How do I create a Hello World Applet?

The code below demonstrate the very basic of Java applet. Applet is a small Java application that can be embedded on the web browser.

package org.kodejava.example.applet;

import java.applet.*;
import java.awt.*;

public class HelloWorldApplet extends Applet {
    public void init() {
    }

    public void start() {
    }

    public void stop() {
    }

    public void destroy() {
    }

    public void paint(Graphics g) {
        g.setColor(Color.GREEN);
        g.drawString("Hello World", 50, 100);
    }
}

To display the applet we need to create an HTML document. Here is a simple example of the document.

<html>
<head>
    <title>Hello World Applet</title>
</head>

<body>
    <applet 
        code="org.kodejava.example.applet.HelloWorldApplet" 
        height="250" 
        width="250">
    </applet>
</body>
</html>

You can now load the applet in your browser or by using the appletviewer utility.

How do I create a HelloWorld Servlet?

Servlet is a Java solution for creating a dynamic web application, it can be compared with the old CGI technology. Using Servlet we can create a web application that can display information from database, receive information from a web form to be stored on the application database.

This example show the very basic of servlet, it returns a hello world html document for the browser. At the very minimum a servlet will have a doPost() and doGet() method which handles the HTTP POST and GET request.

package org.kodejava.example.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

public class HelloWorld extends HttpServlet {

    public HelloWorld() {
        super();
    }

    protected void doGet(HttpServletRequest req, HttpServletResponse res) 
            throws ServletException, IOException {
        doPost(req, res);
    }

    protected void doPost(HttpServletRequest req, HttpServletResponse res) 
            throws ServletException, IOException {
        res.setContentType("text/html");

        PrintWriter writer = res.getWriter();
        writer.println("<html>");
        writer.println("<head><title>Hello World Servlet</title></head>");
        writer.println("<body>Hello World! How are you doing?</body>");
        writer.println("</html>");
        writer.close();
    }
}

For a servlet to works on a servlet container such as Apache Tomcat we need to add or register the servlet in the application’s web.xml file. This configuration tells the container about our servlet class and a url that maps a request to the servlet.

<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4"
         xmlns="http://java.sun.com/xml/ns/j2ee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
    <display-name>kodejava-example</display-name>
    <servlet>
        <description></description>
        <display-name>HelloWorld</display-name>
        <servlet-name>HelloWorld</servlet-name>
        <servlet-class>
            org.kodejava.example.servlet.HelloWorld
        </servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>HelloWorld</servlet-name>
        <url-pattern>/HelloWorld</url-pattern>
    </servlet-mapping>
</web-app>

When the servlet is deployed to the container we can access it from a url in a form of http://localhost:8080/app-name/HelloWorld.

How do I write Hello World program in Java?

Hello World is a classic program example to start with when we begin to learn a new programming language. The main purpose is to know the basic syntax of the language. Below is the Java version of a Hello World program, it simple enough to start.

package org.kodejava.example.basic;

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

The code contains one class called HelloWorld, we declare a class using the class keyword. A main(String[] args) method with a public static void signature and an argument of string array is a special method in Java class. This method is the execution entry point of every Java application. When a class have this method it will be executable. And finally in the body of the method we have a single line of code that write a Hello World string to the console.

The HelloWorld class must be saved in a file named HelloWorld.java, the class file name is case sensitive. In the example we also define a package name for the class. In this case the HelloWorld.java must be placed in a org.kodejava.example.basic directory.

To run the application we need to compile it first. I assume that you have your Java in your path. To compile it type:

javac -cp . org.kodejava.example.basic.HelloWorld.java

The compilation process will result a file called HelloWorld.class, this is the binary version of our program. As you can see that the file ends with .class extension because Java is everything about class. To run it type the command bellow, class name is written without it extension.

java -cp . org.kodejava.example.basic.HelloWorld

And you will see a string Hello World! is printed out in the console as the output of your first Java program, the HelloWorld program.

To install Java Development Kit (JDK) and configure Java environment you can see the following tutorial: