Friday, July 18, 2008

Java

History


The Java language was created by James Gosling in June 1991 for use in one of his many set-top box projects.[4] The language was initially called Oak, after an oak tree that stood outside Gosling's office—and also went by the name Green—and ended up later being renamed to Java, from a list of random words.[5] Gosling's goals were to implement a virtual machine and a language that had a familiar C/C++ style of notation.[6] The first public implementation was Java 1.0 in 1995. It promised "Write Once, Run Anywhere" (WORA), providing no-cost runtimes on popular platforms. It was fairly secure and its security was configurable, allowing network and file access to be restricted. Major web browsers soon incorporated the ability to run secure Java applets within web pages. Java quickly became popular. With the advent of Java 2, new versions had multiple configurations built for different types of platforms. For example, J2EE was for enterprise applications and the greatly stripped down version J2ME was for mobile applications. J2SE was the designation for the Standard Edition. In 2006, for marketing purposes, new J2 versions were renamed 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.[7][8][9] Java remains a de facto standard that is controlled through the Java Community Process.[10] At one time, Sun made most of its Java implementations available without charge although they were proprietary software. Sun's revenue from Java was generated by 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) that is a subset of the SDK, the primary distinction being that in the JRE, the compiler, utility programs, and many necessary header files are not present.

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.[11]



Philosophy

[edit] Primary goals

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

  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 by selecting what were considered the good parts of other object-oriented languages.

[edit] Platform independence

Main article: Java Platform

One characteristic, platform independence, means that 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 most Java compilers by compiling the Java language code halfway (to 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. (In some JVM versions, bytecode can also be compiled to native code, either before or during program execution, resulting in faster execution.) 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 is an explicit compiling stage, at some point, the Java bytecode is interpreted or converted to native machine code by the JIT compiler.

The first implementations of the language used an interpreted virtual machine to achieve portability. These implementations produced programs that ran slower than programs compiled to native executables, for instance 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.

One technique, known as just-in-time compilation (JIT), translates the Java bytecode into native code at the time that the program is run, which results in a program that executes faster than interpreted code but also incurs compilation overhead during execution. More sophisticated VMs use dynamic recompilation, in which the VM can analyze the behavior of the running program and selectively recompile and optimize 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, and can identify the 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 take advantage of the speed of native code without losing portability.

Another technique, commonly known as static compilation, is to compile directly into native code like a more traditional compiler. Static Java compilers, such as GCJ, translate the Java language code to native object code, removing the intermediate bytecode stage. This achieves good performance compared to interpretation, but at the expense of portability; the output of these compilers can only be run on a single architecture. Some see avoiding the VM in this manner as defeating the point of developing in Java; however it can be useful to provide both a generic bytecode version, as well as an optimised native code version of an application.

[edit] Implementations

Sun Microsystems officially licenses the Java Standard Edition platform for Microsoft Windows, Linux, and Solaris. Through a network of third-party vendors and licensees[13], alternative Java environments are available for these and other platforms. To qualify as a certified Java licensee, an implementation on any particular platform must pass a rigorous suite of validation and compatibility tests. This method enables a guaranteed level of compliance and platform through a trusted set of commercial and non-commercial partners.

Sun's trademark license for usage of the Java brand insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support the RMI and JNI interfaces 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.[14] 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. However, Sun and others have made available Java run-time systems at no cost 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.

[edit] Automatic memory management

See also: Garbage collection (computer science)

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 the programmer allocates memory for the creation of objects stored on the heap and the responsibility of later deallocating that memory also resides with the programmer. If the programmer forgets to deallocate memory or writes code that fails to do so, a memory leak occurs and the program can consume an arbitrarily large amount of memory. Additionally, if the program attempts to deallocate the region of memory more than once, the result is undefined and the program may become unstable and may crash. Finally, in non garbage collected environments, there is a certain degree of overhead and complexity of user-code to track and finalize allocations. Often developers may box themselves into certain designs to provide reasonable assurances that memory leaks will not occur.[15]

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 object's 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 unreachable object is eligible for release by the Java garbage collector - it may be freed automatically by the garbage collector at any time. 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.

