Java programming language
Encyclopedia : J : JA : JAV : Java programming language
Java is an object-oriented programming language developed by James Gosling and colleagues at Sun Microsystems in the early 1990s. Unlike conventional languages which are generally designed to be compiled to native code, Java is compiled to a bytecode which is then run (generally using JIT compilation) by a Java virtual machine.
The language itself borrows much syntax from C and C++ but has a much simpler object model and does away with low-level tools like programmer-manipulated pointers.
Java is only distantly related to JavaScript, though they have similar names and share a C-like syntax.
Version history
As with other parts of the Java platform, the Java language has evolved over the years while largely maintaining backwards compatibility.
- JDK 1.0 (January 23, 1996) — Initial release. ([press release])
- JDK 1.1 (February 19, 1997) ([press release])
- * inner classes added to the language
- J2SE 1.2 (December 8, 1998) — Codename Playground. This and subsequent releases through J2SE 5.0 were rebranded Java 2 and the version name "J2SE" (Java 2 Platform, Standard Edition) replaced JDK to distinguish the base platform from J2EE (Java 2 Platform, Enterprise Edition) and J2ME (Java 2 Platform, Micro Edition). Major additions included: ([press release])
- *
strictfpkeyword - J2SE 1.3 (May 8, 2000) — Codename Kestrel. ([press release])([full list of changes])
- J2SE 1.4 (February 6, 2002) — Codename Merlin. This was the first release of the Java platform developed under the Java Community Process as [JSR 59].([press release])([full list of changes])
- J2SE 5.0 (September 30, 2004) — Codename Tiger. (Originally numbered 1.5, which is still used as the internal version number.[link]) Developed under [JSR 176], Tiger added a number of significant new language features: ([press release])([full list of changes])
- * Generics — provides compile-time (static) type safety for collections and eliminates the need for most typecasts. (Specified by [JSR 14].)
- * Metadata — also called annotations, allows language constructs such as classes and methods to be tagged with additional data, which can then be processed by metadata-aware utilities. (Specified by [JSR 175].)
- * Autoboxing/unboxing — automatic conversions between primitive types (such as
int) and primitive wrapper classes (such as ). (Specified by [JSR 201].) - * Enumerations — the
enumkeyword creates a typesafe, ordered list of values (such asDay.MONDAY,Day.TUESDAY, etc.). Previously this could only be achieved by non-typesafe constant integers or manually constructed classes (typesafe enum pattern). (Specified by JSR 201.) - * Varargs — the last parameter of a method can now be declared using a type name followed by three dots (e.g.
void drawtext(String... lines)). In the calling code any number of parameters of that type can be used and they are then placed in an array to be passed to the method. - * Enhanced
forloop — theforloop syntax is extended with special syntax for iterating over each member of either an array or any , such as the standard classes, using a construct of the form:
}
This example iterates over the |
- Java SE 6 — Codename [Mustang]. As of 2006 this is currently in development under [JSR 270]. A beta version was released on February 15, 2006.([press release]) Beta 2 was released on June 15, 2006 and is available at [http://java.sun.com/javase/downloads/ea.jsp]. The final release is expected autumn 2006. New builds including enhancements and bug fixes are released approximately weekly. As of this version, Sun replaced the name "J2SE" with Java SE and dropped the ".0" from the version number.[link]
- Java SE 7 — Codename Dolphin. This is in the early planning stages. The Dolphin Project will start up in late July, with release estimated in 2008.[link]
Philosophy
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 and borrow the good parts of older object-oriented languages like C++.
Object orientation
The first characteristic, object orientation ("OO"), refers to a method of programming and language design. Although there are many interpretations of OO, one primary distinguishing idea is to design software so that the various types of data it manipulates are combined together with their relevant operations. Thus, data and code are combined into entities called objects. An object can be thought of as a self-contained bundle of behavior (code) and state (data). The principle is to separate the things that change from the things that stay the same; often, a change to some data structure requires a corresponding change to the code that operates on that data, or vice versa. This separation into coherent objects provides a more stable foundation for a software system's design. The intent is to make large software projects easier to manage, thus improving quality and reducing the number of failed projects.Another primary goal of OO programming is to develop more generic objects so that software can become more reusable between projects. A generic "customer" object, for example, should in theory have roughly the same basic set of behaviors between different software projects, especially when these projects overlap on some fundamental level as they often do in large organizations. In this sense, software objects can hopefully be seen more as pluggable components, helping the software industry build projects largely from existing and well tested pieces, thus leading to a massive reduction in development times. Software reusability has met with mixed practical results, with two main difficulties: the design of truly generic objects is poorly-understood, and a methodology for broad communication of reuse opportunities is lacking. Some open source communities want to help ease the reuse problem, by providing authors with ways to disseminate information about generally reusable objects and object libraries.
