Introduction to Java

History
Java is a programming language created by James Gosling from Sun Micro systems in 1991. The first public available version of Java (Java 1.0) was released 1995. Over time several version of Java were released which enhanced the language and its libraries. The current version of Java is Java 1.6 also known as Java 6.0.
From the Java programming language the Java platform evolved. The Java platform allows that code is written in other languages then the Java programming language and still runs on the Java virtual machine.

Overview
The Java programming language consists out of a Java compiler, the Java virtual machine, and the Java class libraries. The Java virtual machine (JVM) is a software implementation of a computer that executes programs like a real machine.
The Java compiler translates Java coding into so-called byte-code. The Java virtual machine interprets this byte-code and runs the program.
The Java virtual machine is written specifically for a specific operating system.
The Java run time environment (JRE) consists of the JVM and the Java class libraries.


Characteristics

The target of Java is to write a program once and then run this program on multiple operating systems.
Java has the following properties:

Platform independent: Java programs use the Java virtual machine as abstraction and do not access the operating system directly. This makes Java programs highly portable. A Java program which is standard complaint and follows certain rules can run unmodified all several platforms, e.g. Windows or Linux.
Object-orientated programming language: Except the primitive data types, all elements in Java are objects.
Strongly-typed programming language: Java is strongly-typed, e.g. the types of the used variables must be per-defined and conversion to other objects is relatively strict, e.g. must be done in most cases by the programmer.
Interpreted and compiled language: Java source code is transferred into byte-code which does not depend on the target platform. This byte-code will be interpreted by the Java Virtual machine (JVM). The JVM contains a so called Hotspot-Compiler which translates critical byte-code into native code.
Automatic memory management: Java manages the memory allocation and de-allocation for creating new objects. The program does not have direct access to the memory. The so-called garbage collector deletes automatically object to which no active pointer exists.



The Java syntax is similar to C++. Java is case sensitive, e.g. the variables my Value and my value will be treated as different variables.


Development with Java

The programmer writes Java source code in an text editor which supports plain text. Normally the programmer uses an IDE (integrated development environment) for programming. An IDE support the programmer in the task of writing code, e.g. it provides auto-formatting of the source code, highlighting of the important keywords, etc.
At some point the programmer (or the IDE) calls the Java compiler (javac). The Java compiler creates platform independent code which is called byte code. This byte-code is stored in ".class" files.
Byte code can be executed by the Java run time environment. The Java run time environment (JRE) is a program which knows how to run the byte-code on the operating system. The JRE translates the byte code into native code and executes it, e.g. the native code for Linux is different then the native code for Windows.
By default, the compiler puts each class file in the same directory as its source file. You can specify a separate destination directory with -d