"Java
language" redirects here. For the natural language from the Indonesian
island of Java, see Javanese language.
Not to be
confused with JavaScript.
|
Java
|
|||||||||
 |
|||||||||
|
First appeared
|
1995;
20 years ago[1]
|
||||||||
|
Java
Standard Edition 9 Early Access b46 (1.9.0-ea-b46) / January 20, 2015; 4
months ago
|
|||||||||
|
Implementation
language
|
|||||||||
|
Cross-platform
(multi-platform)
|
|||||||||
|
Website
|
|||||||||
|
|||||||||
|
|||||||||
Java is a general-purpose computer programming language that is concurrent, class-based, object-oriented,[12]
and specifically designed to have as few implementation dependencies as
possible. It is intended to let application developers "write once, run anywhere" (WORA),[13]
meaning that compiled
Java code can run on all platforms that support Java without the need for
recompilation.[14]
Java applications are typically compiled to bytecode
that can run on any Java virtual machine (JVM) regardless of computer architecture. As of 2015, Java is
one of the most popular programming languages
in use,[15][16][17][18]
particularly for client-server web applications, with a reported 9 million
developers.[citation needed] Java was
originally developed by James Gosling at Sun
Microsystems (which has since been acquired by Oracle Corporation) 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 it has
fewer low-level facilities than either of
them.
The original
and reference implementation Java compilers,
virtual machines, and class libraries were originally released by Sun
under proprietary licences. As of May 2007, in compliance with the
specifications of the Java Community Process, Sun relicensed most
of its Java technologies under the GNU General Public License. Others have
also developed alternative implementations of these Sun technologies, such as
the GNU Compiler for Java (bytecode compiler), GNU
Classpath (standard libraries), and IcedTea-Web
(browser plugin for applets).
Contents
- 1 History
- 2 Practices
- 3 Syntax
- 4 Examples
- 5 Special classes
- 6 Criticism
- 7 Use on unofficial software platforms
- 8 Class libraries
- 9 Documentation
- 10 Editions
- 11 See also
- 12 Notes
- 13 References
- 14 External links
History
See also: Java (software platform) § History
Duke, the
Java mascot
James
Gosling, the creator of Java (2008)
The TIOBE programming language
popularity index graph from 2002 to 2015. Over the course of a decade Java
(black) and C (blue) competing for the top position.
James
Gosling, Mike Sheridan, and Patrick
Naughton initiated the Java language project in June 1991.[19]
Java was originally designed for interactive television, but it was too
advanced for the digital cable television industry at the time.[20]
The language was initially called Oak after an oak tree that stood
outside Gosling's office. Later the project went by the name Green and
was finally renamed Java, from Java coffee.[21]
Gosling designed Java with a C/C++-style syntax that system and application
programmers would find familiar.[22]
Sun
Microsystems released the first public implementation as Java 1.0 in 1995.[1]
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. The Java 1.0
compiler was re-written in Java by Arthur
van Hoff to comply strictly with the Java 1.0 language specification.[23]
With the advent of Java 2 (released initially as J2SE 1.2 in December
1998 – 1999), new versions had multiple configurations built for different
types of platforms. J2EE included technologies and APIs for enterprise
applications typically run in server environments, while J2ME featured
APIs optimized for mobile applications. The desktop version was renamed J2SE.
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 JTC 1 standards body and later the Ecma International to formalize Java, but it
soon withdrew from the process.[24][25][26]
Java remains a de facto standard, controlled through the Java Community Process.[27]
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.
On November
13, 2006, Sun released much of Java as free and open-source software,
(FOSS), under the terms of the GNU General Public License (GPL). On May
8, 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.[28]
Sun's
vice-president Rich Green said that Sun's ideal role with regard to Java was as
an "evangelist".[29]
Following Oracle Corporation's acquisition of Sun
Microsystems in 2009-2010, Oracle has described itself as the "steward of
Java technology with a relentless commitment to fostering a community of
participation and transparency".[30]
This did not prevent Oracle from filing a lawsuit against Google shortly after
that for using Java inside the Android SDK (see Google section below). Java
software runs on everything from laptops to data centers, game consoles to scientific supercomputers.[31]
On April 2, 2010, James Gosling resigned from Oracle.[32]
Principles
There were
five primary goals in the creation of the Java language:[14]
- It must be "simple, object-oriented and familiar".
- It must be "robust and secure".
- It must be "architecture-neutral and portable".
- It must execute with "high performance".
