Historical Glimpses of Java
Java is a programming language created by James Gosling from Sun Microsystems (Sun) in 1991. The target of Java is to write a program once and then run this program on multiple operating systems. The first publicly available version of Java (Java 1.0) was released in 1995. Sun Microsystems was acquired by the Oracle Corporation in 2010.
Over time new enhanced versions of Java have been released. The current version of Java is Java 1.8 which is also known as Java 8.
Java is defined by a specification and consists of a programming language, a compiler, core libraries and a runtime (Java virtual machine) The Java runtime allows software developers to write program code in other languages than the Java programming language which still runs on the Java virtual machine. The Java platform is usually associated with the Java virtual machine and the Java core libraries.
The Java language was designed with 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-compliant 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 automatically deletes objects to which no active pointer exists.
The Java syntax is similar to C++. Java is case-sensitive, e.g., variables called myValue and myvalue are treated as different variables.
Java First Program
// a small Java program
Public class HelloWorld {
Public static void main (String [] args) {
System.out.println (“Hello World”);
}
}
Java Virtual Machine
Java programs are compiled by the Java compiler into bytecode. The Java virtual machine interprets this bytecode and executes the Java program.
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.
JRE vs JDK
A Java distribution typically comes in two flavors, the Java Runtime Environment (JRE) and the Java Development Kit (JDK). The JRE consists of the JVM and the Java class libraries. Those contain the necessary functionality to start Java programs. The JDK additionally contains the development tools necessary to create Java programs. The JDK therefore consists of a Java compiler, the Java virtual machine and the Java class libraries.
Development Process with Java
Java source files are written as plain text documents. The programmer typically writes Java source code in an Integrated Development Environment (IDE) for programming. An IDE supports 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 the bytecode instructions. These instructions are stored in .class files and can be executed by the Java Virtual Machine.
Garbage Collector
The JVM automatically re-collects the memory which is not referred to by other objects. The Java garbage collector checks all object references and finds the objects which can be automatically released. While the garbage collector relieves the programmer from the need to explicitly manage memory, the programmer still need to ensure that he does not keep unneeded object references, otherwise the garbage collector cannot release the associated memory. Keeping unneeded object references are typically called memory leaks.
Class Path
The class path defines where the Java compiler and Java runtime look for .class files to load. These 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.
