The Java platform consists of the Java application programming interfaces (APIs) and the Java 1 virtual machine (JVM).
Java APIs are libraries of compiled code that you can use in your programs. They let you add ready-made and customizable functionality to save you programming time.
Before you can write and run the simple Java program in this lesson, you need to install the Java platform on your computer system.
The Java platform is available free of charge from the Java web site. You can use the latest Java® 8 Platform software for Windows, Linux or for Solaris.
The download page contains the information you need to install and configure the Java platform for writing and running Java programs.
- Java SE: Java Platform, Standard Edition (Java SE) lets you develop and deploy Java applications on desktops and servers, as well as in today's demanding embedded environments. Java offers the rich user interface, performance, versatility, portability, and security that today's applications require.
- Java EE: Java Platform, Enterprise Edition (Java EE) is the standard in community-driven enterprise software. Java EE is developed using the Java Community Process, with contributions from industry experts, commercial and open source organizations, Java User Groups, and countless individuals. Each release integrates new features that align with industry needs, improves application portability, and increases developer productivity.Today, Java EE offers a rich enterprise software platform and with over 20 compliant Java EE implementations to choose from, low risk and plenty of options.
- Java ME: Java Platform, Micro Edition (Java ME) provides a robust, flexible environment for applications running on embedded and mobile devices in the Internet of Things: micro-controllers, sensors, gateways, mobile phones, personal digital assistants (PDAs), TV set-top boxes, printers and more. Java ME includes flexible user interfaces, robust security, built-in network protocols, and support for networked and offline applications that can be downloaded dynamically. Applications based on Java ME are portable across many devices, yet leverage each device's native capabilities.
Before you can write and run the simple Java program, you need to install the Java platform on your computer system.
Writing a Program
The easiest way to write a simple program is with a text editor. So, using the text editor of your choice, create a text file with the following text, and be sure to name the text file ExampleProgram.java. Java programs are case sensitive, so if you type the code in yourself, pay particular attention to the capitalization.
//A Very Simple Example
class ExampleProgram {
public static void main(String[] args){
System.out.println("I'm a Simple Program");
}
}
Here is the ExampleProgram.java source code file if you do not want to type the program text in yourself.
Compiling the Program
A program has to be converted to a form the Java VM can understand so any computer with a Java VM can interpret and run the program. Compiling a Java program means taking the programmer-readable text in your program file (also called source code) and converting it to bytecodes, which are platform-independent instructions for the Java VM.
The Java compiler is invoked at the command line on Unix and DOS shell operating systems as follows:
javac ExampleProgram.java
Interpreting and Running the Program
Once your program successfully compiles into Java bytecodes, you can interpret and run applications on any Java VM, or interpret and run applets in any Web browser with a Java VM built in such as Netscape or Internet Explorer. Interpreting and running a Java program means invoking the Java VM byte code interpreter, which converts the Java byte codes to platform-dependent machine codes so your computer can understand and run the program.
The Java interpreter is invoked at the command line on Unix and DOS shell operating systems as follows:
java ExampleProgram
At the command line, you should see:
I'm a Simple Program
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