- It must be "interpreted, threaded, and dynamic".
Versions
Main
article: Java version history
Major
release versions of Java, along with their release dates:
- JDK 1.0 (January 21, 1996)
- JDK 1.1 (February 19, 1997)
- J2SE 1.2 (December 8, 1998)
- J2SE 1.3 (May 8, 2000)
- J2SE 1.4 (February 6, 2002)
- J2SE 5.0 (September 30, 2004)
- Java SE 6 (December 11, 2006)
- Java SE 7 (July 28, 2011)
- Java SE 8 (March 18, 2014)
Practices
Java platform
Main
articles: Java (software platform) and Java virtual machine
One design
goal of Java is portability, which means that programs written for the Java
platform must run similarly on any combination of hardware and operating system
with adequate runtime support. This is achieved by compiling the Java language
code to an intermediate representation called Java
bytecode, instead of directly to architecture-specific machine
code. Java bytecode instructions are analogous to machine code, but they
are intended to be executed 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. Just-in-Time (JIT) compilers were introduced
from an early stage that compile bytecodes to machine code during runtime. Java
is platform independent. But as Java virtual machine must convert Java
bytecode into machine language which depends on the operating
system being used, it is platform dependent.[33]
Implementations
See also: Free Java implementations
Oracle Corporation is the current owner of the
official implementation of the Java SE platform, following their acquisition of
Sun
Microsystems on January 27, 2010. This implementation is based on the
original implementation of Java by Sun. The Oracle implementation is available
for Microsoft Windows, Mac OS X, Linux and Solaris. Because Java lacks any formal
standardization recognized by Ecma International, ISO/IEC, ANSI, or other
third-party standards organization, the Oracle implementation is the de
facto standard.
The Oracle
implementation is packaged into two different distributions: The Java Runtime
Environment (JRE) which contains the parts of the Java SE platform required to
run Java programs and is intended for end users, and the Java Development Kit (JDK), which is intended
for software developers and includes development tools such as the Java
compiler, Javadoc,
Jar, and a debugger.
OpenJDK is
another notable Java SE implementation that is licensed under the GNU GPL. The
implementation started when Sun began releasing the Java source code under the
GPL. As of Java SE 7, OpenJDK is the official Java reference implementation.
The goal of
Java is to make all implementations of Java compatible. Historically, Sun's trademark
license for usage of the Java brand insists that all implementations be
"compatible". This resulted in a legal dispute with Microsoft
after Sun claimed that the Microsoft implementation did not support RMI or JNI and had added platform-specific features
of their own. Sun sued in 1997, and in 2001 won a settlement of US$20 million,
as well as a court order enforcing the terms of the license from Sun.[34]
As a result, Microsoft no longer ships Java with Windows.
Platform-independent
Java is essential to Java EE, and an even more
rigorous validation is required to certify an implementation. This environment
enables portable server-side applications.
Performance
Main
article: Java performance
Programs
written in Java have a reputation for being slower and requiring more memory
than those written in C++.[35][36]
However, Java programs' execution speed improved significantly with the
introduction of Just-in-time compilation in 1997/1998 for Java 1.1,[37]
the addition of language features supporting better code analysis (such as
inner classes, the StringBuilder class, optional assertions, etc.), and
optimizations in the Java virtual machine, such as HotSpot becoming
the default for Sun's JVM in 2000.
Some
platforms offer direct hardware support for Java; there are microcontrollers
that can run Java in hardware instead of a software Java virtual machine, and ARM
based processors can have hardware support for executing Java bytecode through
their Jazelle
option.
Automatic memory management
Java uses an
automatic garbage collector
to manage memory in the object lifecycle. The programmer determines when
objects are created, and the Java runtime is responsible for recovering the
memory once objects are no longer in use. Once no references to an object
remain, the unreachable memory becomes eligible to be freed
automatically by the garbage collector. Something similar to a memory leak
may still occur if a programmer's code holds a reference to an object that is
no longer needed, typically when objects that are no longer needed are stored
in containers that are still in use. If methods for a nonexistent object are
called, a "null pointer exception" is thrown.[38][39]
One of the
ideas behind Java's automatic memory management model is that programmers can
be spared the burden of having to perform manual memory management. In some
languages, memory for the creation of objects is implicitly allocated on the stack, or explicitly allocated and
deallocated from the heap. In the latter case the responsibility of
managing memory resides with the programmer. If the program does not deallocate
an object, a memory leak occurs. If the program attempts to access
or deallocate memory that has already been deallocated, the result is undefined
and difficult to predict, and the program is likely to become unstable and/or
crash. This can be partially remedied by the use of smart
pointers, but these add overhead and complexity. Note that garbage
collection does not prevent "logical" memory leaks, i.e.,
those where the memory is still referenced but never used.
