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Java Computer  Programming Language

Java is a Computer Programming Language that is concurrent, class-based, object-oriented, and specifically designed to have as few implementation dependencies as possible. It is intended to let application developers "write once, run anywhere" (WORA), meaning that code that runs on one platform does not need to be recompiled to run on another.

Java Applications are typically compiled to bytecode (class file) that can run on any Java Virtual Machine (JVM) regardless of computer architecture. Java is, as of 2014, one of the most popular programming languages in use, particularly for client-server web applications, with a reported 9 million developers.

Java was originally developed by James Gosling at Sun Microsystems (which has since merged into Oracle Corporation) and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++, but it has fewer low-level facilities than either of them.

The original and reference implementation Java compilers, virtual machines, and class libraries were developed by Sun from 1991 and first released in 1995. As of May 2007, in compliance with the specifications of the Java Community Process, Sun relicensed most of its Java technologies under the GNU General Public License. Others have also developed alternative implementations of these Sun technologies, such as the GNU Compiler for Java (bytecode compiler), GNU Classpath (standard libraries), and IcedTea-Web (browser plugin for applets).

Principles of Java

James Gosling, the creator of Java

There were five primary goals in the creation of the Java language:[23]

  • It should be "simple, object-oriented and familiar"
  • It should be "robust and secure"
  • It should be "architecture-neutral and portable"
  • It should execute with "high performance"
  • It should be "interpreted, threaded, and dynamic"

Java version history

Major release versions of Java, along with their release dates:

  • JDK 1.0 (January 21, 1996)
  • JDK 1.1 (February 19, 1997)
  • J2SE 1.2 (December 8, 1998)
  • J2SE 1.3 (May 8, 2000)
  • J2SE 1.4 (February 6, 2002)
  • J2SE 5.0 (September 30, 2004)
  • Java SE 6 (December 11, 2006)
  • Java SE 7 (July 28, 2011)
  • Java SE 8 (March 18, 2014)

Practices

Java Software platform and Java virtual Machine

One characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any hardware/operating-system platform. This is achieved by compiling the Java language code to an intermediate representation called Java bytecode, instead of directly to platform-specific machine code. Java bytecode instructions are analogous to machine code, but they are intended to be interpreted by a virtual machine (VM) written specifically for the host hardware. End-users commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a Web browser for Java applets.

Standardized libraries provide a generic way to access host-specific features such as graphics, threading, and networking.

A major benefit of using bytecode is porting. However, the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would. Just-in-Time (JIT) compilers were introduced from an early stage that compile bytecodes to machine code during runtime.

Java Implementations

Oracle Corporation is the current owner of the official implementation of the Java SE platform, following their acquisition of Sun Microsystems on January 27, 2010. This implementation is based on the original implementation of Java by Sun. The Oracle implementation is available for Mac OS X, Windows and Solaris. Because Java lacks any formal standardization recognized by Ecma International, ISO/IEC, ANSI, or other third-party standards organization, the Oracle implementation is the de facto standard.

The Oracle implementation is packaged into two different distributions: The Java Runtime Environment (JRE) which contains the parts of the Java SE platform required to run Java programs and is intended for end-users, and the Java Development Kit (JDK), which is intended for software developers and includes development tools such as the Java compiler, Javadoc, Jar, and a debugger.

OpenJDK is another notable Java SE implementation that is licensed under the GPL. The implementation started when Sun began releasing the Java source code under the GPL. As of Java SE 7, OpenJDK is the official Java reference implementation.

The goal of Java is to make all implementations of Java compatible. Historically, Sun's trademark license for usage of the Java brand insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support RMI or JNI and had added platform-specific features of their own. Sun sued in 1997, and in 2001 won a settlement of US$20 million, as well as a court order enforcing the terms of the license from Sun. As a result, Microsoft no longer ships Windows with Java.

Platform-independent Java is essential to Java EE, and an even more rigorous validation is required to certify an implementation. This environment enables portable server-side applications.

Java Performance

Programs written in Java have a reputation for being slower and requiring more memory than those written in C++.[25][26] However, Java programs' execution speed improved significantly with the introduction of Just-in-time compilation in 1997/1998 for Java 1.1,[27] the addition of language features supporting better code analysis (such as inner classes, the StringBuilder class, optional assertions, etc.), and optimizations in the Java virtual machine itself, such as HotSpot becoming the default for Sun's JVM in 2000.

Some platforms offer direct hardware support for Java; there are microcontrollers that can run Java in hardware instead of a software Java virtual machine, and ARM based processors can have hardware support for executing Java bytecode through their Jazelle option.

Java Automatic Memory Management

Java uses an automatic garbage collector to manage memory in the object lifecycle. The programmer determines when objects are created, and the Java runtime is responsible for recovering the memory once objects are no longer in use. Once no references to an object remain, the unreachable memory becomes eligible to be freed automatically by the garbage collector. Something similar to a memory leak may still occur if a programmer's code holds a reference to an object that is no longer needed, typically when objects that are no longer needed are stored in containers that are still in use. If methods for a nonexistent object are called, a "null pointer exception" is thrown.

