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Thursday, March 26, 2009

Introduction to JAVA

Java is a programming language originally developed by James Gosling at Sun Microsystems 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 has a simpler object model and fewer low-level facilities. Java applications are typically compiled to bytecode that can run on any Java virtual machine (JVM) regardless of computer architecture.

The original and reference implementation Java compilers, virtual machines, and class libraries were developed by Sun from 1995. As of May 2007, in compliance with the specifications of the Java Community Process, Sun made available most of their Java technologies as free software under the GNU General Public License. Others have also developed alternative implementations of these Sun technologies, such as the GNU Compiler for Java and GNU Classpath History

Duke, the Java mascotSee also: Java (Sun) history and Java version historyJames Gosling initiated the Java language project in June 1991 for use in one of his many set-top box projects.[5] The language, initially called Oak after an oak tree that stood outside Gosling's office, also went by the name Green and ended up later renamed as Java, from a list of random words. Gosling aimed to implement a virtual machine and a language that had a familiar C/C++ style of notation.
Sun 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. With the advent of Java 2 (released initially as J2SE 1.2 in December 1998), new versions had multiple configurations built for different types of platforms. For example, J2EE targeted enterprise applications and the greatly stripped-down version J2ME for mobile applications. J2SE designated the Standard Edition. 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 JTC1 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.[9] 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. Sun distinguishes between its Software Development Kit (SDK) and Runtime Environment (JRE) (a subset of the SDK); the primary distinction involves the JRE's lack of the compiler, utility programs, and header files. On 13 November 2006, Sun released much of Java as free and open source software under the terms of the GNU General Public License (GPL). On 8 May 2007 Sun finished the process, making all of Java's core code free and open-source, aside from a small portion of code to which Sun did not hold the copyright.

Philosophy Primary goals
There were five primary goals in the creation of the Java language:[11] It should be "simple, object oriented, and familiar". It should be "robust and secure". It should be "architecture neutral and portable". It should execute with "high performance". It should be "interpreted, threaded, and dynamic".
Java Platform
Java Runtime EnvironmentOne characteristic of Java is portability, which means that computer programs written in the Java language must run similarly on any supported hardware/operating-system platform. One should be able to write a program once, compile it once, and run it anywhere. This is achieved by compiling the Java language code, not to machine code but to Java bytecode – instructions analogous to machine code but intended to be interpreted 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. Standardized libraries provide a generic way to access host specific features such as graphics, threading and networking. In some JVM versions, bytecode can be compiled to native code, either before or during program execution, resulting in faster execution. 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, and Java suffered a reputation for poor performance. This gap has been narrowed by a number of optimisation techniques introduced in the more recent JVM implementations. One such technique, known as just-in-time (JIT) compilation, translates Java bytecode into native code the first time that code is executed, then caches it. This results in a program that starts and executes faster than pure interpreted code can, at the cost of introducing occasional compilation overhead during execution. More sophisticated VMs also use dynamic recompilation, in which the VM analyzes the behavior of the running program and selectively recompiles and optimizes parts of the program. Dynamic recompilation can achieve optimizations superior to static compilation because the dynamic compiler can base optimizations on knowledge about the runtime environment and the set of loaded classes, and can identify hot spots - parts of the program, often inner loops, that take up the most execution time. JIT compilation and dynamic recompilation allow Java programs to approach the speed of native code without losing portability. Another technique, commonly known as static compilation, or ahead-of-time (AOT) compilation, is to compile directly into native code like a more traditional compiler. Static Java compilers translate the Java source or bytecode to native object code. This achieves good performance compared to interpretation, at the expense of portability; the output of these compilers can only be run on a single architecture. AOT could give Java something close to native performance, yet it is still not portable since there are no compiler directives, and all the pointers are indirect with no way to micro manage garbage collection. Java's performance has improved substantially since the early versions, and performance of JIT compilers relative to native compilers has in some tests been shown to be quite similar.[12][13] The performance of the compilers does not necessarily indicate the performance of the compiled code; only careful testing can reveal the true performance issues in any system. One of the unique advantages of the concept of a runtime engine is that even the most serious errors (exceptions) in a Java program should not 'crash' the system under any circumstances, provided the JVM itself is properly implemented. Moreover, in runtime engine environments such as Java there exist tools that attach to the runtime engine and every time that an exception of interest occurs they record debugging information that existed in memory at the time the exception was thrown (stack and heap values). These Automated Exception Handling tools provide 'root-cause' information for exceptions in Java programs that run in production, testing or development environments. Such precise debugging is much more difficult to implement without the run-time support that the JVM offers.

