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Java (programming language)
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Java
Paradigm: Object-oriented, structured, imperative
Appeared in: 1990s
Designed by: Sun Microsystems
Typing discipline: Static, strong, safe, nominative
Major implementations: Numerous
Influenced by: Objective-C, C++, Smalltalk, Eiffel, C#[1]
Influenced: C#, D, J#, Ada 2005
OS: Cross-platform
Website: http://www.java.com/
Java is an object-oriented programming language developed by Sun Microsystems in the early 1990s. Java applications are often compiled to bytecode, which may be compiled to native machine code at runtime.
The language itself borrows much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities. JavaScript, a scripting language, shares a similar name and has similar syntax, but is not related to Java.
Sun Microsystems provides a GNU General Public License implementation of a Java compiler and Java virtual machine, in compliance with the specifications of the Java Community Process, although the class library that is required to run Java programs is not free software.
Contents [hide]
1 History
1.1 Releases
2 Philosophy
2.1 Primary Goals
2.2 Platform independence
2.3 Automatic garbage collection
3 Syntax
3.1 Hello world
3.1.1 Stand-alone application
3.1.2 Applet
3.1.3 Servlet
3.1.4 JavaServer Page
3.1.5 Swing application
4 Criticism
4.1 Performance
4.2 Look and feel
4.3 Lack of OO purity
4.4 Single-paradigm language
5 Resources
5.1 Java Runtime Environment
5.1.1 Components
5.2 APIs
5.3 Licensing
5.4 Extensions and related architectures
6 See also
6.1 Lists
7 Notes
8 References
9 External links
9.1 Sun
9.2 Java Specification Requests
9.3 Resources
9.4 History
9.5 Java Implementations
9.6 Criticism
[edit] History
Duke, the Java mascot.Java was started as a project called "Oak" by James Gosling in June 1991.[2] Gosling's goals were to implement a virtual machine and a language that had a familiar C/++ style of notation. The first public implementation was Java 1.0 in 1995. It promised "Write Once, Run Anywhere" (WORA), providing no-cost runtimes on popular platforms. It was fairly secure and its security was configurable, allowing network and file access to be restricted. Major web browsers soon incorporated the ability to run secure Java "applets" within web pages. Java became popular quickly. With the advent of "Java 2", new versions had multiple configurations built for different types of platform. For example, J2EE was for enterprise applications and the greatly stripped down version J2ME was for mobile applications.
In 1997, Sun approached the ISO/IEC JTC1 standards body and later the Ecma International to formalize Java, but it soon withdrew from the process.[3][4][5] Java remains a proprietary de facto standard that is controlled through the Java Community Process [6]. Sun makes most of its Java implementations available without charge, with revenue being generated by specialized products such as the Java Enterprise System. Sun distinguishes between its Software Development Kit (SDK) and Runtime Environment (JRE) which is a subset of the SDK, the primary distinction being that in the JRE the compiler is not present.
On November 13, 2006, Sun released parts of Java as Free/Open Source Software, under the GNU General Public License. The release of the complete sources under GPL is expected in the first quarter of 2007.
[edit] Releases
Main article: Java version history
The Java project has seen many release versions. They are:
JDK 1.1.4 (Sparkler) September 12, 1997
JDK 1.1.5 (Pumpkin) December 3, 1997
JDK 1.1.6 (Abigail) April 24, 1998
JDK 1.1.7 (Brutus) September 28, 1998
JDK 1.1.8 (Chelsea) April 8, 1999
J2SE 1.2 (Playground) December 4, 1998
J2SE 1.2.1 (none) March 30, 1999
J2SE 1.2.2 (Cricket) July 8, 1999
J2SE 1.3 (Kestrel) May 8, 2000
J2SE 1.3.1 (Ladybird) May 17, 2001
J2SE 1.4.0 (Merlin) February 13, 2002
J2SE 1.4.1 (Hopper) September 16, 2002
J2SE 1.4.2 (Mantis) June 26, 2003
J2SE 5.0 (1.5.0) (Tiger) September 29, 2004
Java SE 6 (1.6.0) (Mustang) December 11, 2006 [4]
Java SE 7 (1.7.0) (Dolphin) anticipated for 2008
[edit] Philosophy
[edit] Primary Goals
There were five primary goals in the creation of the Java language:
It should use the object-oriented programming methodology.
It should allow the same program to be executed on multiple operating systems.
It should contain built-in support for using computer networks.
It should be designed to execute code from remote sources securely.