The use of garbage collection in a language can also affect programming paradigms. If, for example, the developer assumes that the cost of memory allocation/recollection is low, they may choose to more freely construct objects instead of pre-initializing, holding and reusing them. With the small cost of potential performance penalties (inner-loop construction of large/complex objects), this facilitates thread-isolation (no need to synchronize as different threads work on different object instances) and data-hiding. The use of transient immutable value-objects minimizes side-effect programming.

Comparing Java and C++, it is possible in C++ to implement similar functionality (for example, a memory management model for specific classes can be designed in C++ to improve speed and lower memory fragmentation considerably), with the possible cost of adding comparable runtime overhead to that of Java's garbage collector, and of added development time and application complexity if one favors manual implementation over using an existing third-party library. In Java, garbage collection is built-in and 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. Depending on intended application, this can be beneficial or disadvantageous: the programmer is freed from performing low-level tasks, but at the same time loses the option of writing lower level code. Additionally, the garbage collection capability demands some attention to tuning the JVM, as large heaps will cause apparently random stalls in performance.

Java does not support pointer arithmetic as is supported in, for example, C++. This is because the garbage collector may relocate referenced objects, invalidating such pointers. Another reason that Java forbids this is that type safety and security can no longer be guaranteed if arbitrary manipulation of pointers is allowed.

[edit] 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 exclusively as an object oriented language. As a result, almost everything is an object and all code is written inside a class. The exceptions are the intrinsic data types (ordinal and real numbers, boolean values, and characters), which are not classes for performance reasons.

[edit] Hello, world program

This is a minimal Hello world program in Java with syntax highlighting:

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

To execute a Java program, the code is saved as a file named Hello.java. It must first be compiled into bytecode using a Java compiler, which produces a file named Hello.class. This class is then launched.

The above example merits a bit of explanation.

  • All executable statements in Java are 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 that is declared public is required to follow this convention. (In this case, the class Hello is public, 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.
  • 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 keyword static indicates that the method is a static method, associated with the class rather than object instances.
  • The keyword void indicates that the main method does not return any value to the caller.
  • 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.
  • 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) 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.
  • 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 while creating a new line (standard out).

[edit] A more comprehensive example

// OddEven.java
import javax.swing.JOptionPane;

public class OddEven {
   public static void main(String[] args) {
       // This is the main method. It gets called when this class is run through a Java interpreter.
       OddEven number = new OddEven();
       /* This line of code creates a new instance of this class called "number" and
        * initializes it, and the next line of code calls the "showDialog()" method,
        * which brings up a prompt to ask you for a number
        */
       number.showDialog();
   }
   private int input; // A whole number("int" means integer)
       // "input" is the number that the user gives to the computer

   public OddEven() {
       /* This is the constructor method. It gets called when an object of the OddEven type
        * is created.
        */
   }

   public void showDialog() {
       try
       /* This makes sure nothing goes wrong. If something does,
        * the interpreter skips to "catch" to see what it should do.
        */
       {
               input = Integer.parseInt(JOptionPane.showInputDialog("Please Enter A Number"));
               calculate();
               /*
                * The code above 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."
                */
       }
       catch (NumberFormatException e)
       /* This means that there was a problem with the format of the number
        * (Like if someone were to type in 'Hello world' instead of a number).
        */
       {
               System.err.println("ERROR: Invalid input. Please type in a numerical value.");
       }
   }