Platform independence
The second characteristic, platform independence, means that programs written in the Java language must run similarly on diverse hardware. One should be able to write a program 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. 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 compile 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 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. More sophisticated VMs use dynamic recompilation, in which the VM can analyze the behavior of the running program and selectively recompile and optimise 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. 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.
Automatic garbage collection
One possible argument against languages such as C++ is that programmers should be spared the burden of having to perform manual memory management. In C++, the programmer must allocate memory to create any object stored on the heap, and deallocate 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.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 objects' 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. On the whole, Java's automatic garbage collection makes creation and deletion of objects in Java simpler, potentially safer, and often faster than in C++.
Like many comparisons between 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), but doing so requires extra development time and adds considerable complexity to an application. 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.
Syntax
The syntax of Java is largely derived from C++. But unlike C++, which combines the syntax for structured, generic, and object-oriented programming, Java was built from the ground up to be fully object-oriented. Everything in Java is an object (with a few exceptions), and everything in Java is written inside a class.
Hello world
- For an explanation of the tradition of programming "Hello World" see: Hello world program.
Stand-alone application
// Hello.java public class Hello }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
classwhich is declaredpublicis required to follow this convention. (In this case, the class isHello, 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 a sequential integer starting with
0. - Programs to be executed as stand-alone must have a
main()method. - The keyword
voidindicates that the main method does not return anything. - The main method must accept an array of objects. By convention, it is referenced as
argsalthough any other legal identifier name can be used. - The keyword
staticindicates that the method is a class method, associated with the class rather than object instances. Main methods must be static. - The keyword
publicdenotes that a method can be called from code in other classes, or that a class may be used by classes outside the class hierarchy. Main methods must also be public. - The printing facility is part of the Java standard library: The class defines a public static field called . The
outobject is an instance of the class and provides the method for displaying data to the screen (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 . Hellowill 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.
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 }
<!-- Hello.html -->TheHello World Applet
import statements direct the Java compiler to include the and 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) , which provides the applet with the capability to display a graphical user interface (GUI) and respond to user events.
The Hello class overrides the method inherited from the 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 method to display the "Hello, world!" string at a pixel offset of (65, 95) in the applet's display.
An applet is placed in an HTML document using the HTML element. The applet tag has three attributes set: code="Hello" specifies the name of the Applet class and width="200" height="200" sets the pixel width and height of the applet. (Applets may also be embedded in HTML using either the object or embed element, although support for these elements by Web browsers is inconsistent.[link][link])
Servlet
Java servlets are server-side Java EE components that generate responses to requests from clients.
// Hello.java import java.io.*; import javax.servlet.*;
public class Hello extends GenericServlet }The
import statements direct the Java compiler to include all of the public classes and interfaces from the and packages in the compilation.
The Hello class extends the class; the GenericServlet class provides the interface for the server to forward requests to the servlet and control the servlet's lifecycle.
The Hello class overrides the method defined by the interface to provide the code for the service request handler. The service() method is passed a object that contains the request from the client and a object used to create the response returned to the client. The service() method declares that it throws the exceptions and if a problem prevents it from responding to the request.
The method in the response object is called to set the MIME content type of the returned data to "text/html". The method in the response returns a object that is used to write the data that is sent to the client. The method is called to write the "Hello, world!" string to the response and then the method is called to close the print writer, which causes the data that has been written to the stream to be returned to the client.
Swing application
Swing is the advanced graphical user interface library for the Java SE platform.
// Hello.java import javax.swing.*;
public class Hello extends JFrame
public static void main(String[] args) }The
import statement directs the Java compiler to include all of the public classes and interfaces from the package in the compilation.
The Hello class extends the class; the JFrame class implements a window with a title bar with a close control.
The Hello() constructor initializes the frame by first calling the method inherited from JFrame to set the default operation when the close control on the title bar is selected to —this causes the JFrame to be disposed of when the frame is closed (as opposed to merely hidden), which allows the JVM to exit and the program to terminate. Next a new is created for the string "Hello, world!" and the method inherited from the superclass is called to add the label to the frame. The method inherited from the superclass is called to size the window and layout its contents.
