Tuesday, December 29, 2009

History of Java is a programming language


A programming language is an artificial language designed to express computations that can be performed by a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine, to express algorithms precisely, or as a mode of human communication.

Many programming languages have some form of written specification of their syntax (form) and semantics (meaning). Some languages are defined by a specification document. For example, the C programming language is specified by an ISO Standard. Other languages, such as Perl, have a dominant implementation that is used as a reference.

The earliest programming languages predate the invention of the computer, and were used to direct the behavior of machines such as Jacquard looms and player pianos. Thousands of different programming languages have been created, mainly in the computer field, with many more being created every year. Most programming languages describe computation in an imperative style, i.e., as a sequence of commands, although some languages, such as those that support functional programming or logic programming, use alternative forms of description.

History of Java Programming
James Gosling initiated the Java language project in June 1991 for use in one of his many set-top box projects.The language, initially called Oak after an oak tree that stood outside Gosling's office, also went by the name Green and ended up later renamed as Java, from a list of random words. Gosling aimed to implement a virtual machine and a language that had a familiar C/C++ style of notation. Sun Microsystems released the first public implementation as Java 1.0 in 1995. It promised "Write Once, Run Anywhere" (WORA), providing no-cost run-times on popular platforms. Fairly secure and featuring configurable security, it allowed network- and file-access restrictions. Major web browsers soon incorporated the ability to run Java applets within web pages, and Java quickly became popular. With the advent of Java 2 (released initially as J2SE 1.2 in December 1998), new versions had multiple configurations built for different types of platforms. For example, J2EE targeted enterprise applications and the greatly stripped-down version J2ME for mobile applications. J2SE designated the Standard Edition. In 2006, for marketing purposes, Sun renamed new J2 versions as Java EE, Java ME, and Java SE, respectively.

In 1997, Sun Microsystems approached the ISO/IEC JTC1 standards body and later the Ecma International to formalize Java, but it soon withdrew from the process. Java remains a de facto standard, controlled through the Java Community Process. At one time, Sun made most of its Java implementations available without charge, despite their proprietary software status. Sun generated revenue from Java through the selling of licenses for specialized products such as the Java Enterprise System. Sun distinguishes between its Software Development Kit (SDK) and Runtime Environment (JRE) (a subset of the SDK); the primary distinction involves the JRE's lack of the compiler, utility programs, and header files. On November 13, 2006, Sun released much of Java as open source software under the terms of the GNU General Public License (GPL). On May 8, 2007, Sun finished the process, making all of Java's core code available under free software/open-source distribution terms, aside from a small portion of code to which Sun did not hold the copyright.

Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities. Java applications are typically compiled to bytecode (class file) that can run on any Java Virtual Machine (JVM) regardless of computer architecture. This programming language is a general purpose concurrent class based object oriented programming language, specifically designed to have as few implementation dependencies as possible. It is intended to let application developers "write once, run anywhere".

The original and reference implementation Java compilers, virtual machines, and class libraries were developed by Sun from 1995. As of May 2007, in compliance with the specifications of the Java Community Process, Sun relicensed most of their Java technologies under the GNU General Public License. Others have also developed alternative implementations of these Sun technologies, such as the GNU Compiler for Java and GNU Classpath.

Sun Microsystems, Inc. (NASDAQ, JAVA) multinational vendor of computers, computer components, computer software, and information technology services, founded on February 24, 1982. The company is headquartered in Santa Clara, California (part of Silicon Valley), on the former west campus of the Agnews Developmental Center.

Products include computer servers and workstations based on its own SPARC processors as well as AMD's Opteron and Intel's Xeon processors; storage systems; and, a suite of software products including the Solaris operating system, developer tools, Web infrastructure software, and identity management applications. Other technologies of note include the Java platform, MySQL and NFS. Sun is a proponent of open systems in general and Unix in particular, and a major contributor to open source software.

Java refers to a number of computer software products and specifications from Sun Microsystems that together provide a system for developing application software and deploying it in a cross-platform environment. Java is used in a wide variety of computing platforms from embedded devices and mobile phones on the low end, to enterprise servers and supercomputers on the high end. Java is nearly ubiquitous in mobile phones, Web servers and enterprise applications, and while less common on desktop computers, Java applets are often used to provide improved and secure functionalities while browsing the World Wide Web.

Writing in the Java programming language is the primary way to produce code that will be deployed as Java bytecode, though there are bytecode compilers available for other languages such as JavaScript, Python, Ruby and Scala, and a native Java scripting language called Groovy. Java syntax borrows heavily from C and C++ but it eliminates certain low-level constructs such as pointers and has a very simple memory model where every object is allocated on the heap and all variables of object types are references. Memory management is handled through integrated automatic garbage collection performed by the Java Virtual Machine (JVM).

On November 13, 2006, Sun Microsystems made the bulk of its implementation of Java available under the GNU General Public License, although there are still a few parts distributed as precompiled binaries due to copyright issues with Sun-licensed (not owned) code.