Implementations
Sun Microsystems officially licenses the Java Standard Edition platform for Microsoft Windows, Linux, Mac OS X, and Solaris. Through a network of third-party vendors and licensees[14], alternative Java environments are available for these and other platforms. 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 $20 million as well as a court order enforcing the terms of the license from Sun.[15] As a result, Microsoft no longer ships Java with Windows, and in recent versions of Windows, Internet Explorer cannot support Java applets without a third-party plugin. Sun, and others, have made available free Java run-time systems for those and other versions of Windows. Platform-independent Java is essential to the Java EE strategy, and an even more rigorous validation is required to certify an implementation. This environment enables portable server-side applications, such as Web services, servlets, and Enterprise JavaBeans, as well as with embedded systems based on OSGi, using Embedded Java environments. Through the new GlassFish project, Sun is working to create a fully functional, unified open-source implementation of the Java EE technologies. Sun also distributes a superset of the JRE called the Java 2 SDK (more commonly known as the JDK), which includes development tools such as the Java compiler, Javadoc, Jar and debugger.
Automatic memory management
See also: Garbage collection (computer science)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 object 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 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. Either way 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. 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. Where performance or response time is important, explicit memory management and object pools are often used.
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 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 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 are instances of their wrapper classes. Syntax
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 intrinsic data types (ordinal and real numbers, boolean values, and characters), which are not classes for performance reasons. Java suppresses several features (such as operator overloading and multiple inheritance) for classes in order to simplify the language and to prevent possible errors and anti-pattern design. Examples
Hello worldThe traditional Hello world program can be written in Java as: /* * Outputs "Hello, World!" and then exits */ public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, World!"); }}By convention, source files are 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 declared private 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 was 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. 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() 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 Java Beans 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 alternate 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.javaimport 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() { //Code not shown } // 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 = new Integer(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 dialog returns a String that is converted to an int by the Integer.parseInt(String) method. 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 = new Integer(JOptionPane.showInputDialog("Please Enter A Number")); is a statement that converts the type of String to the primitive type int by taking advantage of the wrapper class Integer. Special Classes Applet
Main article: Java appletJava applets are programs that are embedded in other applications, typically in a Web page displayed in a Web browser. // Hello.javaimport javax.swing.JApplet;import java.awt.Graphics; public class Hello extends JApplet { 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 paint() 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. http://www.w3.org/TR/html4/strict.dtd"> An applet is placed in an HTML document using the
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[18], although support for these elements by
Web browsers is inconsistent.[19] 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 paint method is called by the AWT event dispatching thread
whenever the display needs the applet to draw itself. ServletMain article: Java ServletJava 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.javaimport 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(); pw.println("Hello, world!");
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. JavaServer PageMain article: JavaServer
PagesJavaServer Pages (JSPs) 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 . 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 applicationMain 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 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
java.awt.BorderLayout;import javax.swing.*; public class Hello extends JFrame {
public Hello() { super("hello");
setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE); setLayout(new
BorderLayout()); add(new JLabel("Hello, world!")); pack(); }
public static void main(String[] args) { new Hello().setVisible(true);
}}The first import statement directs the Java compiler to include the
BorderLayout class from the java.awt package in the compilation; the second
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, the layout of the frame is set to a
BorderLayout; this tells Swing how to arrange the components that will be added
to the frame. 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, in the manner indicated
by the BorderLayout.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. GenericsMain article: Generics in
JavaPrior to 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.
Class librariesJava libraries are the compiled byte codes of source
code developed by the JRE implementor to support application development in
