Java programming language
The Java language is an object-oriented programming language created by James Gosling and other engineers at Sun Microsystems. It was developed in 1991, as part of the Green Project, and officially announced on May 23, 1995, at SunWorld; being released in November. Gosling and friends initially designed Java, which was called Oak at first (in honour of a tree outside Gosling's office), to replace C++ (although the feature set better resembles that of Objective C). Sun controls the Java specification and holds a trademark on the Java name.
There were four primary goals in the creation of the Java language:
The first characteristic, object orientation ("OO"), refers to a method of programming and language design.
The main idea of OO is to design software around the "things" (ie. objects) it manipulates, rather than the actions it performs. This is based on the notion that the former change less frequently and radically than the latter, making such objects (actually the entities containing data) the 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.
The second characteristic, platform independence, means that programss 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 compiling Java language code "halfway" to bytecode--simplified machine instructions that conform to a set standard.
The code is then run on a virtual machine (VM), a program written in native code on the host hardware that translates generic Java bytecode into usable code on the hardware. Further, standardized libraries are provided to allow access to features of the host machines (such as graphics and networking) in unified ways. The Java language also includes support for multi-threaded programs.
Sun's license for Java insists that all implementations be "compatible". some vendors, notably Microsoft, have insisted on adding platform-specific features (for example, in Microsoft's case, for Windows) and Sun has reacted strongly. After Microsoft insisted on retaining its incompatible variations, Sun sued and won both damages (some $20 million dollars) and a court order enforcing the terms of the license from Sun.
Microsoft has taken the position that a compliant Java, meeting the terms of the license as ordered by the court, is inadequate and has decided to leave out any Java system at all in future versions of Windows. This means that stock versions of Internet Explorer in such versions of Windows will break for Web sites using Java applets. Sun and others have made available Java run-time systems at no cost for those versions of Windows without Java. The necessity to download them is a consequence of the litigation and Microsoft's decision to comply with the court's order by leaving out Java support.
The first implementations of the language used an interpreted virtual machine to achieve portability, and many implementations still do. These implementations produce programs that run more slowly than the fully-compiled programs created by the typical C++ compiler and some later Java-language compilers, so the language suffered a reputation for producing slow programs. More recent implementations of the Java VM produce programs that run much faster, using multiple techniques. There is still a speed difference, but it is much smaller.
The first technique is to simply compile directly into native code like a more traditional compiler, skipping bytecodes entirely. This achieves great performance, but at the expense of portability. Another technique, the just-in-time compiler or "JIT", compiles the Java bytecodes into native code at the time that the program is run.
More sophisticated VMs even 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. Both of these techniques allow the program 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 is a matter of some controversy. 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, or Enterprise Java Beans.
The Java platform was one of the first systems to provide wide support for the execution of code from remote sources. An applet could run within a user's browser, executing code downloaded from a remote HTTP server.
The remote code runs in a highly restricted "sandbox", which protects the user from misbehaving or malicious code; publishers could apply for a certificate that they could use to digitally sign applets as "safe", giving them permission to break out of the sandbox and access the local filesystem and network, presumably under user control.
Most consider Java technology to deliver reasonably well on all
these promises. The language is not, however, without drawbacks.
Java tends to be more high-level than similar languages (such as C++), which means that the Java language lacks features such as hardware-specific data types and low-level pointers to arbitrary memory.
Although these features are frequently abused or misused by programmers, and make it harder (though far from impossible) to write a reliable, secure program, they are also powerful tools. However, the Java Native Interface (JNI) does provide a way for a Java program to call non-Java code in order to leverage the capabilities of other languages when necessary.
Some also see a shortcoming in its lack of multiple inheritance, a powerful feature of several object-oriented languages such as C++, Eiffel, and CLOS. Java subclasses have only one chance to inherit implementation by choosing one superclass, which can lead to false dilemmas (should HouseBoat inherit House or Boat?) and rules out certain implementation techniques such as mixins. However, proponents believe Java's multiple inheritance of interfaces is a good trade-off, providing most of the benefits of full multiple inheritance while sidestepping thorny situations in which a class inherits multiple conflicting implementations.
Java's paradigm of bytecode interpretation is widely seen to provide portability only at the expense of performance degradation relative to native code.
Dynamic compilation helps tremendously, but still typically cannot beat statically compiled code. This is because, when compilation occurs at run-time and thus counts against the performance of the program, the compiler must use its powerful, expensive optimizations very sparingly. However, precisely because dynamic compilers are present at runtime, they can take advantage of dynamic program behaviour, and can even recompile portions of the program as conditions change. Therefore, dynamic compilers can perform optimizations that static compilers cannot; and there are some programs for which dynamically compiled code is faster than statically compiled. (For instance, long-running programs whose behaviour depends on input data may fall into this category.)
It is not unreasonable to suppose that this set of programs will grow as dynamic-compilation technology improves.
