The JRE is a collection of software that allows a computer system to run a Java application. The software collection consists of the Java Virtual Machines (JVMs) that interpret Java bytecode into machine code, standard class libraries, user interface toolkits, and a variety of utilities.

The JDK is a programming environment for compiling, debugging, and running Java applications, Java Beans, and Java applets. The JDK includes the JRE with the addition of the Java Programming Language and additional development tools and tool APIs. Oracle’s JDK supports Mac OS X, Solaris, Linux (Oracle, Suse, Red Hat, Ubuntu, and Debian (on ARM)), and Microsoft Windows (Server 2008 R2, Server 2012, Vista, 7, and 8). Additional operating system and special purpose JVMs, JDKs, and JREs are freely available from Java Virtual Machine. Browsers supported are Internet Explorer 9+, Mozilla Firefox, Chrome on Windows and Safari 5.x.

Table 10-1 lists versions of the JDK provided by Oracle. Download the most recent version at Oracle’s website, where you can also download older versions.

Java SE version 6 reached Oracle’s End of Public Updates in March 2013.

Java source files are created with text editors such as jEdit, TextPad, Vim, Notepad++, or one provided by a Java Integrated Development Environment (IDE). The source files must have the .java extension and the same name as the public class name contained in the file. If the class has package-private access, the class name can differ from the filename.

Therefore, a source file named HelloWorld.java would correspond to the public class named HelloWorld, as represented in the following example (all nomenclature in Java is case-sensitive):

  1 package com.oreilly.tutorial;
  2 import java.time.*;
  3 // import java.time.ZoneId;;
  4 // import java.time.Clock;
  5
  6 public class HelloWorld
  7 {
  8   public static void main(String[] args)
  9   {
 10     ZoneId zi = ZoneId.systemDefault();
 11     Clock c = Clock.system(zi);
 12     System.out.print("From: "
 13       + c.getZone().getId());
 13     System.out.println(", "Hello, World!"");
 14   }
 15 }

In line 1, the class HelloWorld is contained in the package com.oreilly.tutorial. This package name implies that com/oreilly/tutorial is a directory structure that is accessible on the class path for the compiler and the runtime environment. Packaging source files is optional, but it is recommended to avoid conflicts with other software packages.

In line 2, the import declaration allows for the JVM to search for classes from other packages. Here, the asterisk allows for all classes in the java.time package to be made available. However, you should always explicitly include classes so that dependencies are documented; including the statements import java.time. ZoneId; and import java.time.Clock;, which as you see are currently commented out, would have been a better choice than simply using import java.time.*;. Note that import statements are not needed at all, as one may include the full package name before each class name, but this is not an ideal way to code.

In line 6, there must be only one top-level public class defined in a source file. In addition, the file may include multiple top-level package-private classes.

Looking at line 8, we note that Java applications must have a main method. This method is the entry point into a Java program, and it must be defined. The modifiers must be declared public, static, and void. The arguments parameter provides a string array of command-line arguments.

In lines 12 and 13, the statements provide calls to the System.out.print and System.out.println methods to print out the supplied text to the console window.

A JDK provides several command-line tools that aid in software development. Commonly used tools include the compiler, launcher/interpreter, archiver, and documenter. Find a complete list of tools at Oracle.com.

The classpath is an argument set, used by several command-line tools, that tells the JVM where to look for user-defined classes and packages. Classpath conventions differ among operating systems.

On Microsoft Windows operating systems, directories within paths are delineated with backslashes, and the semicolon is used to separate the paths:

-classpath homeXClasses;dirYClasses;.

On POSIX-compliant operations systems (e.g., Solaris, Linux, and Mac OS X), directories within paths are delineated with forward slashes and the colon is used to separate the paths:

-classpath /home/XClasses/:dir/YClasses/:.

The CLASSPATH environmental variable can also be set to tell the Java compiler where to look for class files and packages:

rem Windows
set CLASSPATH=classpath1;classpath2

# Linux, Solaris, Mac OS X
# (May vary due to shell specifics)
setenv CLASSPATH classpath1:classpath2