Respected22
08-26-2009, 10:40 PM
What is Java?
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.
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 made available most of their Java technologies as free software 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.
James Gosling initiated the Java language project in June 1991 for use in one of his many set-top box projects.[7] 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.[8] Gosling aimed to implement a virtual machine and a language that had a familiar C/C++ style of notation.[9]
Sun 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.[10] Java remains a de facto standard, controlled through the Java Community Process.[11] 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 13 November 2006, Sun released much of Java as free and open source software under the terms of the GNU General Public License (GPL). On 8 May 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.[12]
Principles
There were five primary goals in the creation of the Java language:[13]
It should be "simple, object oriented, and familiar".
It should be "robust and secure".
It should be "architecture neutral and portable".
It should execute with "high performance".
It should be "interpreted, threaded, and dynamic".
Evangelism
While much of Sun's direct revenue from Java comes from licensing and JCP membership dues, some believe that Java is primarily a vehicle to promote hardware server and/or Solaris OS sales for Sun internally, and Sun's vice-president Rich Green has said that Sun's ideal role with regards to Java is as an "evangelist".
Java Platform
Main articles: Java Platform and Java Runtime Environment
One characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any supported hardware/operating-system platform. This is achieved by compiling the Java language code, not to machine code but to Java bytecode – instructions ****ogous to machine code but intended to be interpreted by a virtual machine (VM) written specifically for the host hardware. End-users commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a Web browser for Java applets.
Standardized libraries provide a generic way to access host specific features such as graphics, threading and networking.
A major benefit of using bytecode is porting. However, the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would, and Java suffered a reputation for poor performance. This gap has been narrowed by a number of optimization techniques introduced in the more recent JVM implementations.
By convention, source files are named after the public class they contain, appending the suffix .java, for example, HelloWorld.java. It must first be compiled into bytecode, using a Java compiler, producing a file named HelloWorld.class. Only then can it be executed, or 'launched'. The java source file may only contain one public class but can contain multiple classes with less than public access and any number of public inner classes.
A class that is declared private may be stored in any .java file. The compiler will generate a class file for each class defined in the source file. The name of the class file is the name of the class, with .class appended. For class file generation, anonymous classes are treated as if their name was the concatenation of the name of their enclosing class, a $, and an integer.
The keyword public denotes that a method can be called from code in other classes, or that a class may be used by classes outside the class hierarchy. The class hierarchy is related to the name of the directory in which the .java file is.
The keyword static in front of a method indicates a static method, which is associated only with the class and not with any specific instance of that class. Only static methods can be invoked without a reference to an object. Static methods cannot access any method variables that are not static.
The keyword void indicates that the main method does not return any value to the caller. If a Java program is to exit with an error code, it must call System.exit() explicitly.
The method name "main" is not a keyword in the Java language. It is simply the name of the method the Java launcher calls to pass control to the program. Java classes that run in managed environments such as applets and Enterprise Java Beans do not use or need a main() method. A java program may contain multiple classes that have main methods, which means that the VM needs to be explicitly told which class to launch from.
The main method must accept an array of String objects. By convention, it is referenced as args although any other legal identifier name can be used. Since Java 5, the main method can also use variable arguments, in the form of public static void main(String... args), allowing the main method to be invoked with an arbitrary number of String arguments. The effect of this alternate declaration is semantically identical (the args parameter is still an array of String objects), but allows an alternate syntax for creating and passing the array.
The Java launcher launches Java by loading a given class (specified on the command line or as an attribute in a JAR) and starting its public static void main(String[]) method. Stand-alone programs must declare this method explicitly. The String[] args parameter is an array of String objects containing any arguments passed to the class. The parameters to main are often passed by means of a command line.
Printing is part of a Java standard library: The System class defines a public static field called out. The out object is an instance of the PrintStream class and provides many methods for printing data to standard out, including println(String) which also appends a new line to the passed string.
Examples:
//This is an example of a single line comment using two forward slashes
/* This is an example of a multiple line comment using the forward slash
and asterisk. This type of comment can be used to hold a lot of information
but it is very important to remember to close the comment. */
/*
* Outputs "Hello, world!" and then exits
*/
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world!");
}
}
My Sources: Wikipedia
Thanks,
~Respected
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.
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 made available most of their Java technologies as free software 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.
