Introduction

Java is a computer programming language, which was invented by Sun Microsystems and now has been merged into Oracle Corporation. Java was originally designed by a Canadian computer scientist James Gosling when he worked at Sun Microsystems.

Java is a distinguished language due to many features that other languages do not have. It is a static language but it does not run by the native code, like C or C++. It is interpreted at runtime but it is not dynamic typed like Perl, Python, PHP, etc. It is compiled into a platform neutral byte code, which is then interpreted by the Java Virtual Machine (JVM).

+-------+    +--------+    +-----+
| .java | -> | .class | -> | JVM |
+-------+    +--------+    +-----+

Setup JDK

JDK, which is short of Java Development Kit, is a set of tools used to develop Java programs. Before one can compile and run a Java program, the JDK should be installed. Here there are two commands users should get familiar:

1. javac

   javac is a program which compiles the Java source code into byte code.

2. java

   java is a program which start up a JVM and run Java byte code.

So the above figure should be like this:

+-------+             +--------+
| .java | -> javac -> | .class | -> java
+-------+             +--------+

Download and Installation

There are many distributions of JDK. The two representatives of them are from Oracle or OpenJDK project. They are free to download and use. For the convince, I assume the JDK is version 7 from Oracle in the following.

Setup System Environment Variables

This questions and answers give out a clear definition and usage about environment variables. Please refer them here.

For Windows users with GUI available, this post is more friendly.

Add JAVA_HOME variable. JAVA_HOME variable is used by many famous java programs such as maven, ant, hadoop, etc.

JAVA_HOME=< the folder path where you installed JDK >

Add the < the folder path where you installed JDK >\bin to your existing PATH. Please note that do not replace your PATH variable in case you will not run other programs.

Write your first Java program

Here is the first java program, which does nothing than print a line Hello, world! in the console.

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

The following are the steps of building and run this program:

1. Save the above code into a file named HelloWorldApp.java
2. Open your console and go to the folder contains HelloWorldApp.java
3. Compile the code by javac HelloWorldApp.java, then if successful, there should be a file named HelloWorldApp.class
4. Run the code by java HelloWorldApp
5. The program should print Hello, world! on your screen.

One step further

In the above code, HelloWorldApp is the class name. It should also be the file name, since it is the top level public class in the file. The method public static void main is the entry method for a Java program and the method arguments is String[] args, in which String[] refers the type as an array of String and args is the variable name. This arguments could be given from the command line when starting the program, like java HelloWorldApp XXX.

So let’s change the source code like this:

public class HelloWorldApp {
    public static void main(String[] args) {
        System.out.println("Hello, " + args[0]);
    }
}

Repeat the above steps of building this program: javac HelloWorldApp.java. Run the program: java HelloWorldApp XXX. Then, the program should print Hello, XXX on your screen.

Practice

Write your third program which could print a figure in the console like the following:

         *
        ***
       *****
      *******
     *********
    ***********
   *************
  ***************
 *****************
*******************