   public void calculate() {
       if (input % 2 == 0)
               System.out.println("Even");
       /* 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)
        */
       else
               System.out.println("Odd");
   }
}
  • The import statement imports the JOptionPane class from the javax.swing package.
  • The OddEven class declares a single private field of type int named input. Every instance of the OddEven class has its own copy of the input field. The private declaration means that no other class can access (read or write) the input field.
  • OddEven() is a public constructor. Constructors have the same name as the enclosing class they are declared in, and unlike a method, have no return type. A constructor is used to initialize an object that is a newly created instance of the class. The dialog returns a String that is converted to an int by the Integer.parseInt(String) method.
  • The calculate() method is declared without the static keyword. This means that the method is invoked using a specific instance of the OddEven class. (The reference used to invoke the method is passed as an undeclared parameter of type OddEven named this.) The method tests the expression input % 2 == 0 using the if keyword to see if the remainder of dividing the input field belonging to the instance of the class by two is zero. If this expression is true, then it prints Even; if this expression is false it prints Odd. (The input field can be equivalently accessed as this.input, which explicitly uses the undeclared this parameter.)
  • OddEven number = new OddEven(); declares a local object reference variable in the main method named number. This variable can hold a reference to an object of type OddEven. The declaration initializes number by first creating an instance of the OddEven class, using the new keyword and the OddEven() constructor, and then assigning this instance to the variable.
  • The statement number.showDialog(); calls the calculate method. The instance of OddEven object referenced by the number local variable is used to invoke the method and passed as the undeclared this parameter to the calculate method.
  • For simplicity, error handling has been ignored in this example. Entering a value that is not a number will cause the program to crash. This can be avoided by catching and handling the NumberFormatException thrown by Integer.parseInt(String).

[edit] 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 java.applet.Applet;
import java.awt.Graphics;

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

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) from the upper-left corner in the applet's display.


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

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[16], although support for these elements by Web browsers is inconsistent.[17] 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.

[edit] 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.

[edit] 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.

[edit] Swing application

Main article: Swing (Java)

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

[edit] Criticism

Main article: Criticism of Java

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.[18][19][20] 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.

The default look and feel of GUI applications written in Java using the Swing toolkit is very different from native applications. 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,[citation needed] Swing in Java SE 6 addresses this problem by using more native widget drawing routines of the underlying platforms. Alternatively, third party toolkits such as wx4j, Qt Jambi or SWT may be used for increased integration with the native windowing system.

As in C++ and some other object-oriented languages, variables of Java's primitive types were not originally 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.

Java suppresses several features (such as operator overloading and multiple inheritance) for classes in order to simplify the language, to "save the programmers from themselves", and to prevent possible errors and anti-pattern design. This has been a source of criticism,[citation needed] relating to a lack of low-level features, but some of these limitations may be worked around. Java interfaces have always had multiple inheritance.

[edit] Resources

[edit] Java Runtime Environment

Main article: 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, Jar and debugger.

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.

[edit] Components

[edit] APIs

See also: Free Java implementations#Class library

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.

[edit] See also

[edit] Notes

  1. ^ "The Java Language Environment" (May 1996).
  2. ^ "The Java Language Specification, 2nd Edition".
  3. ^ 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]
  4. ^ Jon Byous, Java technology: The early years. Sun Developer Network, no date [ca. 1998]. Retrieved April 22, 2005.
  5. ^ http://blogs.sun.com/jonathan/entry/better_is_always_different.
  6. ^ Heinz Kabutz, Once Upon an Oak. Artima, Retrieved April 29, 2007.
  7. ^ Java Study Group
  8. ^ Why Java Was - Not - Standardized Twice
  9. ^ 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. ^ Java SE - Licensees
  14. ^ James Niccolai (January 23, 2001). "Sun, Microsoft settle Java lawsuit", JavaWorld, IDG. Retrieved on 2008-07-09.
  15. ^ Stroustrup: C++ Style and Technique FAQ
  16. ^ Using the applet Tag (The Java Tutorials > Deployment > Applets)
  17. ^ Deploying Applets in a Mixed-Browser Environment (The Java Tutorials > Deployment > Applets)
  18. ^ Performance of Java versus C++, J.P.Lewis and Ulrich Neumann, Computer Graphics and Immersive Technology Lab, University of Southern California
  19. ^ The Java is Faster than C++ and C++ Sucks Unbiased Benchmark
  20. ^ FreeTTS - A Performance Case Study, Willie Walker, Paul Lamere, Philip Kwok

[edit] References

[edit] External links

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