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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 namedHelloWorldApp.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 mainmethods, 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 Stringarguments. The effect of this alternate declaration is semantically identical (the args parameter is still an array ofString 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, includingprintln(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.

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 theOddEven 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 ifkeyword 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 asthis.userInput, which both explicitly use the undeclared this parameter.)
• OddEven number = new OddEven(); declares a local object reference variable in the main method namednumber. 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 thecalculate 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 classInteger.

Special classes

Main article: Java appletApplet

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.Graphicsclasses 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 objecttag 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 andjavax.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 theserver to forward requests to the servlet and control the servlet's lifecycle.

The Hello class overrides the service(ServletRequest, ServletResponse) method defined by the Servletinterface to provide the code for the service request handler. The service() method is passed: a ServletRequestobject 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 HTTPrequests 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 first import includes all the public classes and interfaces from the javax.swing package.

The Hello class extends the JFrame class; the JFrame class implements a window with a title bar and a closecontrol.

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 JFrameto 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 theadd(Component) method inherited from the Container superclass is called to add the label to the frame. Thepack() 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 theAndroid 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

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:Main article: Java Class Library

• 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

See also: Free Java implementations § Class library

Java editions
Java Card Micro Edition (ME) Standard Edition (SE) Enterprise Edition (EE) JavaFX (Merged to Java SE 8) PersonalJava (discontinued)
v t e

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 calledpackages. Each package contains a set of related interfaces, classes andexceptions. 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

• 
  Comparison of Java with other languages[edit
• ]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 programming languages
• Comparison of Java and C++
• Comparison of C# and Java