It should be easy to use by selecting what was considered the good parts of other object-oriented languages.
To achieve the goals of networking support and remote code execution, Java programmers sometimes find it necessary to use extensions such as CORBA, Internet Communications Engine, or OSGi.
[edit] Platform independence
One characteristic, platform independence, means that programs written in the Java language must run similarly on any supported hardware/operating-system platform. One should be able to write a program once, compile it once, and run it anywhere.
This is achieved by most Java compilers by compiling the Java language code "halfway" to bytecode (specifically Java bytecode)—simplified machine instructions specific to the Java platform. The code is then run on a virtual machine (VM), a program written in native code on the host hardware that interprets and executes generic Java bytecode. (In some JVM versions, bytecode can also be compiled to native code, resulting in faster execution.) Further, standardized libraries are provided to allow access to features of the host machines (such as graphics, threading and networking) in unified ways. Note that, although there's an explicit compiling stage, at some point, the Java bytecode is interpreted or converted to native machine instructions by the JIT compiler.
There are also implementations of Java compilers that translate the Java language code to native object code, such as GCJ, removing the intermediate bytecode stage, but the output of these compilers can only be run on a single architecture.
Sun's license for Java insists that all implementations be "compatible". This resulted in a legal dispute with Microsoft after Sun claimed that the Microsoft implementation did not support the RMI and JNI interfaces and had added platform-specific features of their own. Sun sued and won both damages (some $20 million) and a court order enforcing the terms of the license from Sun. In response, Microsoft no longer ships Java with Windows, and in recent versions of Windows, Internet Explorer cannot support Java applets without a third-party plugin. However, Sun and others have made available Java run-time systems at no cost for those and other versions of Windows.
The first implementations of the language used an interpreted virtual machine to achieve portability. These implementations produced programs that ran more slowly than programs compiled to native executables, for instance written in C or C++, so the language suffered a reputation for poor performance. More recent JVM implementations produce programs that run significantly faster than before, using multiple techniques.
The first technique is to simply compile directly into native code like a more traditional compiler, skipping bytecodes entirely. This achieves good performance, but at the expense of portability. Another technique, known as just-in-time compilation (JIT), translates the Java bytecodes into native code at the time that the program is run which results in a program that executes faster than interpreted code but also incurs compilation overhead during execution. More sophisticated VMs use dynamic recompilation, in which the VM can analyze the behavior of the running program and selectively recompile and optimize critical parts of the program. Dynamic recompilation can achieve optimizations superior to static compilation because the dynamic compiler can base optimizations on knowledge about the runtime environment and the set of loaded classes, and can identify the "hot spots" (parts of the program, often inner loops, that take up most of execution time). JIT compilation and dynamic recompilation allow Java programs to take advantage of the speed of native code without losing portability.
Portability is a technically difficult goal to achieve, and Java's success at that goal has been mixed. Although it is indeed possible to write programs for the Java platform that behave consistently across many host platforms, the large number of available platforms with small errors or inconsistencies led some to parody Sun's "Write once, run anywhere" slogan as "Write once, debug everywhere".
Platform-independent Java is however very successful with server-side applications, such as Web services, servlets, and Enterprise JavaBeans, as well as with Embedded systems based on OSGi, using Embedded Java environments.
[edit] Automatic garbage collection
One idea behind Java's automatic memory management model is that programmers should be spared the burden of having to perform manual memory management. In some languages the programmer allocates memory to create any object stored on the heap and is responsible for later manually deallocating that memory to delete any such objects. If a programmer forgets to deallocate memory or writes code that fails to do so in a timely fashion, a memory leak can occur: the program will consume a potentially arbitrarily large amount of memory. In addition, if a region of memory is deallocated twice, the program can become unstable and may crash. Finally, in non garbage collected environments, there is a certain degree of overhead and complexity of user-code to track and finalize allocations. Often developers may box themselves into certain designs to provide reasonable assurances that memory leaks will not occur.
In Java, this potential problem is avoided by automatic garbage collection. The programmer determines when objects are created, and the Java runtime is responsible for managing the object's lifecycle. The program or other objects can reference an object by holding a reference to it (which, from a low-level point of view, is its address on the heap). When no references to an object remain, the Java garbage collector automatically deletes the unreachable object, freeing memory and preventing a memory leak. Memory leaks may still occur if a programmer's code holds a reference to an object that is no longer needed—in other words, they can still occur but at higher conceptual levels.