One of the ideas behind Java's automatic memory management model is that programmers can be spared the burden of having to perform manual memory management. In some languages, memory for the creation of objects is implicitly allocated on the stack, or explicitly allocated and deallocated from the heap. In the latter case the responsibility of managing memory resides with the programmer. If the program does not deallocate an object, 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. Note that garbage collection does not prevent "logical" memory leaks, i.e. those where the memory is still referenced but never used.

Garbage collection may happen at any time. Ideally, it will occur when a program is idle. It is guaranteed to be triggered if there is insufficient free memory on the heap to allocate a new object; this can cause a program to stall momentarily. Explicit memory management is not possible in Java.

Java does not support C/C++ style pointer arithmetic, where object addresses and unsigned integers (usually long integers) can be used interchangeably. This allows the garbage collector to relocate referenced objects and ensures type safety and security.

As in C++ and some other object-oriented languages, variables of Java's primitive data types are not objects. Values of primitive types are either stored directly in fields (for objects) or on the stack (for methods) rather than on the heap, as is commonly true for objects (but see escape analysis). This was a conscious decision by Java's designers for performance reasons. Because of this, Java was not considered to be a pure object-oriented programming language. However, as of Java 5.0, autoboxing enables programmers to proceed as if primitive types were instances of their wrapper class.

Java contains multiple types of garbage collectors. By default, HotSpot uses the concurrent mark sweep collector, also known as the CMS garbage collector or CMS. However, there are also several other garbage collectors that can be used to manage the heap. For 90% of applications in Java, the CMS garbage collector is sufficient. Oracle aims to replace CMS with the Garbage-first collector (G1).

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 primitive data types (i.e. integers, floating-point numbers, boolean values, and characters), which are not classes for performance reasons.

Unlike C++, Java does not support operator overloading or multiple inheritance for classes. This simplifies the language and aids in preventing potential errors and anti-pattern design.

Java uses similar commenting methods to C++. There are three different styles of comments: a single line style marked with two slashes (//), a multiple line style opened with /* and closed with */, and the Javadoc commenting style opened with /** and closed with */. The Javadoc style of commenting allows the user to run the Javadoc executable to compile documentation for the program.

Example:

// This is an example of a single line comment using two slashes 

/* This is an example of a multiple line comment using the slash and asterisk.

 This type of comment can be used to hold a lot of information or deactivate

 code, but it is very important to remember to close the comment. */ 

/**

 * This is an example of a Javadoc comment; Javadoc can compile documentation

 * from this text.

 */ 

/** Finally, an example of a method written in Java, wrapped in a class. */

package fibsandlies;

import java.util.HashMap; 

public class FibCalculator extends Fibonacci implements Calculator {

    private static HashMap<Integer, Integer> memoized = new HashMap<Integer, Integer>();

    static {

        memoized.put(1, 1);

        memoized.put(2, 1);

    } 

    /** Given a non-negative number FIBINDEX, returns

     *  the Nth Fibonacci number, where N equals FIBINDEX.

     *  @param fibIndex The index of the Fibonacci number

     *  @return The Fibonacci number itself

     */

    @Override

    public static int fibonacci(int fibIndex) {

        if (memoized.containsKey(fibIndex)) {

            return memoized.get(fibIndex);

        } else {

            int answer = fibonacci(fibIndex - 1) + fibonacci(fibIndex - 2);

            memoized.put(fibIndex, answer);

            return answer;

        }

    }

Java Special Classes

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(final 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 (additionally indicated with the annotation, supported as of JDK 1.5, Override) inherited from the Container superclass to provide the code to display the applet. The paintComponent() method is passed a Graphics object that contains the graphic context used to display the applet. The paintComponent() method calls the graphic context drawString(String, int, int) method to display the "Hello, world!" string at a pixel offset of (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.class" 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,[32] although support for these elements by Web browsers is inconsistent.[33] However, the applet tag is deprecated, so the object tag is preferred where supported. 

The host application, typically a Web browser, instantiates the Hello applet and creates an AppletContext for the applet. Once the applet has initialized itself, it is added to the AWT display hierarchy. The paintComponent() method is called by the AWT event dispatching thread whenever the display needs the applet to draw itself.

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(final ServletRequest request, final ServletResponse response)

    throws ServletException, IOException {

        response.setContentType("text/html");

        final PrintWriter pw = response.getWriter();

        try {

            pw.println("Hello, world!");

        } finally {

            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. Packages make Java well suited for large scale applications.

The Hello class extends the GenericServlet class; the GenericServlet class provides the interface for the server to forward requests to the servlet and control the servlet's lifecycle.

The Hello class overrides the service(ServletRequest, ServletResponse) method defined by 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.

Java Server Pages

JavaServer Pages (JSP) 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 Java Application

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 GUI 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 javax.swing.*;

 

public class Hello extends JFrame {

    public Hello() {

        super("hello");

        super.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

        super.add(new JLabel("Hello, world!"));

        super.pack();

        super.setVisible(true);

    }

 

    public static void main(final String[] args) {

        new Hello();

    }

}

The first 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 Java virtual machine to exit and the program to terminate. Next, 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.

The main() method is called by the Java virtual machine 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 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

bject, 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. In addition to enabling more efficient code, certain runtime exceptions are converted to compile-time errors, a characteristic known as type safety.

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