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Java is an object-oriented programming language developed by James Gosling and colleagues at Sun Microsystems in the early 1990s. Java is designed to be compiled to bytecode which is interpreted at runtime. This is unlike most conventional programming languages, which either compile source code to native (machine) code or interprete source code at runtime. The language itself borrows much 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.
History Java was started as a project called "Oak" by James Gosling in June 1991. Gosling's goals were to implement a virtual machine and a language that had a familiar C-like notation but with greater uniformity and simplicity than C/C++. The first public implementation was Java 1.0 in 1995. It promised "Write Once, Run Anywhere" (WORA), providing free 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 platforms. 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. Java remains a proprietary de facto standard that is controlled through the Java Community Process . 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. Releases The Java project has undergone a number of version changes, large and small, most with a codename. As of September 2004, they include: Philosophy There were five primary goals in the creation of the Java language: 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. 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 translating 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 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. 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 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 penalities (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. 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 to be virtually fully object-oriented: everything in Java is an object with the exceptions of atomic datatypes (ordinal and real numbers, boolean values, and characters) 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 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 The above example merits a bit of explanation. 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).void indicates that the main method does not return any value to the caller.args although any other legal identifier name can be used.static indicates that the method is a class method, associated with the class rather than object instances.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.public static void main(String) method. Stand-alone programs must declare this method explicitly.out object is an instance of the class and provides the method for displaying data to the screen (standard out).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.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 The 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) from the upper-left corner in the applet's display.An applet is placed in an HTML document using 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.**)The host application, typically a Web browser, instantiates the Hello applet and creates an for the applet. Once the applet has initialized itself, it is added to the AWT display hierarchy. The paint method is called by the AWT event dispatching thread whenever the display needs the applet to draw itself.Servlet Java Servlet technology provides Web developers with a simple, consistent mechanism for extending the functionality of a Web server and for accessing existing business systems. Servlets are server-side Java EE components that generate responses (typically HTML pages) to requests (typically HTTP requests) from clients. A servlet can almost be thought of as an applet that runs on the server side—without a face. // Hello.java import java.io. 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. JSP JavaServer Pages (JSPs) are server-side Java EE components that generate responses, typically HTML pages, to HTTP requests from clients. JSPs embed Java code in an HTML page. A JSP is compiled to a Java servlet the first time it is accessed. After that the generated servlet creates the response. // Hello.jsp <%@ page import="java.util. <% String javaVariable = "Hello World"; %> <%= javaVariable %> The " <%@ page import="java.util." directive is converted to a Java import statement.
Java declaration between the two " <% ... %>" delimiters is executed during the response generation. The content of the Java variable between the two " <%= ... %>" delimiters is embedded in the HTML response.Swing application Swing is the advanced graphical user interface library for the Java SE platform. // Hello.java import java.awt.BorderLayout; import javax.swing. public class Hello extends JFrame 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 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 above paragraph needs to be modified due to some changes in the code sample, . once the paragraph is modified, please remove this text.) 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. Java does not universally accommodate all programming styles or environments. See Java criticisms for a thorough analysis of common criticisms about the language. Performance Partly due to the early versions of the language, Java is often perceived as significantly slower and more memory-consuming than natively compiled languages such as C or C++. In general, interpreted languages require additional instructions to execute, and can potentially run several times slower than directly compiled machine code. There also is a significant overhead in both time and space, since many Java interpreters and runtimes take up many megabytes and take many seconds to start up. This arguably makes small Java utility programs uncompetitive with programs compiled in other compiled languages. To be fair, several tests have indicated that the Java overhead is less noticeable for sophisticated or large Java programs. Also, Java programs' execution speed can benefit from JIT compiler post-passes, but those add additional startup overhead in time and space. Still, Java's performance has increased substantially since the early versions, and performance of JIT compilers relative to native compilers has in some tests been shown to be quite similar. The performance of the compilers, however, does not necessarily indicate the performance of the compiled code; only careful testing can reveal the true performance issues in any system. 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 of Swing. Clones of Windows, GTK and Motif are supplied by Sun. Apple also provides an Aqua look and feel for Mac OS X. Though prior implementations of these look and feels have been considered lacking, Swing in Java SE 6 addresses this problem by using more native widget drawing routines of the underlying platforms. Alternatively, third party toolkits such as wx4j or SWT may be used for increased integration with the native windowing system. Lack of OO purity 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 for improved flexibility. Single-paradigm language Java is predominantly a single-paradigm language. However, with the addition of static imports in Java 5.0 the procedural paradigm is better accommodated than in earlier versions of Java. 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 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: The classes in the Java APIs are organized into separate groups called packages. Each package contains a set of related interfaces, classes and exceptions. Refer to the separate platforms for a description of the packages available. 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; when and how are still to be defined. Extensions and related architectures The extensions to standard Java are typically in javax. packages. They are not included in the JDK or JRE. Extensions and architectures closely tied to the Java programming language include:
See also Lists Notes Sun Java Specification Requests There are several JSRs related to the Java Language and core API packages. Resources Java Integrated Development Environments The following Integrated Development Environments (IDEs) can be used to create Java programs. History Java Implementations Proprietary: Open source: Criticism | |||||||||||||||||||||||||
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