Garbage
collection may happen at any time. Ideally, it will occur when a program is
idle. It is guaranteed to be triggered if there is insufficient free memory on
the heap to allocate a new object; this can cause a program to stall
momentarily. Explicit memory management is not possible in Java.
Java does
not support C/C++ style pointer arithmetic, where object
addresses and unsigned integers (usually long integers) can be used
interchangeably. This allows the garbage collector to relocate referenced
objects and ensures type safety and security.
As in C++
and some other object-oriented languages, variables of Java's primitive data types are not objects. Values of
primitive types are either stored directly in fields (for objects) or on the stack (for methods) rather than on
the heap, as is commonly true for objects (but see escape
analysis). This was a conscious decision by Java's designers for
performance reasons. Because of this, Java was not considered to be a pure
object-oriented programming language. However, as of Java 5.0, autoboxing enables
programmers to proceed as if primitive types were instances of their wrapper
class.
Java
contains multiple types of garbage collectors. By default,[citation needed] HotSpot uses
the parallel
scavenge garbage collector. However, there are also several other garbage
collectors that can be used to manage the heap. For 90% of applications in
Java, the Concurrent Mark-Sweep garbage
collector is sufficient.[40]
Oracle aims to replace CMS with the Garbage-first collector
(G1).
Syntax
Main
article: Java
syntax
The syntax
of Java is largely derived from C++. Unlike C++, which combines the syntax for structured,
generic, and object-oriented programming, Java was built almost exclusively as
an object-oriented language.[14]
All code is written inside classes, and every data item is an object, with the
exception of the primitive data types, i.e. integers, floating-point
numbers, boolean values, and characters, which are not
objects for performance reasons.
Unlike C++,
Java does not support operator overloading[41]
or multiple inheritance for classes,
though multiple inheritance is supported for interfaces.[42]
This simplifies the language and aids in preventing potential errors and anti-pattern
design.
Java uses
comments similar to those of C++. There are three different styles of comments:
a single line style marked with two slashes (//), a multiple line style opened with /* and closed with */, and the Javadoc
commenting style opened with /** and closed with */. The Javadoc style of commenting allows the user to run the Javadoc
executable to create documentation for the program.
Example:
// This is an example of a single line comment using
two slashes
/* This is an example of a multiple line comment using
the slash and asterisk.
This type of
comment can be used to hold a lot of information or deactivate
code, but it is
very important to remember to close the comment. */
package fibsandlies;
import java.util.HashMap;
/**
* This is an
example of a Javadoc comment; Javadoc can compile documentation
* from this
text. Javadoc comments must immediately precede the class, method, or field
being documented.
*/
public class FibCalculator extends Fibonacci implements
Calculator {
private static
Map<Integer, Integer> memoized = new HashMap<Integer, Integer>();
/**
* The main
method written as follows is used by the JVM as a starting point for the
program.
*/
public static
void main(String[] args) {
memoized.put(1,
1);
memoized.put(2, 1);
System.out.println(fibonacci(12));
//Get the 12th Fibonacci number and print to console
}
/**
* An
example of a method written in Java, wrapped in a class.
* Given a
non-negative number FIBINDEX, returns
* the Nth
Fibonacci number, where N equals FIBINDEX.
* @param
fibIndex The index of the Fibonacci number
* @return
The Fibonacci number
*/
public static
int fibonacci(int fibIndex) {
if (memoized.containsKey(fibIndex))
{
return
memoized.get(fibIndex);
} else {
int
answer = fibonacci(fibIndex - 1) + fibonacci(fibIndex - 2);
memoized.put(fibIndex, answer);
return
answer;
}
}
}
Examples
Hello World
The
traditional "Hello, world!" program
can be written in Java as:[43]
class HelloWorldApp {
public static
void main(String[] args) {
System.out.println("Hello
World!"); // Prints the string to the console.
}
}
To compare
this to other programming languages see the list of "Hello World!" program
examples.