James Gosling initiated the Java language project in June 1991 for use in one of his many set-top box projects.[7] 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.[8] Gosling aimed to implement a virtual machine and a language that had a familiar C/C++ style of notation.[9]
Sun 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.[10] Java remains a de facto standard, controlled through the Java Community Process.[11] 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 13 November 2006, Sun released much of Java as free and open source software under the terms of the GNU General Public License (GPL). On 8 May 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.[12]
Principles
There were five primary goals in the creation of the Java language:[13]
It should be "simple, object oriented, and familiar".
It should be "robust and secure".
It should be "architecture neutral and portable".
It should execute with "high performance".
It should be "interpreted, threaded, and dynamic".
Evangelism
While much of Sun's direct revenue from Java comes from licensing and JCP membership dues, some believe that Java is primarily a vehicle to promote hardware server and/or Solaris OS sales for Sun internally, and Sun's vice-president Rich Green has said that Sun's ideal role with regards to Java is as an "evangelist".
Java Platform
Main articles: Java Platform and Java Runtime Environment
One characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any supported hardware/operating-system platform. This is achieved by compiling the Java language code, not to machine code but to Java bytecode – instructions ****ogous to machine code but intended to be interpreted by a virtual machine (VM) written specifically for the host hardware. End-users commonly use a Java Runtime Environment (JRE) installed on their own machine for standalone Java applications, or in a Web browser for Java applets.
Standardized libraries provide a generic way to access host specific features such as graphics, threading and networking.
A major benefit of using bytecode is porting. However, the overhead of interpretation means that interpreted programs almost always run more slowly than programs compiled to native executables would, and Java suffered a reputation for poor performance. This gap has been narrowed by a number of optimization techniques introduced in the more recent JVM implementations.
By convention, source files are named after the public class they contain, appending the suffix .java, for example, HelloWorld.java. It must first be compiled into bytecode, using a Java compiler, producing a file named HelloWorld.class. Only then can it be executed, or 'launched'. The java source file may only contain one public class but can contain multiple classes with less than public access and any number of public inner classes.
A class that is declared private may be stored in any .java file. The compiler will generate a class file for each class defined in the source file. The name of the class file is the name of the class, with .class appended. For class file generation, anonymous classes are treated as if their name was the concatenation of the name of their enclosing class, a $, and an integer.
The keyword public denotes that a method can be called from code in other classes, or that a class may be used by classes outside the class hierarchy. The class hierarchy is related to the name of the directory in which the .java file is.
The keyword static in front of a method indicates a static method, which is associated only with the class and not with any specific instance of that class. Only static methods can be invoked without a reference to an object. Static methods cannot access any method variables that are not static.
The keyword void indicates that the main method does not return any value to the caller. If a Java program is to exit with an error code, it must call System.exit() explicitly.
The method name "main" is not a keyword in the Java language. It is simply the name of the method the Java launcher calls to pass control to the program. Java classes that run in managed environments such as applets and Enterprise Java Beans do not use or need a main() method. A java program may contain multiple classes that have main methods, which means that the VM needs to be explicitly told which class to launch from.
The main method must accept an array of String objects. By convention, it is referenced as args although any other legal identifier name can be used. Since Java 5, the main method can also use variable arguments, in the form of public static void main(String... args), allowing the main method to be invoked with an arbitrary number of String arguments. The effect of this alternate declaration is semantically identical (the args parameter is still an array of String objects), but allows an alternate syntax for creating and passing the array.
The Java launcher launches Java by loading a given class (specified on the command line or as an attribute in a JAR) and starting its public static void main(String[]) method. Stand-alone programs must declare this method explicitly. The String[] args parameter is an array of String objects containing any arguments passed to the class. The parameters to main are often passed by means of a command line.
Printing is part of a Java standard library: The System class defines a public static field called out. The out object is an instance of the PrintStream class and provides many methods for printing data to standard out, including println(String) which also appends a new line to the passed string.
Examples:
//This is an example of a single line comment using two forward slashes
/* This is an example of a multiple line comment using the forward slash
and asterisk. This type of comment can be used to hold a lot of information
but it is very important to remember to close the comment. */
/*
* Outputs "Hello, world!" and then exits
*/
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world!");
}
}
My Sources: Wikipedia
Thanks,
~Respected