performance of the program can be comparable to that achieved by a native compiler, even on numerical tasks. Although Java does not support manual inlining of method calls, many JIT compilers perform this optimization at load time and can exploit information from the runtime environment to guide more effective transformations, such as profile-directed inlining. Dynamic recompilation, as provided by Sun's HotSpot JVM, can exceed the performance of the static compilation available in most other languages by exploiting information that is only available at runtime.

Java was designed with an emphasis on security and portability, and does not support direct access to the machine architecture and address space. Java does not support inline assembly language, however, applications can drop down to native code to access these features using Java Native Interface (JNI) libraries.

Resources

Java Runtime Environment

The Java Runtime Environment or JRE is the software required to run any application deployed on the Java Platform. End-users commonly use a JRE in software packages and Web browser plugins. Sun also distributes a superset of the JRE called the Java 2 SDK (more commonly known as the JDK), which includes development tools such as the Java compiler, Javadoc, and debugger.

Components

APIs

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.

In 2004, IBM and BEA publicly supported the notion of creating an official open source implementation of Java but as of 2006, Sun Microsystems has refused that.

The extensions to standard Java are typically in javax.* packages. They are not included in the JDK or JRE. Extensions and architectures closely tied to the Java programming language include:

  • Java EE (previously J2EE) (Java Platform, Enterprise Edition—for distributed enterprise applications)
  • Java ME (previously J2ME) (Java Platform, Micro Edition—for PDAs and cellular phones)
  • JMF (Java Media Framework)
  • JNDI (Java Naming and Directory Interface)
  • JSML (Java Speech API Markup Language)
  • JDBC (Java Database Connectivity)
  • JDO (Java Data Objects)
  • JAI (Java Advanced Imaging)
  • JAIN (Java API for Integrated Networks)
  • JDMK (Java Dynamic Management Kit)
  • Jini (a network architecture for the construction of federated distributed systems)
  • Jiro
  • Java Card
  • JavaSpaces
  • JML (Java Modeling Language)
  • JMI (Java Metadata Interface)
  • JMX (Java Management Extensions)
  • JSP (JavaServer Pages)
  • JSF (JavaServer Faces)
  • JNI (Java Native Interface)
  • JXTA (Open Protocols for Peer-to-Peer (P2P) Virtual Network)
  • Java 3D (A high level API for 3D graphics programming)
  • JOGL (Java OpenGL—A low level API for 3D graphics programming, using OpenGL)
  • LWJGL (Light Weight Java Game Library—A low level API providing access to OpenGL, OpenAL and various input devices)
  • OSGi (Dynamic Service Management and Remote Maintenance)

See also

Lists

References

Notes

  1. ^ The device was named Star7 after a telephone feature activated by *7 on a telephone keypad, which enabled users to answer the telephone anywhere.
The Wikibook Programming has a page on the topic of: Java
The Wikibook Java Programming has a page on the topic of: Java

Sun

Java Specification Requests

There are several JSRs related to the Java Language and core API packages.

Tutorials

Resources

Java IDEs

History

Alternatives

Criticism

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