Source files
must be named after the public class they contain, appending the suffix .java, for example, HelloWorldApp.java. It must first be compiled into
bytecode, using a Java compiler, producing a file named HelloWorldApp.class. Only then can it be executed, or
'launched'. The Java source file may only contain one public class, but it can
contain multiple classes with other than public access and any number of public
inner
classes. When the source file contains multiple classes, make one class
'public' and name the source file with that public class name.
A class that is not declared public 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 were 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 located.
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 class members that are not also 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 JavaBeans 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 it allows an alternative 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.
The string
"Hello World!" is automatically converted to a String object by the
compiler.
A more comprehensive example
 |
This
section does not cite any references or sources. Please help
improve this section by adding citations to reliable sources.
Unsourced material may be challenged and removed. (May 2013)
|
 |
This
article contains instructions, advice, or how-to
content. The purpose of Wikipedia is to present facts, not to train.
Please help improve
this article either by rewriting the how-to content or by moving
it to Wikiversity, Wikibooks or Wikivoyage (May 2013)
|
// OddEven.java
import javax.swing.JOptionPane;
public class OddEven {
private int
userInput; // a whole number("int" means integer)
/**
* This is
the constructor method. It gets called when an object of the OddEven type
* is being created.
*/
public
OddEven() {
/*
* In
most Java programs constructors can initialize objects with default values, or
create
* other
objects that this object might use to perform its functions. In some Java
programs, the
*
constructor may simply be an empty function if nothing needs to be initialized
prior to the
*
functioning of the object. In this program's case, an empty constructor would
suffice.
* A
constructor must exist; however, if the user doesn't put one in then the
compiler
* will
create an empty one.
*/
}
/**
* This is
the main method. It gets called when this class is run through a Java
interpreter.
* @param
args command line arguments (unused)
*/
public static
void main(final String[] args) {
/*
* This
line of code creates a new instance of this class called "number"
(also known as an
*
Object) and initializes it by calling the constructor. The next line of code
calls
* the
"showDialog()" method, which brings up a prompt to ask you for a
number.
*/
OddEven
number = new OddEven();
number.showDialog();
}
public void
showDialog() {
/*
*
"try" makes sure nothing goes wrong. If something does,
* the
interpreter skips to "catch" to see what it should do.
*/
try {
/*
*
The code below brings up a JOptionPane, which is a dialog box
*
The String returned by the "showInputDialog()" method is converted
into
*
an integer, making the program treat it as a number instead of a word.
*
After that, this method calls a second method, calculate() that will
*
display either "Even" or "Odd."
*/
userInput = Integer.parseInt(JOptionPane.showInputDialog("Please
enter a number."));
calculate();
} catch (final NumberFormatException e) {
/*
*
Getting in the catch block means that there was a problem with the format of
*
the number. Probably some letters were typed in instead of a number.
*/
System.err.println("ERROR: Invalid
input. Please type in a numerical value.");
}
}
/**
* When this
gets called, it sends a message to the interpreter.
* The
interpreter usually shows it on the command prompt (For Windows users)
* or the
terminal (For *nix users).(Assuming it's open)
*/
private void
calculate() {
if ((userInput
% 2) == 0) {
JOptionPane.showMessageDialog(null,
"Even");
} else {
JOptionPane.showMessageDialog(null,
"Odd");
}
}
}
- The import statement imports the JOptionPane class from the javax.swing package.
- The OddEven class declares a single private field of type int named userInput. Every instance of the OddEven class has its own copy of the userInput field. The private declaration means that no other class can access (read or write) the userInput field.
- OddEven() is a public constructor. Constructors have the same name as the enclosing class they are declared in, and unlike a method, have no return type. A constructor is used to initialize an object that is a newly created instance of the class.
- The calculate() method is declared without the static keyword. This means that the method is invoked using a specific instance of the OddEven class. (The reference used to invoke the method is passed as an undeclared parameter of type OddEven named this.) The method tests the expression userInput % 2 == 0 using the if keyword to see if the remainder of dividing the userInput field belonging to the instance of the class by two is zero. If this expression is true, then it prints Even; if this expression is false it prints Odd. (The calculate method can be equivalently accessed as this.calculate and the userInput field can be equivalently accessed as this.userInput, which both explicitly use the undeclared this parameter.)
- OddEven number = new OddEven(); declares a local object reference variable in the main method named number. This variable can hold a reference to an object of type OddEven. The declaration initializes number by first creating an instance of the OddEven class, using the new keyword and the OddEven() constructor, and then assigning this instance to the variable.