The main() method is called by the JVM when the program starts. It instantiates a new Hello frame and causes it to be displayed by calling the method inherited from the superclass with the boolean parameter true. Note that once the frame is displayed, exiting the main method does not cause the program to terminate because the AWT event dispatching thread remains active until all of the Swing top-level windows have been disposed.
Criticism
Java was intended to serve as a novel way to manage software complexity. Most consider Java technology to deliver reasonably well on this promise. However, Java is not without flaws, and it does not universally accommodate all programming styles, environments, or requirements (see Java criticisms for a thorough analysis of common criticisms about the language).
- Performance : Partly due to the early versions of the language, Java has often been perceived as significantly slower, and more memory-consuming, than natively compiled languages such as C or C++. Performance has increased a lot since then, and relative performance of JIT compilers as compared to native compilers can be quite close, and is often a subject of debate.
- Look and feel : The default look and feel of GUI applications written in Java using the Swing toolkit is very different from native applications. It is possible to specify a different look and feel through the pluggable look and feel system and clones of Windows, Mac and Motif are supplied by Sun but some consider these to be far from perfect. Java SE 6 addresses this problem to better match underlying platforms.
- Language choices :
- *Java designers decided not to implement certain features present in other languages, including:
- ** multiple inheritance
- ** operator overloading
- ** class properties
- ** tuples
- *Java's primitive types are not objects. Primitive types hold their values in the stack rather than being references to values. This was a conscious decision by Java's designers for performance reasons. Because of this, Java is not considered to be a pure object-oriented programming language. However, as of Java 5.0, autoboxing enables programmers to write as if primitive types are their wrapper classes, and freely interchange between them, giving high performance with flexibility.
- *Java is predominantly a single-paradigm language. The addition of static imports in Java 5.0 accomodates the procedural paradigm better than earlier versions of Java.
- Classpath : Installing the JRE requires explicitly setting the classpath and Java is unforgiving if you do it wrong. This confuses a lot of people new to Java.
Resources
Java Runtime Environment
The Java Runtime Environment or JRE is the software required to run any application deployed on the Java Platform. End-users commonly use a JRE in software packages and Web browser plugins. Sun also distributes a superset of the JRE called the Java 2 SDK (more commonly known as the JDK), which includes development tools such as the Java compiler, Javadoc, and debugger.Components
- Java libraries that are the compiled byte codes of source code developed by the JRE implementor to support application development in Java. Examples of these libraries are:
- * The core libraries, which include:
- ** Collection libraries which implement data structures such as lists, dictionaries, trees and sets
- ** XML Parsing libraries
- ** Security
- ** Internationalization and localization libraries
- * The integration libraries, which allow the application writer to communicate with external systems. These libraries include:
- ** The Java Database Connectivity (JDBC) API for database access
- ** Java Naming and Directory Interface (JNDI) for lookup and discovery
- ** RMI and CORBA for distributed application development
- * User Interface libraries, which include:
- ** The (heavyweight, or native) Abstract Windowing Toolkit (AWT), which provides GUI components, the means for laying out those components and the means for handling events from those components
- ** The (lightweight) Swing libraries, which are built on AWT but provide (non-native) implementations of the AWT widgetry
- ** APIs for audio capture, processing, and playback
- A platform dependent implementation of Java virtual machine (JVM) which is the means by which the byte codes of the Java libraries and third party applications are executed
- Plugins, which enable applets to be run in Web browsers
- Java Web Start, which allows Java applications to be efficiently distributed to end users across the Internet
- Licensing and documentation
APIs
Sun has defined three platforms targeting different application environments and segmented many of its APIs so that they belong to one of the platforms. The platforms are:- Java Platform, Micro Edition (Java ME) — targeting environments with limited resources,
- Java Platform, Standard Edition (Java SE) — targeting workstation environments, and
- Java Platform, Enterprise Edition (Java EE) — targeting large distributed enterprise or Internet environments.
The set of APIs is controlled by Sun Microsystems in cooperation with others through the Java Community Process program. Companies or individuals participating in this process can influence the design and development of the APIs. This process has been a subject of controversy.
In 2004, IBM and BEA publicly supported the notion of creating an official open source implementation of Java. As of 2006, Sun has agreed that they will eventually open source Java, the When and How are still to be defined.