JDK vs. JRE

• JDK stands for Java Development Kit, which contains the compiler executable javac.exe. The java.exe executable compiles the source code (.java files) into an intermediate form called as byte code ( .class files). If a source file has more than one class, each class is compiled into a separate .class file. 

• JRE stands for Java Runtime Environment and contains the JVM (the interpreter java.exe executable). The java.exe executable reads the byte code (.class file), translates it into the native language of the host machine and execute it. 

Source - Compile - Execute flow

• A java program is written in a .java file, which is called as source code file.

• javac.exe (part of JDK) which compiles the source code source files (.java files) into a .class file.

• JVM java.exe (part of JDK and JRE) reads the byte code and translates it into the native language of the host machine on the fly, and execute.

First Java Program (Hello.java)

A class can have variables (properties) and methods (functions).

Let us write a simple class called Hello with a single method called main, in a file Hello.java:

public class Hello {

  public static void main(String[] args)

  {

     System.out.println("Hello");

  }

}

Save the file Hello.java. 

Note: Classes and methods are explained in other tutorials. The scope of this tutorial is only to execute and see the output to verify environment. 

Compiling the program

To compile source code file (Hello.java) from command line, go to command prompt and go to the folder where your .java file is present (e.g. using 'CD path' where path is the path to your folder.)

javac Hello.java

• A file called Hello.class will be created.

Executing the program

You can execute the class file (Hello.class) from command line as:

java Hello

• Don't mention .class.

• Rather than writing the code in command line, you can use an IDE like Eclipse for easy writing and execution. But during the initial learning days, I would suggest you use command line for a better understanding of the basics.