- The statement number.showDialog(); calls the calculate method. The instance of OddEven object referenced by the number local variable is used to invoke the method and passed as the undeclared this parameter to the calculate method.
- userInput = Integer.parseInt(JOptionPane.showInputDialog("Please Enter A Number")); is a statement that converts the type of String to the primitive data type int by using a utility function in the primitive wrapper class Integer.
Special classes
|
|
This
article contains instructions, advice, or how-to
content. The purpose of Wikipedia is to present facts, not to train.
Please help improve
this article either by rewriting the how-to content or by moving
it to Wikiversity, Wikibooks or Wikivoyage (January
2012)
|
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 javax.swing.JApplet;
import java.awt.Graphics;
public class Hello extends JApplet {
public void
paintComponent(final Graphics g) {
g.drawString("Hello,
world!", 65, 95);
}
}
The import statements direct the Java
compiler to include the javax.swing.JApplet 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. JApplet) instead of the fully qualified
class name (FQCN, i.e. javax.swing.JApplet).
The Hello class extends (subclasses) the JApplet (Java Applet) class; the JApplet class provides the framework for
the host application to display and control the lifecycle
of the applet. The JApplet class is a JComponent (Java Graphical 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 paintComponent(Graphics) method (additionally indicated with
the annotation, supported as of JDK 1.5, Override) inherited from the Container superclass to provide the
code to display the applet. The paintComponent() method is passed a Graphics object that contains the graphic
context used to display the applet. The paintComponent() 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.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML
4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<!-- Hello.html -->
<html>
<head>
<title>Hello
World Applet</title>
</head>
<body>
<applet
code="Hello.class" width="200" height="200">
</applet>
</body>
</html>
An applet is
placed in an HTML
document using the <applet> HTML element. The applet tag has three attributes set: code="Hello" specifies the name of the JApplet 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,[44]
although support for these elements by web browsers is inconsistent.[45]
However, the applet tag is deprecated,
so the object tag is
preferred where supported.
The host
application, typically a Web browser, instantiates the Hello applet and creates an AppletContext for the applet. Once the applet has
initialized itself, it is added to the AWT display hierarchy. The paintComponent() method is called by the AWT event dispatching thread whenever the
display needs the applet to draw itself.
Servlet
Main
article: Java 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.*;
import javax.servlet.*;
public class Hello extends GenericServlet {
public void
service(final ServletRequest request, final ServletResponse response)
throws
ServletException, IOException {
response.setContentType("text/html");
final PrintWriter pw = response.getWriter();
try {
pw.println("Hello,
world!");
} finally
{
pw.close();
}
}
}
The import statements direct the Java compiler
to include all the public classes and interfaces from the java.io and javax.servlet packages in the compilation.
Packages make Java well suited for large scale applications.
The Hello class extends the GenericServlet 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 service(ServletRequest,
ServletResponse) method
defined by the Servlet interface to provide the code for the service
request handler. The service() method is passed: a ServletRequest object that contains the request
from the client and a ServletResponse object used to create the response
returned to the client. The service() method declares that it throws the exceptions ServletException and IOException if a problem prevents it from
responding to the request.
The setContentType(String) method in the response object is
called to set the MIME
content type of the returned data to "text/html". The getWriter() method in the response returns a PrintWriter object that is used to write the
data that is sent to the client. The println(String) method is called to write the "Hello,
world!" string to the response and then the close() 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.
JavaServer Pages
Main
article: JavaServer Pages
JavaServer
Pages (JSP) 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 by
using the special delimiters <% and %>. A JSP is
compiled to a Java servlet, a Java application in its own right, the
first time it is accessed. After that, the generated servlet creates the
response.
Swing application
Main
article: Swing (Java)
Swing
is a graphical user interface library for the Java SE platform. 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. Where prior
implementations of these looks and feels may have been considered lacking,
Swing in Java SE 6 addresses this problem by using more native GUI widget drawing routines of the
underlying platforms.
This example
Swing application creates a single window with "Hello, world!"
inside:
// Hello.java (Java SE 5)
import javax.swing.*;
public class Hello extends JFrame {
public Hello()
{
super("hello");
super.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
super.add(new
JLabel("Hello, world!"));
super.pack();
super.setVisible(true);
}
public static
void main(final String[] args) {
new
Hello();
}
}
The Hello class extends the JFrame class; the JFrame class implements a window with a title bar
and a close control.
