Java (programming language)
"Java
language" redirects here. For the natural language from the Indonesian
island of Java, see Javanese language.
Not to be
confused with JavaScript.
First appeared
|
1995; 20 years ago
|
Java Standard Edition 8 Update 65
(1.8.0_65) / October 20, 2015; 5 days ago
|
|
Java Standard Edition 9 Early
Access b85 (1.9.0-ea-b85) / September 28, 2015; 27 days ago
|
|
Implementation
language
|
|
Cross-platform
(multi-platform)
|
|
Website
|
|
Major
implementations
|
|
Influenced
by
|
|
Ada 83, C++, C#, Eiffel, Generic
Java, Mesa, Modula-3, Oberon, Objective-C,
UCSD
Pascal,[9][10]
|
|
Influenced
|
|
|
|
Java is a
general-purpose computer programming language that is concurrent, class-based, object-oriented, and specifically
designed to have as few implementation dependencies as possible. It is intended
to let application developers "write once, run anywhere"
(WORA),meaning that compiled Java code can run on all platforms that support
Java without the need for recompilation. 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, 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).
The latest
version is Java 8, the only supported version, currently.
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
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
(blue) and C (black) 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. 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. Gosling designed Java with a C/C++-style syntax that system and
application programmers would find familiar.
Sun
Microsystems released the first public implementation as Java 1.0 in 1995.
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. 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.
Sun's vice-president Rich Green said
that Sun's ideal role with regard to Java was as an "evangelist".
Following Oracle Corporation's acquisition of Sun
Microsystems in 2009–10, Oracle has described itself as the "steward of
Java technology with a relentless commitment to fostering a community of
participation and transparency". 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.
On April 2, 2010, James Gosling resigned from Oracle.
Principles
There were five primary goals in the
creation of the Java language:
- 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
Currently only Java 8 is supported
("publicly"). 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
Java Control Panel, version 7
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.
Standard libraries provide a generic
way to access host-specific features such as graphics, threading, and networking.
The use of universal bytecode makes
porting simple. However, the overhead of interpreting bytecode into machine
instructions makes interpreted programs almost always run more slowly than
native executables.
However, just-in-time (JIT) compilers that compile
bytecodes to machine code during runtime were introduced from an early stage.
Java itself is platform-independent, and is adapted to the particular platform
it is to run on by a Java virtual machine for it, which translates
the Java
bytecode into the platform's machine language.[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 (still works for XP, while only later versions currently
"publicly" supported), 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. 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++. However, Java programs' execution speed improved significantly with the
introduction of just-in-time compilation in 1997/1998 for Java 1.1, 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 (while its support is mostly dropped in current implementations of ARM).
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.
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. Oracle aims to replace CMS with the Garbage-first collector
(G1).
Syntax
Main article: Java
syntax
The syntax of Java is largely
influenced by C++.
Unlike C++, which combines the syntax for structured, generic, and
object-oriented programming, Java was built almost exclusively as an
object-oriented language. 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. Java
reuses some popular aspects of C++ (such as printf() method).
Unlike C++, Java does not support operator overloading or multiple inheritance for classes,
though multiple inheritance is supported for interfaces. 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" program
The traditional "Hello, world!" program
can be written in Java as:
class
HelloWorldApp {
public static void main(String[] args) {
System.out.println("Hello
World!"); // Prints the string to the console.
}
}
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. This is called an access level modifier. Other access level
modifiers include the keywords private , and protected.
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. Methods that are not
designated static are instance methods, and require a specific instance of a
class to operate.
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.
Comprehensive
example
//
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
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, although support for these
elements by web browsers is inconsistent. 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, speed, the handling of unsigned numbers, the
implementation of floating-point arithmetic, and a history of security
vulnerabilities in the primary Java VM implementation HotSpot.
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 or more recently Android
Runtime which actually compiles applications to native machine code upon
installation.
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. District Judge William Haskell Alsup ruled on May 31, 2012,
that APIs cannot be copyrighted, but this was reversed by the United States
Court of Appeals for the Federal Circuit in May 2014.
Class
libraries
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:
- IO/NIO
- Networking
- Reflection
- Concurrency
- Generics
- Scripting/Compiler
- Functional Programming (Lambda, Streaming)
- Collection libraries that implement data structures such as lists, dictionaries, trees, sets, queues and double-ended queue, or stacks
- XML Processing (Parsing, Transforming, Validating) 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
- 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
- JavaFX
- 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[by whom?]. 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 */.
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