Java. Examples of these libraries are: The core libraries, which include:
Collection libraries that implement data structures such as lists, dictionaries,
trees and sets 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 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 Java virtual machine (JVM) that is the means by
which the byte codes 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.
DocumentationJavadoc 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. Whereas normal comments in Java and C are set off with /* and */,
the multi-line comment tags, Javadoc comments have an extra asterisk at the
beginning, so that the tags are /** and */. ExamplesThe
following is an example of java code commented with simple Javadoc-style
comments:/** * A program that does useful things. */public class Program
{ /** * A main method. * @param args The arguments */ public static
void main(String[] args) { //do stuff } } EditionsSee
also: Free Java implementations#Class libraryJava editions Java Card Micro
Edition (ME) Standard Edition (SE) Enterprise Edition (EE) PersonalJava
(discontinued) 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
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.The set of APIs 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.Sun also provided an edition called PersonalJava
that has been superseded by later, standards-based Java ME configuration-profile
pairings.Criticism of Java
Language choices
GenericsWhen generics
were added to Java 5.0, there was already a large framework of classes (many of
which were already deprecated), so generics were chosen to be implemented using
erasure to allow for migration compatibility and re-use of these existing
classes. This limited the features that could be provided by this addition as
compared to other languages. Unsigned integer
typesJava lacks
native unsigned integer types which are very useful in some situations (i.e.
dealing with binary data manipulation). Although it is possible to circumvent
this problem with extra code and using larger data types, it makes using Java
cumbersome for these tasks. If abstracted using functions, function calls become
necessary for many operations which are native to some other
languages. Floating point arithmetic
While Java's floating point arithmetic is largely based on
IEEE 754 (Standard for Binary Floating-Point Arithmetic), certain features are
not supported even when using the strictfp modifier, such as Exception Flags and
Directed Roundings — capabilities mandated by IEEE Standard 754. Many so-called
"Java gotchas" are not problems with Java per se, but problems that are
inevitable whenever using floating point arithmetic. Look and
feel This section does
not cite any references or sources. Please help improve this article by adding
citations to reliable sources. Unverifiable material may be challenged and
removed. (August 2008)The look and feel of GUI applications written in
Java using the Swing platform may look different from native applications. While
programmers can choose to use the AWT toolkit that displays native widgets (and
thus look like the operating platform), the AWT toolkit is unable to meet
advanced GUI programming needs by wrapping around advanced widgets and not
sacrificing portability across the various supported platforms, each of which
have vastly different APIs especially for higher-level widgets. If an
alternative GUI toolkit is used, such as SWT, it is possible for a Java
application to have native look and feel whilst also having access to advanced
widgets.The Swing toolkit – written completely in Java – both creates the
problem of having a different look and feel from native applications, and avoids
the problem of being limited by native toolkit capabilities because it
reimplements widgets using only the most basic drawing mechanisms that are
guaranteed available on all platforms. Unfortunately, the default installations
of the JRE (as of August 2006) do not use the system's "native" look and feel,
instead defaulting to the built-in Metal Look and Feel. If the programmer
doesn't take care to set the native look and feel, users will have applications
whose appearance is vastly different from that of their native applications.
Apple Computer's own optimized version of the Java Runtime, which is included
within the Mac OS X distribution, by default does set the default and implements
its "Aqua" look-and-feel, giving Swing applications on the Macintosh a similar
appearance to native software. Even in this environment, the programmer must
still do some extra work to ensure that that application looks like an Aqua one
(for example, they must set system properties to ensure the menubar is rendered
in the OS X menubar and not in the application window as it would be on other
platforms). PerformanceFurther information: Java performanceJava's
performance has improved substantially since the early versions. Performance of
JIT compilers relative to native compilers has in some tests been shown to be
quite similar.In a paper written in 1999 by Lutz Prechelt it is outlined
that, statistically, programmer efficiency and experience has a bearing many
standard deviations greater on run-time and memory usage than language choice.
This paper specifically uses Java as a basis for the comparison, due to its then
bad reputation. Sun Microsystems have taken considerable trouble to address
these problems, and regularly produce white papers on this topic. A more recent
study (2003–4) gives Java a comparable performance to C++. However, the validity
of this study is disputed. Java bytecode can either be interpreted at run time
by a virtual machine, or it can be compiled at load time or runtime into machine
code which runs directly on the computer's hardware. Interpretation is slower
than native execution, and compilation at load time or runtime has an initial
performance penalty for the compilation.