The Hello() constructor initializes
the frame by first calling the superclass constructor, passing the parameter "hello", which is used as the window's
title. It then calls the setDefaultCloseOperation(int) method inherited from JFrame to set the default operation when
the close control on the title bar is selected to WindowConstants.EXIT_ON_CLOSE — this causes the JFrame to be disposed of when the frame is
closed (as opposed to merely hidden), which allows the Java virtual machine to
exit and the program to terminate. Next, a JLabel is created for the string "Hello,
world!" and the add(Component) method inherited from the Container superclass is called to add the
label to the frame. The pack() method inherited from the Window superclass is called to size the
window and lay out its contents.
The main() method is called by the Java
virtual machine when the program starts. It instantiates a new Hello frame and causes it to be displayed
by calling the setVisible(boolean) method inherited from the Component superclass with the boolean
parameter true. 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.
Generics
Main
article: Generics in Java
In 2004, generics were added to the Java language, as
part of J2SE 5.0. Prior to the introduction of generics, each variable
declaration had to be of a specific type. For container classes, for example,
this is a problem because there is no easy way to create a container that
accepts only specific types of objects. Either the container operates on all
subtypes of a class or interface, usually Object, or a different container class has to be created for
each contained class. Generics allow compile-time type checking without having
to create many container classes, each containing almost identical code. In
addition to enabling more efficient code, certain runtime exceptions are
converted to compile-time errors, a characteristic known as type safety.
Criticism
Main
article: Criticism of Java
Criticisms
directed at Java include the implementation of generics,[46]
speed,[47]
the handling of unsigned numbers,[48]
the implementation of floating-point arithmetic,[49]
and a history of security vulnerabilities in the primary Java VM implementation
HotSpot.[50]
Use on unofficial software platforms
The Java
programming language requires the presence of a software platform in order for
compiled programs to be executed. A well-known unofficial Java-like software
platform is the Android software platform, which allows the use of Java 6 and
some Java 7 features, uses a different standard library (Apache
Harmony reimplementation), different bytecode language and different
virtual machine, and is designed for low-memory devices such as smartphones
and tablet computers.
The Android operating system makes
extensive use of Java-related technology.
Google
See also: Oracle
v. Google
Google and Android,
Inc. have chosen to use Java as a key pillar in the creation of the Android operating system, an open source
mobile operating system. Although the
Android operating system, built on the Linux
kernel, was written largely in C, the Android SDK uses the Java language as
the basis for Android applications. However, Android does not use the Java
virtual machine, instead using Java bytecode as an intermediate step and
ultimately targeting Android's own Dalvik virtual machine.
Android also
does not provide the full Java SE standard library, although the Android class
library does include an independent implementation of a large subset of it.
This led to a legal dispute between Oracle and Google. On May 7, 2012, a San
Francisco jury found that if APIs could be copyrighted, then Google had infringed
Oracle's copyrights by the use of Java in Android devices.[51]
District Judge William Haskell Alsup ruled on May 31, 2012,
that APIs cannot be copyrighted,[52]
but this was reversed by the Ninth Circuit Court of Appeals in May 2014.[53][54]
Class libraries
|
|
This
section needs additional citations for verification. Please help improve
this article by adding citations to reliable
sources. Unsourced material may be challenged and removed. (December
2014)
|
Main
article: Java Class Library
The Java Class Library is the standard
library, developed to support application development in Java. It 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.[when?] The
class library contains features such as:
- The core libraries, which include:
- Collection libraries that implement data structures such as lists, dictionaries, trees, sets, queues and double-ended queue, or stacks[55]
- XML Processing (Parsing, Transforming, Validating) libraries
- Security[56]
- Internationalization and localization libraries[57]
- 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
- JMX for managing and monitoring applications
- User interface libraries, which include:
- The (heavyweight, or native) Abstract Window 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 the Java virtual machine that is the means by which the bytecodes 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.
Documentation
Main
article: Javadoc
Javadoc is a
comprehensive documentation system, created by Sun
Microsystems, used by many Java developers. It provides developers with an
organized system for documenting their code. Javadoc comments have an extra
asterisk at the beginning, i.e. the delimiters are /** and */, whereas the normal multi-line comments in Java are
set off with the delimiters /* and */.[58]
Editions
|
Java
editions
|
|
|
|
Sun has
defined and supports four editions of Java targeting different application
environments and segmented many of its APIs so that they belong to one
of the platforms. The platforms are:
- Java Card for smartcards.[59]
- Java Platform, Micro Edition (Java ME) — targeting environments with limited resources.[60]
- Java Platform, Standard Edition (Java SE) — targeting workstation environments.[61]
- Java Platform, Enterprise Edition (Java EE) — targeting large distributed enterprise or Internet environments.[62]
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.[relevant
to this section? – discuss]
Sun also
provided an edition called PersonalJava that has been superseded by later,
standards-based Java ME configuration-profile pairings.