Java's heavy use of heap-allocated objects causes programs to consume more memory than similar programs written in lower-level languages, where data storage can be optimized at a fine granularity. Because it treats almost everything as an object, Java encourages a style in which objects contain many references to other objects rather than raw data, making the objects larger and the data less localized. Both of these effects tend to work against modern memory hierarchies, whose caching schemes thrive on small working sets and good locality.
There are some who believe that, for certain projects, object orientation makes work harder instead of easier. This particular complaint is not unique to the Java language but applies to other object-oriented languages as well.
An example of a hello world program in the Java language follows:
Java does not have goto statements, as their use is widely considered poor programming practice. There are, however, some uses for goto that are considered acceptable, and Java has facilities to duplicate such functionality. Note that goto is a reserved word and as such cannot be used for identifiers.
Java provides two statements to exit from a loop in the midst of an iteration. The statement
Java's break and continue statements are more powerful than the C and C++ constructs of the same name, in that they are capable of exiting more than one loop-nesting level. Specifically, to jump out of a particular loop, one can label the loop, and then affix the label to the break or continue statement.
To achieve similar functionality in C and C++ would require the use of a goto statement.
For instance:
The statement
With
Overview
Object orientation
Platform independence
Secure execution of remote code
Evaluation
Language
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello world!");
}
}
Control structures
Loops
while (Boolean expression) {
statement(s)
}
do {
statement(s)
} while (Boolean expression);
for (initialisation ; termination condition ; incrementing expr) {
statement(s)
}
Conditional statements
if (Boolean expression) {
statement(s)
}
if (Boolean expression) {
statement(s)
} else {
statement(s)
}
With else if arbitrarily complex if-then-constructions may be built.if (Boolean expression) {
statement(s)
} else if (Boolean expression) {
statement(s)
} else if (Boolean expression) {
statement(s)
} else {
statement(s)
}
switch (integer expression) {
case constant integer expr:
statement(s)
break;
...
default:
statement(s)
break;
}
Exception handling
try {
statement(s)
} catch (exception type) {
statement(s)
} catch (exception type) {
statement(s)
} finally {
statement(s)
}
Unstructured control flow
Early exit from loops
continue;
terminates the current iteration of a loop and starts the next one, behaving as a goto jumping to the top of the loop body. Similarly, the statement
break;
exits the loop, terminating the current iteration and executing no more iterations. The effect is that of a goto jumping just past the end of the loop.outer: while (true) {
inner: while (true) {
break; // Exits the innermost loop
break inner; // Also exits the innermost loop
break outer; // Exits the outermost loop
}
}
Early exit from methods
return;
terminates a method.return aValue;
aValue may be given back to the calling method.
| Variable Type | Description |
|---|---|
| byte | 8-bit signed (two's complement) integer |
| short | 16-bit signed (two's complement) integer |
| int | 32-bit signed (two's complement) integer |
| long | 64-bit signed (two's complement) integer |
| float | 32-bit single-precision floating point (IEEE 754 standard) |
| double | 64-bit double-precision floating point
(IEEE 754 standard) |
| char | 16-bit single Unicode character |
| boolean | true or false |
Characters use the 16-bit Unicode encoding (namely UTF-16). It contains all of the usual characters, but also includes character sets for many languages other than English, including Greek, Cyrillic, Chinese, Arabic, etc.
Java programs can use all of these characters, although most editors do not have built-in support for character sets other than the usual ASCII characters.
Arrays and strings are not primitive types: they are objects.
One thing that Java lets one do is create an interface that classes will then implement. For example, you can create an interface like this:
Interfaces in Java work differently to the concept of the interface in other object-oriented programming languages - they behave much more like the concept of the Objective-C protocol.
Following code snippet shows how user input is accepted from a console and displayed back :
The Java platform and language began as an internal project at Sun Microsystems in the December 1990 timeframe. Patrick Naughton, an engineer at Sun, had become increasingly frustrated with the state of Sun's C++ and C APIs and tools. While considering moving to NeXT, Patrick was offered a chance to work on new technology and thus the Stealth Project was started.
The Stealth Project was soon renamed to the Green Project with James Gosling and Mike Sheridan joining Patrick Naughton. They, together with some other engineers, began work in a small office on Sand Hill Road in Menlo Park, California to develop a new technology. The team originally considered C++ as the language to use, but many of them as well as Bill Joy found C++ and the available APIs problematic for several reasons.
Their platform was an embedded platform and had limited resources. Many members found that C++ was too complicated and developers often misused it. They found C++'s lack of garbage collection to also be a problem. Security, distributed programming, and threadinging support was also required. Finally, they wanted a platform that could be easily ported to all types of devices.
According to the available accounts, Bill Joy had ideas of a new language combining the best of MESA and C. He proposed, in a paper called Further, to Sun that its engineers should produce an object-oriented environment based on C++. James Gosling's frustrations with C++ began while working on Imagination, an SGML editor. Initially, James attempted to modify and extend C++, which he referred to as C++ ++ --, but soon abandoned that in favor of creating an entirely new language, called Oak named after the oak tree that stood just outside his office.
