1.1. History
Java is a programming language created by James Gosling from Sun Microsystems in 1991. The first public available version of Java (Java 1.0) was released 1995. Over time several version of Java were release which improved and enhanced the language and it libraries. The current version of Java is Java 1.6 also known as Java 6.0. Java is a de facto standard that is controlled through a standard process, called the Java Community Process.
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.
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 runtime environment (JRE) consists of the JVM and the Java class libraries.
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 program do in general not access directly system resources but use the Java virtual machine as abstraction. 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. types of the elements must be pre-defined and conversion to other objects is relatively restricted.
- Interpreted and compiled language: Java source code is transfered 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 myValue and myvalue will be treated as different variables.
The programmer writes java source code which can be done in any text editor which supports plain text as output. Normally the programmer uses an IDE (integrated development environment) for programming. An IDE support usually the programmer in his tasks, 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. The java compiler creates platform independent code which is called bytecode.
Bytecode can be executed by the java runtime environment. The java runtime environment (JRE) is a program which knows how to run the bytecode on the operating system. The JRE translates the bytecode into native code, e.g. the native code for Linux is different then the native code for Windows and executes it.
The classpath is the connection between the Java compiler and Java interpreter. It defines where the compiler and interpreter look for .class files to load.
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
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