See also
|
|
||||
 |
||||
 |
||||
- Dalvik
- JavaOne
- Javapedia
- List of Java virtual machines
- List of Java APIs
- List of JVM languages
- Graal, a project aiming to implement a high performance Java dynamic compiler and interpreter
Comparison of Java with other languages
Notes
1. "The
History of Java Technology". Retrieved October 6, 2012.
2. "Java 7 and
Java 8 Releases by date". Retrieved March 3, 2015.
3. Java 5.0 added several new language
features (the enhanced for loop, autoboxing, varargs
and annotations), after they were introduced in the
similar (and competing) C# language [1] [2]
4. Gosling, James; and McGilton, Henry
(May 1996). "The
Java Language Environment".
5. Gosling, James; Joy, Bill; Steele,
Guy; and Bracha, Gilad. "The
Java Language Specification, 2nd Edition".
6. "The
A-Z of Programming Languages: Modula-3". Computerworld.com.au.
Retrieved 2010-06-09.
7. Niklaus
Wirth stated on a number of public occasions, e.g. in a lecture at the
Polytechnic Museum, Moscow in September, 2005 (several independent first-hand
accounts in Russian exist, e.g. one with an audio recording: Filippova, Elena
(September 22, 2005). "Niklaus
Wirth's lecture at the Polytechnic Museum in Moscow".), that the Sun
Java design team licenced the Oberon compiler sources a number of years prior
to the release of Java and examined it: a (relative) compactness, type safety,
garbage collection, no multiple inheritance for classes -- all these key
overall design features are shared by Java and Oberon.
8. Patrick
Naughton cites Objective-C as a strong influence on the design of the
Java programming language, stating that notable direct derivatives include Java
interfaces (derived from Objective-C's protocol)
and primitive wrapper classes. [3]
9. TechMetrix Research (1999). "History of Java"
(PDF). Java
Application Servers Report. The project went ahead under the name
"green" and the language was based on an old model of UCSD Pascal,
which makes it possible to generate interpretive code
10. "A Conversation with
James Gosling – ACM Queue". Queue.acm.org. 2004-08-31. Retrieved 2010-06-09.
11. "Facebook
Q&A: Hack brings static typing to PHP world". InfoWorld.
2014-03-26. Retrieved 2015-01-11.
12. Gosling
et al. 2014, p. 1.
13. "Write
once, run anywhere?". Computer
Weekly. 2002-05-02. Retrieved 2009-07-27.
14. "1.2
Design Goals of the Java™ Programming Language". Oracle. 1999-01-01.
Retrieved 2013-01-14.
15. McMillan, Robert (2013-08-01). "Is Java
Losing Its Mojo?". wired.com. Java is on the wane, at least according to one
outfit that keeps on eye on the ever-changing world of computer programming
languages. For more than a decade, it has dominated the Tiobe Programming
Community Index — a snapshot of software developer enthusiasm that looks at
things like internet search results to measure how much buzz different
languages have. But lately, Java has been slipping.
16. RedMonk
Index on redmonk.com (Stephen O'Grady, January 2015)
17. "Programming
Language Popularity". langpop.com. 2013-10-25. Retrieved 2015-04-02.
Normalized Comparison: 1st C, 2nd Java, 3rd PHP
18. "TIOBE
Programming Community Index". 2015. Retrieved 2015-04-03.
19. Byous, Jon (c. 1998). "Java
technology: The early years". Sun Developer Network. Sun
Microsystems. Archived from the original on
April 20, 2005. Retrieved 2005-04-22.
20. Object-oriented programming "The History of Java
Technology". Sun Developer Network. c. 1995. Retrieved 2010-04-30.
21. "So why did they
decide to call it Java?", Kieron Murphy, JavaWorld.com, 10/04/96
22. Kabutz, Heinz; Once Upon an
Oak. Artima. Retrieved April 29, 2007.
23. Object-oriented
Programming with Java: Essentials and Applications. Tata McGraw-Hill
Education. p. 34.
24. "JSG - Java Study
Group". open-std.org.
