Hello world

// Hello.java

public class Hello {

    public static void main(String[] args) {

        System.out.println("Hello, World!");

    }

}

 

 

 

 

 

 

To execute this program, one first saves the above code as a file named Hello.java. One then compiles it to bytecode:

$ javac Hello.java

which produces a file named Hello.class . This class is then launched with the java launcher (usually named java, java.exe, or some variant depending on the operating system).

$ java Hello

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.

· The keyword void indicates that the main method does not return any value to the caller.

· 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 keyword static indicates that the method is a static method, associated with the class rather than object instances.

· 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 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 containg any arguments passed to the class. The parameter to main are often passed by means of a command line.

· 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 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).

· 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.

 

An example that better demonstrates object-oriented programming:

// OddEven.java

import javax.swing.JOptionPane;

 

public class OddEven {

    private int input;

 

    public OddEven() {

        input = Integer.parseInt(JOptionPane.showInputDialog("Please Enter A Number"));

    }

       

    public void calculate() {

        if (input % 2 == 0)

            System.out.println("Even");

        else

            System.out.println("Odd");

    }

 

    public static void main(String[] args) {

        OddEven number = new OddEven();

        number.calculate();

    }

}

 

· 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. In this case, the constructor initializes the input field to the value entered into a JOptionPane input dialog. The dialog returns a String which 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 undeclare 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 instantiating an instance of the OddEven class using the new keyword and then calling the OddEven() constructor to initialize the newly created object.

· The statement number.calculate(); 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.

 

 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.

<!-- Hello.html -->

<html>

  <head>

    <title>Hello World Applet</title>

  </head>

  <body>

    <applet code="Hello" width="200" height="200">

    </applet>

  </body>

</html>

 

An applet is placed in an HTML document using the <applet> HTML element. The applet tag has three attributes set: code="Hello" specifies the name of the 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.However, the applet tag is deprecated, so the object tag is preferred where supported.

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

 Servlet

 

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");

       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 Generic Servlet 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 get Writer() method in the response returns a PrintWriter object that is used to write the data that is sent to the client. The println(String) method is called to write the "Hello, world!" string to the response and then the close() method is called to close the print writer, which causes the data that has been written to the stream to be returned to the client.

 

JavaServer Page

 

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. A JSP is compiled to a Java servlet the first time it is accessed. After that the generated servlet creates the response.

 

 

 

                         

 

 

 

 

 

                                                   

 

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