The use of garbage collection in a language can also affect programming paradigms. If, for example, the developer assumes that the cost of memory allocation/recollection is low, they may choose to more freely construct objects instead of pre-initializing, holding and reusing them. With the small cost of potential performance penalties (inner-loop construction of large/complex objects), this facilitates thread-isolation (no need to synchronize as different threads work on different object instances) and data-hiding. The use of transient immutable value-objects minimizes side-effect programming.
Comparing Java and C++, it is possible in C++ to implement similar functionality (for example, a memory management model for specific classes can be designed in C++ to improve speed and lower memory fragmentation considerably), with the possible cost of extra development time and some application complexity. In Java, garbage collection is built-in and virtually invisible to the developer. That is, developers may have no notion of when garbage collection will take place as it may not necessarily correlate with any actions being explicitly performed by the code they write. Depending on intended application, this can be beneficial or disadvantageous: the programmer is freed from performing low-level tasks, but at the same time loses the option of writing lower level code.
[edit] Syntax
Main article: Java syntax
The syntax of Java is largely derived from C++. However, unlike C++, which combines the syntax for structured, generic, and object-oriented programming, Java was built from the ground up as an object oriented language. As a result, almost everything is an object and all code is written inside a class. The exceptions are the intrinsic datatypes (ordinal and real numbers, boolean values, and characters), which are not classes for performance reasons.
[edit] Hello world
The quote "Hello, world" comes from the very first computer program.
For an explanation of the tradition of programming "Hello World" see: Hello world program.
[edit] Stand-alone application
This is a minimal usage of Java, but it does not demonstrate object-oriented programming well. No object is explicitly created since the keyword new is never used.
// Hello.java
public class Hello {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}
To execute this program, one first saves the above code as a file named Hello.java. One then compiles it to bytecode:
$ javac Hello.java
which produces a file named Hello.class . This class is then launched with the java launcher (usually named java, java.exe, or some variant depending on the operating system).
$ java Hello
Hello, World!
$
The above example merits a bit of explanation.
Everything in Java is written inside a class, including stand-alone programs.
Source files are by convention named the same as the class they contain, appending the mandatory suffix .java. A class which is declared public is required to follow this convention. (In this case, the class is Hello, therefore the source must be stored in a file called Hello.java).
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 void indicates that the main method does not return any value to the caller.
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 varargs (variable arguments), in the form of public static void main(String... args), allowing the main method to be invoked with an arbitrary number of arguments.)
The keyword static indicates that the method is a static method, associated with the class rather than object instances.
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 java launcher launches java by loading a given class (specified on the command line) and starting its public static void main(String[]) method. Stand-alone programs must declare this method explicitly. The 'String []' parameter represents any arguments passed to the class (often by means of a command line).
The method name "main" is not part of the java language - it is not a keyword. It is simply that the java launcher uses it. Java classes that run in managed environments such as applets and Enterprise Java Beans do not use or need a main() method.
The printing facility is part of the 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 the method println(String) for displaying data to the screen while creating a new line (standard out).
Standalone programs are run by giving the Java runtime the name of the class whose main method is to be invoked. For example, at a Unix command line java -cp . Hello will start the above program (compiled into Hello.class) from the current directory. The name of the class whose main method is to be invoked can also be specified in the MANIFEST of a Java archive (Jar) file.
[edit] Applet
Main article: Java applet
Java applets are programs that are embedded in other applications, typically in a Web page displayed in a Web browser.
// Hello.java
import java.applet.Applet;
import java.awt.Graphics;
public class Hello extends Applet {
public void paint(Graphics gc) {
gc.drawString("Hello, world!", 65, 95);
}
}
The import statements direct the Java compiler to include the java.applet.Applet and java.awt.Graphics classes in the compilation. The import statement allows these classes to be referenced in the source code using the simple class name (i.e. Applet) instead of the fully qualified class name (i.e. java.applet.Applet).
The Hello class extends (subclasses) the Applet class; the Applet class provides the framework for the host application to display and control the lifecycle of the applet. The Applet class is an Abstract Windowing Toolkit (AWT) Component, which provides the applet with the capability to display a graphical user interface (GUI) and respond to user events.
The Hello class overrides the paint(Graphics) method inherited from the Container superclass to provide the code to display the applet. The paint() method is passed a Graphics object that contains the graphic context used to display the applet. The paint() method calls the graphic context drawString(String, int, int) method to display the "Hello, world!" string at a pixel offset of (65, 95) from the upper-left corner in the applet's display.
Hello World Applet
An applet is placed in an HTML document using the