Java Interview questions

1.Is the JDBC-ODBC Bridge multi-threaded? - No. The JDBC-ODBC Bridge does not support concurrent access from different threads. The JDBC-ODBC Bridge uses synchronized methods to serialize all of the calls that it makes to ODBC. Multi-threaded Java programs may use the Bridge, but they won’t get the advantages of multi-threading.


2.Does the JDBC-ODBC Bridge support multiple concurrent open statements per connection? - No. You can open only one Statement object per connection when you are using the JDBC-ODBC Bridge.


3.What is cold backup, hot backup, warm backup recovery? - Cold backup (All these files must be backed up at the same time, before the databaseis restarted). Hot backup (official name is ‘online backup’) is a backup taken of each tablespace while the database is running and is being accessed by the users.


4.When we will Denormalize data? - Data denormalization is reverse procedure, carried out purely for reasons of improving performance. It maybe efficient for a high-throughput system to replicate data for certain data. What is the advantage of using PreparedStatement? - If we are using PreparedStatement the execution time will be less. The PreparedStatement object contains not just an SQL statement, but the SQL statement that has been precompiled. This means that when the PreparedStatement is executed,the RDBMS can just run the PreparedStatement’s Sql statement without having to compile it first.


5.What is a “dirty read”?Quite often in database processing, we come across the situation wherein one transaction can change a value, and a second transaction can read this value before the original change has been committed or rolled back. This is known as a dirty read scenario because there is always the possibility that the first transaction may rollback the change, resulting in the second transaction having read an invalid value. While you can easily command a database to disallow dirty reads, this usually degrades the performance of your application due to the increased locking overhead. Disallowing dirty reads also leads to decreased system concurrency.


6.What is Metadata and why should I use it?Metadata (’data about data’) is information about one of two things: Database information (java.sql.DatabaseMetaData), or Information about a specific ResultSet (java.sql.ResultSetMetaData). Use DatabaseMetaData to find information about your database, such as its capabilities and structure. Use ResultSetMetaData to find information about the results of an SQL query, such as size and types of columns.


7.Different types of Transaction Isolation Levels?The isolation level describes the degree to which the data being updated is visible to other transactions. This is important when two transactions are trying to read the same row of a table. Imagine two transactions: A and B. Here three types of inconsistencies can occur:


Dirty-read: A has changed a row, but has not committed the changes. B reads the uncommitted data but his view of the data may be wrong if A rolls back his changes and updates his own changes to the database.Non-repeatable read: B performs a read, but A modifies or deletes that data later. If B reads the same row again, he will get different data.Phantoms:A does a query on a set of rows to perform an operation. B modifies the table such that a query of A would have given a different result. The table may be inconsistent. TRANSACTION_READ_UNCOMMITTED : DIRTY READS, NON-REPEATABLE READ AND PHANTOMS CAN OCCUR. TRANSACTION_READ_COMMITTED : DIRTY READS ARE PREVENTED, NON-REPEATABLE READ AND PHANTOMS CAN OCCUR. TRANSACTION_REPEATABLE_READ : DIRTY READS , NON-REPEATABLE READ ARE PREVENTED AND PHANTOMS CAN OCCUR. TRANSACTION_SERIALIZABLE : DIRTY READS, NON-REPEATABLE READ AND PHANTOMS ARE PREVENTED.



8.What is 2 phase commit?A 2-phase commit is an algorithm used to ensure the integrity of a committing transaction. In Phase 1, the transaction coordinator contacts potential participants in the transaction. The participants all agree to make the results of the transaction permanent but do not do so immediately. The participants log information to disk to ensure they can complete In phase 2 f all the participants agree to commit, the coordinator logs that agreement and the outcome is decided. The recording of this agreement in the log ends in Phase 2, the coordinator informs each participant of the decision, and they permanently update their resources.


