Java

A little Bit of History

Java was created in 1991 by James Gosling et al. of Sun Microsystems. Initially called Oak,  in honor  of  the tree outside Gosling's  window,  its  name was changed to Java because there was already a language called Oak.

The original motivation for Java was the need for platform independent language that could  be  embedded  in  various  consumer  electronic  products  like  toasters  and refrigerators. One of the first projects developed using Java was a personal hand-held remote control named Star 7.

At about the same time, the World Wide Web and the Internet were gaining popularity. Gosling et. al. realized that Java could be used for Internet programming.

What is Java Technology?

A programming language

As a programming language, Java can create all kinds of applications that you could create using any conventional programming language.

A development environment

As a  development environment, Java technology provides you with a large suite of tools: a compiler, an interpreter, a documentation generator, a class file packaging tool, and so on.

An application environment

Java technology applications  are typically general-purpose programs that  run on any machine where the Java runtime environment (JRE) is installed.

A deployment environment

There are two main deployment environments: First, the JRE supplied by the Java 2 Software Development Kit (SDK) contains the complete set of class files for all the Java technology packages, which includes basic language classes, GUI component classes, and so  on.  The  other  main  deployment  environment  is  on  your  web  browser.  Most commercial browsers supply a Java technology interpreter and runtime environment.

Some Features of Java

The Java Virtual Machine

The Java Virtual Machine is an imaginary machine that is implemented by emulating software on a real machine. The JVM provides the hardware platform specifications to which you compile all Java technology code. This specification enables the Java software to be platform-independent because the compilation is done for a generic machine known as the JVM.

A bytecode is a special machine language that can be understood by the Java Virtual Machine (JVM). The bytecode is independent of any particular computer hardware, so any computer with a Java interpreter can execute the compiled Java program, no matter what type of computer the program was compiled on.

Garbage Collection

Many programming languages allows a programmer to allocate memory during runtime. However, after using that allocated memory, there should be a way to deallocate that memory  block  in  order  for  other  programs  to  use  it  again.  In  C,  C++ and  other languages the programmer is responsible for this. This can be difficult  at times since there  can  be  instances  wherein  the  programmers  forget  to  deallocate  memory  and therefor result to what we call memory leaks.

In Java, the programmer is freed from the burden of having to deallocate that memory themselves  by  having  what  we  call  the  garbage  collection  thread.  The  garbage collection thread is responsible for freeing any memory that can be freed. This happens automatically during the lifetime of the Java program.

Code Security

Code security  is  attained in Java  through  the  implementation of  its  Java Runtime Environment (JRE). The JRE runs code compiled for a JVM and performs class loading (through the class loader), code verification (through the bytecode verifier) and finally code execution.

The Class Loader is responsible for loading all classes needed for the Java program. It adds security by separating the namespaces for the classes of the local file system from those that are imported from network sources. This limits any Trojan horse applications since local  classes are always loaded first.  After  loading all  the classes,  the memory layout of the executable is then determined. This adds protection against unauthorized access to restricted areas of  the code since the memory layout  is determined during runtime.

After loading the class and layouting of memory, the bytecode verifier then tests the format of the code fragments and checks the code fragments for illegal code that can violate access rights to objects.

After all of these have been done, the code is then finally executed.

The first step in creating a Java program is by writing your programs in a text editor.  Examples of text editors you can use are notepad, vi, emacs, etc. This file is stored in a disk file with the extension .java.

After creating and saving your Java program, compile the program by using the Java Compiler. The output of this process is a file of Java bytecodes with the file extension .class.

The .class file is then interpreted by the Java interpreter that converts the bytecodes into the machine language of the particular computer you are using.

Resource  : Meruvian Education