Kamis, 21 Juli 2011

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. All code is written inside a class, and everything is an object, with the exception of the primitive data types (integers, floating-point numbers, boolean values, and characters), which are not classes for performance reasons.
Unlike C++, Java does not support operator overloading and multiple inheritance for classes in order to simplify the language and to prevent possible errors and anti-pattern design.
Java uses similar commenting methods to 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 compile 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. */
 
/**
 * This is an example of a Javadoc comment; Javadoc can compile documentation
 *  from this text.
 */
Hello world
The traditional Hello world program can be written in Java as:[32]


 
 
class HelloWorldApp {
    public static void main(String[] args) {
        System.out.println("Hello World!"); // Display the string.
    }
}
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, HelloWorld.java. It must first be compiled into bytecode, using a Java compiler, producing a file named HelloWorld.class.  Only then can it be executed, or 'launched'. The java source file may  only contain one public class but can contain multiple classes with less  than public access and any number of public inner classes.

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 method  variables that are not 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 JavaBean 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 allows an alternative syntax for creating and passing the array.

The Java launcher launches Java by loading a given class (specified on the command line or as an attribute in a JAR) and starting its public static void main(String[]) method. Stand-alone programs must declare this method explicitly. The String[] args parameter is an array of String objects containing any arguments passed to the class. The parameters to main are often passed by means of a command line.

Printing is part of a Java standard library: The System class defines a public static field called out. The out object is an instance of the PrintStream class and provides many methods for printing data to standard out, including println(String) which also appends a new line to the passed string.

The string "Hello, world!" is automatically converted to a String object by the compiler.




A more comprehensive example
 
 
// OddEven.java
import javax.swing.JOptionPane;
 
public class OddEven {
    // "input" is the number that the user gives to the computer
    private int input; // 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, even if
     * it is empty. 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.
    public static void main(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."
             */
            input = Integer.parseInt(JOptionPane.showInputDialog("Please Enter A Number"));
            calculate();
        } catch (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 Linux users).(Assuming it's open)
     */
    private void calculate() {
        if (input % 2 == 0) {
            System.out.println("Even");
        } else {
            System.out.println("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 input. Every instance of the OddEven class has its own copy of the input field. The private declaration means that no other class can access (read or write) the input 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 input % 2 == 0 using the if keyword to see if the remainder of dividing the input 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 input field can be equivalently accessed as this.input, which explicitly uses 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.
    • 

  • input = 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
[edit] 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 {
 
    @Override
    public void paintComponent(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 (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 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" 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,[33] although support for these elements by Web browsers is inconsistent.[34] 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.


[edit] 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(ServletRequest request, 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 of the public classes and interfaces from the java.io and javax.servlet packages in the compilation.

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.


[edit] 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.


[edit] 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");
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        add(new JLabel("Hello, world!"));
        pack();
        setVisible(true);
 
    }
 
    public static void main(String[] args) {
        new Hello();
    }
}
The first import includes all of 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 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 JVM 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 JVM 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.


[edit] 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 a large number  of container classes, each containing almost identical code.


[edit] Criticism
Main article: Criticism of Java
A number of criticisms have been leveled at Java for various design  choices in the language and platform. Such criticisms include the  implementation of generics [35], the handling of unsigned numbers[36], the implementation of floating-point arithmetic[37], and security vulnerabilities[clarification needed].


[edit] Class libraries














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