Like many stealth projects working on new technology, the team worked long hours and by the summer of 1992, they were able to demo portions of the new platform including the Green OS, Oak the language, the libraries, and the hardware. Their first attempt focused on building a PDA-like device having a highly graphical interface and a smart agent called Duke to assist the user.
The device was named Star7 after a telephone feature activated by *7 on a telephone keypad. The feature enabled users to answer the telephone anywhere. The PDA device itself was demonstrated on September 3, 1992.
In November of that year, the Green Project was spun off to become a wholly owned subsidiary of Sun Microsystems: FirstPerson, Inc. The team relocated to Palo Alto. The FirstPerson team was interested in building highly interactive devices and when Time Warner issued an RFP for a set-top box, FirstPerson changed their target and responded with a proposal for a set-top box platform. However, the cable industry felt that their platform gave too much control to the user and FirstPerson lost their bid to SGI. An additional deal with 3DO for a set-top box also failed to materialize. FirstPerson was unable to generate any interest within the cable TV industry for their platform. Following their failures, the company, FirstPerson, was rolled back into Sun.
In June and July of 1994, after a 3 day brain storm session with John Gage, James Gosling, Bill Joy, Patrick Naughton, Wayne Rosing, and Eric Schmidt, the team re-targeted yet again its efforts, this time to use the technology for the Internet. They felt that with the advent of the Mosaic web browser, the Internet was on its way to evolving into the same highly interactive vision that they had had for the cable TV network. Patrick Naughton wrote a small web browser, WebRunner, as a prototype. WebRunner would later be renamed HotJava.
It was also in 1994, that Oak was renamed Java. An IP (intellectual property) search revealed that Oak had already been trademarked for another language so the team searched for a new name. The name Java was coined at a local coffee shop frequented by some of the members. It is not clear whether the name is an acronymn or not. Most likely, it is not, however some accounts claim that it stands for the names of James Gosling, Arthur Van Hoff, and Andy Bechtolsheim.
In October of 1994, HotJava and the Java platform was demoed for Sun executives. Java 1.0a was made available for download in 1994, but the first public release of Java and the HotJava web browser came on May 23, 1995, at the SunWorld conference. The announcement was made by John Gage, the Director of Science for Sun Microsystems. His announcement was accompanied by a surprise announcement by Marc Andreessen, Executive Vice President of Netscape, that Netscape would be including Java support in its browsers.
In January of 1996, the JavaSoft business group was formed by Sun Microsystems to develop the technology.
Sun has defined 3 platforms targeting different application environments and segmented many of its APIs so that they belong to one of the platforms. The platforms are:
The set of APIs is controlled by Sun Microsystems in cooperation with others through its Java Community Process program. Companies or individuals participating in this process can influence the design and development of the APIs, but Sun retains ownership and control of the APIs. This process has been a subject of controversy.
The Java language has undergone several changes since JDK 1.0 as well as numeral additions of packages to the standard library:
Extensionss and architectures closely tied to the Java programming language include:
There is an interpreted version of Java called beanshell which may be used as a shell scripting language.
The interpreter may be embedded in a Java application to make it scriptable.
Characters
Interfaces and Classes
public interface Deleteable {
public void delete ();
}
This interface says that anything that is Deleteable will have the exact method delete(). There are many uses to this concept because you can have a class like the following:public class Fred implements Deleteable {
//Must include the delete () method
}
Then, in another class, something like this can be said.public void deleteAll (Deleteable[] list) {
for (int i = 0; i < list.length; i++)
list[i].delete();
}
The feature is a result of compromise. The designers of Java decided not to support multinheritance but interface can be used to simulate multinheritance in some occasions.Input/Output
public static void main(String[] args) throws java.io.IOException {
char a;
System.out.println("Welcome user, please enter a letter");
a = (char) System.in.read();
System.out.println("Your letter was " + a);
}
Early history
Java meets the Internet
APIs
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.Version history
In addition to the language changes, much more dramatic changes have been made to the Java class library over the years, which has grown from a few hundred classes in Java 1.0 to over three thousand in Java 1.5. Entire new APIs, such as Swing (Java) and Java2D, have been introduced, and many of the original 1.0 classes and methods have been deprecated.Interpreted version
Related free software
See also
External links
Programming languages
Ada | Algol | APL | BASIC | C | C++ | C# | Cg | COBOL | Common Lisp | ColdFusion | Delphi | Eiffel | Forth | FORTRAN | Haskell | Java | JavaScript | Jython | Lisp | Logo | Mesa | ML | Modula-2 | Oberon | Objective-C | Objective Caml | Pascal | Perl | PHP | PL/I | PostScript | Powerbuilder | Prolog | Python | QBASIC | REXX | Ruby | Scheme | Smalltalk | Tcl/Tk | Visual Basic