27. "Java Community Process
website". Jcp.org. 2010-05-24. Retrieved 2010-06-09.
28. "JAVAONE:
Sun – The bulk of Java is open sourced". GrnLight.net. Retrieved 2014-05-26.
30. "Oracle and
Java". oracle.com. Oracle Corporation. Retrieved 2010-08-23.
Oracle has been a leading and substantive supporter of Java since its emergence
in 1995 and takes on the new role as steward of Java technology with a
relentless commitment to fostering a community of participation and
transparency.
31. "Learn About Java Technology".
Oracle. Retrieved 21 November 2011.
32. Gosling,
James (April 9, 2010). "Time to
move on...". On a New Road. Retrieved 2011-11-16.
33. "Is
the JVM (Java Virtual Machine) platform dependent or platform independent? What
is the advantage of using the JVM, and having Java be a translated language?".
Programmer Interview. Retrieved 2015-01-19.
34. Niccolai, James (January 23, 2001). "Sun,
Microsoft settle Java lawsuit". JavaWorld (International Data Group). Retrieved 2008-07-09.
35. Jelovic, Dejan. "Why Java will
always be slower than C++". Retrieved 2008-02-15.
38. "NullPointerException".
Oracle. Retrieved 2014-05-06.
39. "Exceptions
in Java". Artima.com. Retrieved 2010-08-10.
40. "Java
HotSpot VM Options". Oracle.com. 2010-09-07. Retrieved 2012-06-30.
41. "Operator
Overloading (C# vs Java)". C# for Java Developers. Microsoft.
Retrieved 10 December 2014.
42. "Multiple
Inheritance of State, Implementation, and Type". The Java™
Tutorials. Oracle. Retrieved 10 December 2014.
43. "Lesson:
A Closer Look at the "Hello World!" Application". The
Java™ Tutorials > Getting Started. Oracle Corporation. Retrieved 2011-04-14.
44. "Using
applet, object and embed Tags". oracle.com. Retrieved 2010-10-14.
45. "Deploying
Applets in a Mixed-Browser Environment". oracle.com. Retrieved 2010-10-14.
46. Arnold, Ken. "Generics
Considered Harmful". java.net. Retrieved 4 July 2011.
47. Jelovic, Dejan. "Why Java Will
Always Be Slower than C++". www.jelovic.com. Retrieved 17 October 2012.
48. Owens, Sean R. "Java
and unsigned int, unsigned short, unsigned byte, unsigned long, etc. (Or
rather, the lack thereof)". Archived from the original
on 2004-11-09. Retrieved 2011-07-04.
49. Kahan, William. "How Java’s
Floating-Point Hurts Everyone Everywhere" (PDF). Electrical Engineering &
Computer Science, University of California at Berkeley. Retrieved 4 June 2011.
51. Mullin, Joe. "Google
guilty of infringement in Oracle trial; future legal headaches loom". Law
& Disorder. Ars Technica. Retrieved 8 May 2012.
52. Joe Mullin (May 31, 2012). "Google
wins crucial API ruling, Oracle’s case decimated". Ars Technica.
Retrieved 2012-06-01.
53. Rosenblatt, Seth (May 9, 2014). "Court
sides with Oracle over Android in Java patent appeal". CNET.
Retrieved 2014-05-10.
54. "ORACLE
AMERICA, INC., Plaintiff - Appellant, v. GOOGLE INC., Defendant - Cross -
Appellant." (PDF). Court of Appeals for the Federal Circuit. May 9, 2014. Retrieved 2014-05-10.
55. "Collections
Framework Overview". Java Documentation. Oracle. Retrieved 18
December 2014.
56. "Java™
Security Overview". Java Documentation. Oracle. Retrieved 18
December 2014.
57. "Trail:
Internationalization". The Java™ Tutorials. Oracle. Retrieved 18
December 2014.
58. "How
to Write Doc Comments for the Javadoc Tool". Oracle Technology
Network. Oracle. Retrieved 18 December 2014.
59. "Java
Card Overview". Oracle Technology Network. Oracle. Retrieved 18
December 2014.
60. "Java
Platform, Micro Edition (Java ME)". Oracle Technology Network.
Oracle. Retrieved 18 December 2014.
61. "Java
SE". Oracle Technology Network. Oracle. Retrieved 18 December
2014.
62. "Java
Platform, Enterprise Edition (Java EE)". Oracle Technology Network.
Oracle. Retrieved 18 December 2014.
Tidak ada komentar:
Posting Komentar