An edition of the Java platform is the name for a bundle of related programs, or platform, from Sun which allow for developing and running programs written in the Java programming language. The platform is not specific to any one processor or operating system, but rather an execution engine (called a virtual machine) and a compiler with a set of standard libraries that are implemented for various hardware and operating systems so that Java programs can run identically on all of them.
  1. Java Card: refers to a technology that allows small Java-based applications (applets) to be run securely on smart cards and similar small memory footprint devices.
  2. Java ME (Micro Edition): Specifies several different sets of libraries (known as profiles) for devices which are sufficiently limited that supplying the full set of Java libraries would take up unacceptably large amounts of storage.
  3. Java SE (Standard Edition): For general purpose use on desktop PCs, servers and similar devices.
  4. Java EE (Enterprise Edition): Java SE plus various APIs useful for multi-tier client-server enterprise applications.
As of September 2009[update], the current version of the Java Platform is specified as either 1.6.0 or 6 (both refer to the same version). Version 6 is the product version, while 1.6.0 is the developer version.

The Java Platform consists of several programs, each of which provides a distinct portion of its overall capabilities. For example, the Java compiler, which converts Java source code into Java bytecode (an intermediate language for the Java Virtual Machine (JVM)), is provided as part of the Java Development Kit (JDK). The Java Runtime Environment (JRE), complementing the JVM with a just-in-time (JIT) compiler, converts intermediate bytecode into native machine code on the fly. Also supplied are extensive libraries, pre-compiled in which are several other components, some available only in certain editions.

The essential components in the platform are the Java language compiler, the libraries, and the runtime environment in which Java intermediate bytecode "executes" according to the rules laid out in the virtual machine specification.

Java Virtual Machine
The heart of the Java Platform is the concept of a "virtual machine" that executes Java bytecode programs. This bytecode is the same no matter what hardware or operating system the program is running under. There is a JIT compiler within the Java Virtual Machine, or JVM. The JIT compiler translates the Java bytecode into native processor instructions at run-time and caches the native code in memory during execution.

The use of bytecode as an intermediate language permits Java programs to run on any platform that has a virtual machine available. The use of a JIT compiler means that Java applications, after a short delay during loading and once they have "warmed up" by being all or mostly JIT-compiled, tend to run about as fast as native programs. Since JRE version 1.2, Sun's JVM implementation has included a just-in-time compiler instead of an interpreter.

Although Java programs are platform independent, the code of the Java Virtual Machine (JVM) that execute these programs is not; every supported operating platform has its own JVM.

In computer science, the syntax of a programming language is the set of rules that define the combinations of symbols that are considered to be correctly structured programs in that language. The syntax of a language defines its surface form. Text-based programming languages are based on sequences of characters, while visual programming languages are based on the spatial layout and connections between symbols (which may be textual or graphical).

The lexical grammar of a textual language specifies how characters must be chunked into tokens. Other syntax rules specify the permissible sequences of these tokens and the process of assigning meaning to these token sequences is part of semantics.

The syntactic analysis of source code usually entails the transformation of the linear sequence of tokens into a hierarchical syntax tree (abstract syntax trees are one convenient form of syntax tree). This process is called parsing, as it is in syntactic analysis in linguistics. Tools have been written that automatically generate parsers from a specification of a language grammar written in Backus-Naur form, e.g., Yacc (yet another compiler compiler).

The syntax of textual programming languages is usually defined using a combination of regular expressions (for lexical structure) and Backus-Naur Form (for grammatical structure) to inductively specify syntactic categories (nonterminals) and terminal symbols. Syntactic categories are defined by rules called productions, which specify the values that belong to a particular syntactic category.

Terminal symbols are the concrete characters or strings of characters (for example keywords such as define, if, let, or void) from which syntactically valid programs are constructed.

Below is a simple grammar, based on Lisp, which defines productions for the syntactic categories expression, atom, number, symbol, and list:
  • expression ::= atom | list
  • atom  ::= number | symbol
  • number  ::= [+-]?['0'-'9']+
  • symbol  ::= ['A'-'Z''a'-'z'].*
  • list  ::= '(' expression* ')'

This grammar specifies the following:
  • an expression is either an atom or a list;
  • an atom is either a number or a symbol;
  • a number is an unbroken sequence of one or more decimal digits, optionally preceded by a plus or minus sign; 
  • a symbol is a letter followed by zero or more of any characters (excluding whitespace); and
  • a list is a matched pair of parentheses, with zero or more expressions inside it.
Here the decimal digits, upper- and lower-case characters, and parentheses are terminal symbols.
The following are examples of well-formed token sequences in this grammar: '12345', '()', '(a b c232 (1))'
The grammar needed to specify a programming language can be classified by its position in the Chomsky hierarchy. The syntax of most programming languages can be specified using a Type-2 grammar, i.e., they are context-free grammars.

However, there are exceptions. In some languages like Perl and Lisp the specification (or implementation) of the language allows constructs that execute during the parsing phase. Furthermore, these languages have constructs that allow the programmer to alter the behavior of the parser. This combination effectively blurs the distinction between parsing and execution, and makes syntax analysis an undecidable problem in these languages, meaning that the parsing phase may not finish. For example, in Perl it is possible to execute code during parsing using a BEGIN statement, and Perl function prototypes may alter the syntactic interpretation, and possibly even the syntactic validity of the remaining code.

Similarly, Lisp macros introduced by the defmacro syntax also execute during parsing, meaning that a Lisp compiler must have an entire Lisp run-time system present. In contrast C macros are merely string replacements, and do not require code execution.

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