Java (programming language)

Java is a general-purpose computer programming language that isconcurrent, 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 thatcompiled Java code can run on all platforms that support Java without the need for recompilation.^[14] Java applications are typically compiled to bytecodethat 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 beenacquired by Oracle Corporation) and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntaxfrom 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).

1 History
- 1.1 Principles
- 1.2 Versions
2 Practices
- 2.1 Java platform
  - 2.1.1 Implementations
  - 2.1.2 Performance
- 2.2 Automatic memory management
3 Syntax
4 Examples
- 4.1 Hello World
- 4.2 A more comprehensive example
5 Special classes
- 5.1 Applet
- 5.2 Servlet
- 5.3 JavaServer Pages
- 5.4 Swing application
- 5.5 Generics
6 Criticism
7 Use on unofficial software platforms
- 7.1 Google
8 Class libraries
9 Documentation
10 Editions
11 See also
- 11.1 Comparison of Java with other languages
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 popularplatforms. Fairly secure and featuring configurable security, it allowed network- and file-access restrictions. Major web browsers soon incorporated the ability to run Java appletswithin web pages, and Java quickly became popular. The Java 1.0 compiler was re-writtenin 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, whileJ2ME featured APIs optimized for mobile applications. The desktop version was renamedJ2SE. 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

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 aJava 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 convertJava bytecode into machine language which depends on the operating system being used, it is platform dependent.^[33]

Implementations

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 theGarbage-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;
        }
    }
}

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