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 2016, Java is one of the most popular programming languages in use, particularly for client-server web applications, with a reported 9 million developers. Java was originally developed by James Gosling, a Canadian, 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 original features from SmallTalk, with a syntax similar to 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 licenses. 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 11, released on September 25, 2018. Java 11 is a currently supported long-term support (LTS) version ("Oracle Customers will receive Oracle Premier Support"); Oracle released for the "legacy" Java 8 LTS the last "public update", which is free for commercial use, in January 2019. Oracle will still support Java 8 with public updates for personal use up to at least December 2020. Oracle (and others) "highly recommend that you uninstall older versions of Java", because of serious risks due to unresolved security issues. Since Java 9 is no longer supported, Oracle advises its users to "immediately transition" to Java 11. Oracle extended support for Java 6 ended in December 2018.
History
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Duke, the Java mascot
James Gosling, the creator of Java, in 2008
The TIOBE programming language popularity index graph from 2002 to 2018. Over the course of a decade, Java (blue) and C (black) competed for the top position.
James Gosling, Mike Sheridan, and Patrick Naughton initiated the Java language project in June 1991.
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 1996. 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. 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. 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 its Java virtual machine (JVM) 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 its JVM'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 Micro-systems 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.
In January 2016, Oracle announced that Java run-time environments based on JDK 9 will discontinue the browser plugin.
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
As of 20 March 2018, both Java 8 and 11 are officially supported. Major release versions of Java, along with their release dates:
- JDK 1.0 (January 23, 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)
- Java SE 9 (September 21, 2017)
- Java SE 10 (March 20, 2018)
- Java SE 11 (September 25, 2018)
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 smart-cards.
- Java Platform, Micro Edition (Java ME) – targeting environments with limited resources.
- Java Platform, Standard Edition (Java SE) – targeting workstation environments.
- Java Platform, Enterprise Edition (Java EE) – targeting large distributed enterprise or Internet environments.
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.
Sun also provided an edition called Personal Java that has been superseded by later, standards-based Java ME configuration-profile pairings.
Execution System
Java JVM and Bytecode
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 run time 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 made
interpreted programs almost always run more slowly than native executables. Just-in-time
(JIT) compilers that compile byte-codes 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.
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. With Java 1.5, the
performance was improved with the addition of the java.util.concurrent
package, including lock free implementations of the ConcurrentMaps and other multi-core collections, and it was improved further with Java 1.6.
Non-JVM
Some
platforms offer direct hardware support for Java; there are micro
controllers that can run Java bytecode in hardware instead of a software
Java virtual machine, and some ARM-based processors could have hardware support for executing Java bytecode through their Jazelle option, though support has mostly been 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
non-existent 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 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 can be arithmetically manipulated (e.g. by
adding or subtracting an offset). 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 either stored directly in fields (for objects) or on the stack (for methods) rather than on the heap, as is commonly true for non-primitive data types. This was a conscious decision by Java's designers for performance reasons.
Java contains multiple types of garbage collectors. By default, 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 (CMS) garbage collector is sufficient. Oracle aims to replace CMS with the Garbage-First collector (G1).
Having solved the memory management problem does not relieve the
programmer of the burden of handling properly other kind of resources,
like network or database connections, file handles, etc., especially in
the presence of exceptions. Paradoxically, the presence of a garbage
collector has faded the necessity of having an explicit destructor
method in the classes, thus rendering the management of these other
resources more difficult!
Syntax
Dependency graph of the Java Core classes (created with jdeps and Gephi).
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 the
printf method).
Unlike C++, Java does not support operator overloading or multiple inheritance for classes, though multiple inheritance is supported for interfaces.
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 and can be read by
some integrated development environments (IDEs) such as Eclipse to allow developers to access documentation within the IDE.
"Hello world" example
The traditional "Hello, world!" program can be written in Java as:
public 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 modifier 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 (to the args parameter which 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.
Example with methods
// 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;
}
}
}
Special classes
Applet
Java applets were programs that were embedded
in other applications, typically in a Web page displayed in a web
browser. The Java applet API is now deprecated since Java 9 in 2017.
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.
The Java servlet API has to some extent been superseded by two standard Java technologies for web services:
- the Java API for RESTful Web Services (JAX-RS 2.0) useful for AJAX, JSON and REST services, and
- the Java API for XML Web Services (JAX-WS) useful for SOAP Web Services.
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
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 macOS.
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.
JavaFX application
JavaFX is a software platform for creating and delivering desktop applications, as well as rich Internet applications (RIAs) that can run across a wide variety of devices. JavaFX is intended to replace Swing as the standard GUI library for Java SE, but both will be included for the foreseeable future. JavaFX has support for desktop computers and web browsers on Microsoft Windows, Linux, and macOS. JavaFX does not have support for native OS look and feels.
Generics
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 prevented from
occurring, by issuing compile-time errors. If Java prevented all runtime
type errors (ClassCastException's) from occurring, it would be type safe.
In 2016, the type system of Java was proven unsound.
Criticism
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.
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. 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
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 */.
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 are currently officially supported), macOS, 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.
Use outside the Java platform
The Java programming language requires the presence of a software platform in order for compiled programs to be executed.
Oracle supplies the Java platform for use with Java. The Android SDK is an alternative software platform, used primarily for developing Android applications with its own GUI system. The Eclipse IDE platform supports Java, but provides its own GUI system SWT.
Android
The Android operating system makes extensive use of Java-related technology.
The Java language is a key pillar in Android, an open source mobile operating system. Although Android, built on the Linux kernel, is written largely in C, the Android SDK
uses the Java language as the basis for Android applications. The
bytecode language supported by the Android SDK is incompatible with Java
bytecode and runs on its own virtual machine, optimized for low-memory
devices such as smartphones and tablet computers.
Depending on the Android version, the bytecode is either interpreted by the Dalvik virtual machine or compiled into native code by the Android Runtime.
Android does not provide the full Java SE standard library,
although the Android SDK does include an independent implementation of a
large subset of it. It supports Java 6 and some Java 7 features,
offering an implementation compatible with the standard library (Apache Harmony).
Controversy
The use of Java-related technology in Android 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.
On May 26, 2016, the district court decided in favor of Google, ruling
the copyright infringement of the Java API in Android constitutes fair
use.
