Introduction to Java

History

• Java is a programming language created by James Gosling from Sun Microsystems in 1991. The first publicly available version of Java (Java 1.0) was released in 1995.
• Over time new enhanced versions of Java have been released. The current version of Java is Java 1.7 which is also known as Java 7.
• From the Java programming language the Java platform evolved. The Java platform allows that the program code is written in other languages than the Java programming language and still runs on the Java virtual machine.
  

. Java Virtual machine

• The Java virtual machine (JVM) is a software implementation of a computer that executes programs like a real machine.
• The Java virtual machine is written specifically for a specific operating system, e.g. for Linux a special implementation is required as well as for Windows.
• Java programs are compiled by the Java compiler into so-called bytecode. The Java virtual machine interprets this bytecode and executes the Java program.
  

Java Runtime Environment vs. Java Development Kit

• Java comes in two flavors, the Java Runtime Environment (JRE) and the Java Development Kit (JDK).
• The Java runtime environment (JRE) consists of the JVM and the Java class libraries and contains the necessary functionality to start Java programs.
• The JDK contains in addition the development tools necessary to create Java programs. The JDK consists therefore of a Java compiler, the Java virtual machine, and the Java class libraries.
  

Characteristics of Java

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 on all supported 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 pre-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 the bytecode format which does not depend on the target platform. These bytecode instructions will be interpreted by the Java Virtual machine (JVM). The JVM contains a so called Hotspot-Compiler which translates performance critical bytecode instructions into native code instructions.

• 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 objects to which no active pointer exists.

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

Development Process with Java

The programmer writes Java source code in a text editor which supports plain text. Normally the programmer uses an Integrated Development Environment (IDE) for programming. An IDE supports the programmer in the task of writing code, e.g. it provides auto-formating 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 the bytecode instructions. . These instructions are stored in .class files and can be executed by the Java Virtual Machine.

Classpath

The classpath defines where the Java compiler and Java runtime look for .class files to load. This instructions can be used in the Java program.

For example if you want to use an external Java library you have to add this library to your classpath to use it in your program.

Your first Java program

Write source code

The following Java program is developed under Microsoft Windows. The process on other operating system should be similar but is not covered in this description.

Select a new directory which will contain your Java code. I will use the c:\temp\java which will be called javadir in the following description.

Open a text editor which supports plain text, e.g. Notepad under Windows and write the following source code. You can start Notepad via Start → Run → Notepad and by pressing enter.

</p><pre class="programlisting"><em class="hl-comment" style="color: #080">// A small Java program </em> <span class="hl-keyword">public</span> <span class="hl-keyword">class</span> HelloWorld { <span class="hl-keyword">public</span> <span class="hl-keyword">static</span> <span class="hl-keyword">void</span> main(String[] args) { System.out.println(<span class="hl-string" style="color:blue">"Hello World"</span>); } } </pre><p>

Save the source code in your javadir directory with the HelloWorld.java filename. The name of a Java source file must always equals the class name (within the source code) and end with the .java extension. In this example the filename must be HelloWorld.java because the class is called HelloWorld.