9.How do you handle your own transaction ?Connection Object has a method called setAutocommit(Boolean istrue) - Default is true. Set the Parameter to false , and begin your transaction.


10.hat is the normal procedure followed by a java client to access the db? - The database connection is created in 3 steps:a. Find a proper database URLb. Load the database driverc. Ask the Java DriverManager class to
open a connection to your databaseIn java code, the steps are realized in code as follows: i. Create a properly formatted JDBR URL for your database. ii. A JDBC URL has the form jdbc:someSubProtocol://myDatabaseServer/theDatabaseName. iii. Class.forName(”my.database.driver”); iv. Connection conn =
DriverManager.getConnection(”a.JDBC.URL”, “databaseLogin”,”databasePassword”);


11What is a data source?A DataSource class brings another level of abstraction than directly using a connection object. Data source can be referenced by JNDI. Data Source may point to RDBMS, file System , any DBMS etc.


12.What are collection pools? What are the advantages?A connection pool is a cache of database connections that is maintained in memory, so that the connections may be reused.


13.How do you get Column names only for a table (SQL Server)?Write the Query.select name from syscolumns where id=(select id from sysobjects where name='user_hdr') order by colid --user_hdr is the table name.

What's the difference between a queue and a stack?

Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule

You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces?
Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.

What makes a plugin secure?

What makes a plugin secure?Here are some characteristics of a secure plugin:1. It is digitally signed by the publisher by a third party issued certificate. This way the user at least knows where the plugin in coming from.2. Inform the user of any actions which can lead to any changes in the system before hand and allow user to reject the proposed actions.3. Give user the option to save current system settings such that the user can go back to it after the plugin has finished execution if the user so desires.Items 2 and 3 above are sometimes the responsiblity of the application installing and using the plugin and not the plugin itself. From the end user perspective they are the same. So the the word plugin here can mean either just the plugin or the plugin together with web application driving it.Both java applets and activex components are plugins. Both are written to the disk and saved for future use. ActiveX components can be used by applications other than the web browser while for most practical purposes, java applets can only be used from a web browser (there are stanalone java applet hosting programs out there but they are mostly used by developers and not by end users). The ActiveX component installation requires modification to system registery and hence the end user must have administrative privilege in order to install it while Java applets don't need any adminitrative privilege for installation. One can argue that this makes a signed Java Applet a bigger security threat than an ActiveX component since even a user with minimum previlege can download and run it without the knowledge of the system adminstrator. The only way to avoid this situation is to set the browser to not allow Java Applets all together.
Pragma may mean:

In computer science or software engineering, a compiler directive communicating additional "pragmatic" or implementation-specific information


Describe #pragma directive of c programming language?
Answer:

Pragma is implementation specific directive i.e. each pragma directive has different implementation rule and use. There are many type of pragma directive and varies from one compiler to another compiler .If compiler does not recognize particular pragma the it ignore the pragma statement without showing any error or warning message and execute the whole program assuming this pragma statement is not present,

e.g.

Suppose any arbitrary pragma directive is #pragma world:





#pragma world

void main()

{

printf(“C is powerful language “);

}

Output: C is powerful language

Explanation:

Since #pragma world is unknown for Turbo c compiler so it ignore this directive without showing error or warning message and execute the whole program assuming #pragma world statement is not present.

List of pragma directive:



1. #pragma startup

2. #pragma exit

3. #pragma warn

4. #pragma option

5. #pragma inline

6. #pragma argsused

7. #pragma hdrfile

8. #pragma hdrstop

9. #pragma saveregs

Sunday, March 22, 2009

8086 Architecture Complete pdf

http://www.howardhuang.us/teaching/cs232/06-Intel-8086-architecture.pdf

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This is a pdf file open in Adobe Acrobat you have to install adobe acrobat in ur computer to see the contents

Friday, March 20, 2009

Recurssion and type of recurssion

download word file from



http://www.easy-share.com/1904126890/recurrsion.doc



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RECURSION - A method of programming whereby a function directly or indirectly calls itself. Recursion is often presented as an alternative to iteration.



LINEAR RECURSION - Recursion where only one call is made to the function from within the function (thus if we were to draw out the recursive calls, we would see a straight, or linear, path).

A linear recursive function is a function that only makes a single call to itself each time the function runs (as opposed to one that would call itself multiple times during its execution). The factorial function is a good example of linear recursion.

;Scheme(define (factorial n)(if (= n 0)1(* n (factorial (- n 1)))))

//C++int factorial (int n){if ( n == 0 )return 1;return n * factorial(n-1); // or factorial(n-1) * n}





TAIL RECURSION - A recursive procedure where the recursive call is the last action to be taken by the function. Tail recursive functions are generally easy to transform into iterative functions.

Tail recursion is a form of linear recursion. In tail recursion, the recursive call is the last thing the function does. Often, the value of the recursive call is returned. As such, tail recursive functions can often be easily implemented in an iterative manner; by taking out the recursive call and replacing it with a loop, the same effect can generally be achieved. In fact, a good compiler can recognize tail recursion and convert it to iteration in order to optimize the performance of the code.

A good example of a tail recursive function is a function to compute the GCD, or Greatest Common Denominator, of two numbers:

;Scheme(define (gcd m n)(cond((< m n) (gcd n m))((= (remainder m n) 0) n)(else (gcd n (remainder m n)))))

//C++int gcd(int m, int n){int r; if (m < n) return gcd(n,m); r = m%n;if (r == 0) return(n);else return(gcd(n,r));}





BINARY RECURSION - A recursive function which calls itself twice during the course of its execution.

Some recursive functions don't just have one call to themself, they have two (or more). Functions with two recursive calls are referred to as binary recursive functions.

The mathematical combinations operation is a good example of a function that can quickly be implemented as a binary recursive function. The number of combinations, often represented as nCk where we are choosing n elements out of a set of k elements, can be implemented as follows:

;Scheme(define (choose n k)(cond((= k 0) 1)((= k n) 1)(else (+ (choose (- n 1) k) (choose (- n 1) (- k 1)) ) ))

//C++int choose(int n, int k){if (k == 0 n == k) return(1);else return(choose(n-1,k) + choose(n-1,k-1));}



EXPONENTIAL RECURSION - Recursion where more than one call is made to the function from within itself. This leads to exponential growth in the number of recursive calls.

An exponential recursive function is one that, if you were to draw out a representation of all the function calls, would have an exponential number of calls in relation to the size of the data set (exponential meaning if there were n elements, there would be O(an) function calls where a is a positive number).

A good example an exponentially recursive function is a function to compute all the permutations of a data set. Let's write a function to take an array of n integers and print out every permutation of it.

//C++void print_array(int arr[], int n){int i;for(i=0; i


NESTED RECURSION

In nested recursion, one of the arguments to the recursive function is the recursive function itself! These functions tend to grow extremely fast. A good example is the classic mathematical function, Ackermann's function. It grows very quickly (even for small values of x and y, Ackermann(x,y) is extremely large) and it cannot be computed with only definite iteration (a completely defined for() loop for example); it requires indefinite iteration (recursion, for example).

Try computing ackerman(4,2) by hand... have fun!

//C++int ackerman(int m, int n){if (m == 0) return(n+1);else if (n == 0) return(ackerman(m-1,1));else return(ackerman(m-1,ackerman(m,n-1)));}



MUTUAL RECURSION

A recursive function doesn't necessarily need to call itself. Some recursive functions work in pairs or even larger groups. For example, function A calls function B which calls function C which in turn calls function A.

A simple example of mutual recursion is a set of function to determine whether an integer is even or odd. How do we know if a number is even? Well, we know 0 is even. And we also know that if a number n is even, then n - 1 must be odd. How do we know if a number is odd? It's not even!

//C++int is_even(unsigned int n){if (n==0) return 1;else return(is_odd(n-1));} int is_odd(unsigned int n){return (!is_even(n));}

Recursion is powerful! Of course, this is just an illustration. The above situation isn't the best example of when we'd want to use recursion instead of iteration or a closed form solution. A more efficient set of function to determine whether an integer is even or odd would be the following:

//C++int is_even(unsigned int n){if (n % 2 == 0) return 1;else return 0;} int is_odd(unsigned int n){if (n % 2 != 0) return 1;else return 0;}



Thursday, March 19, 2009

Debug Monitor

A debug monitor is very powerful graphical or console mode tool that monitors all the activities that are handled by the WinDriver Kernel. You can use the debug monitor to see how each command that is sent to the kernel is executed.




A debug monitor, simply put, is a tool that helps to find and reduce the number of bugs and defects in a computer program or any electrical device within or attached to the computer in order to make it act the way it should. While the driver is being created and downloaded, the debug monitor helps it work properly. For example, when an armored car drives up to a bank and the guards have to transfer money from the truck to the bank, there are special guards that stand watch to make sure no one tries to rob them thus making the transaction go smoothly. Those guards could be the debug monitors in the computer industry.






If the debugging monitor locates a bug or defect in any of the equipment, it will first try to reproduce the problem which will allow a programmer to view each string that was within the bug or defect range and try to fix it. A programmer is a technician who has learned the basic format of computers that make them run. These are strings of technical information that most people using computers will never see. For example, using a clock. The general public will plug in the clock and use it to tell time but will not open it up to see how it works. That is saved for the people who fix clocks. They are the programmers of clocks in the computer industry.
The programmer will delete strings or add new ones and then use the debug monitor to re-create the driver download to see if he fixed the problem. This can be a tedious task with all the processes that run in the computer, but the debug monitor helps to make it a lot easier.










debug monitor
n. A piece of embedded software that has been designed specifically for use as a debugging tool. It usually resides in ROM and communicates with a debugger via a serial port or network connection. The debug monitor provides a set of primitive commands to view and modify memory locations and registers, create and remove breakpoints, and execute your program. A remote debugger with knowledge of the command format communicates with the debug monitor and combines these primitives to fulfill higher-level requests like program download and single-step.
debugger
n. A tool used to test and debug software. A typical remote debugger runs on a host computer and connects to the target through a serial port or over a network. Using the debugger, you can download software to the target for immediate execution. You can also set breakpoints in the code and examine the contents of specific memory locations and registers.








Friday, March 13, 2009

Flowchart of Second pass of Assembler

Flowchart of First pass of Assembler

Important questions from 3rd and 4th unit in C programming

Important questions from 3rd and 4th unit in C programming

Unit 3:

Ques-1 :
Scope and visibility of a variable.

Ques 2:
What is Recurrsion, types and example.

Ques 3:
difference between iteration and recurrsion .

Ques 4:
any program related to recurrsion


Unit 4

Ques 1:
Description or difference "malloc()","calloc()","free()"

Ques 2:
Self refrencial Structure.

Ques 3:
program multiple return or function returning pointer

Ques 4:
Pointer v/s Array

Wednesday, March 11, 2009

Algorithm of second pass of Assembler

Algorithm of Second pass of assembler


Step 1: Set Location Counter to beginning

Step 2: Scan next line of code, if pseudo instruction present then go to step 3, else go to step 8.

Step 3:Check whether origin present , if yes then go to step 4, else go to step 5

Step 4: Set Location Counter and go to Step 2.

Step 5: Check whether end occur if yes then go to last step 17 ,else go to step 6.

Step 6:Convert operand to binary and store in location given by LC.

Step 7:Increment LC and go to step 2.

Step 8:Check whether Memory Reference Instruction or not , if yes then go to Step 9, else go to Step 13.

Step 9: Get operand code and set 2 -4 bits of instruction format.

Step 10:Search address symbol table for binary equivalent of symbolic address and set bits 5 – 16 of instruction format.

Step 11: Check the mode , I is present of not, if present then set first bit to 1, else set first bit to 0.

Step 12: Assembly all parts of binary instruction and store in location given by LC, and go to step 7.

Step 13:Check whether valid NON-MRI instruction or not, if valid then go to next , else go to step 16.

Step 14: Store binary equivalent of instruction in location given by LC.

Step 15:Go to step 7.

Step 16: Error in Code , and go to step 7.

Step 17: done , Stop

Algorithm of first pass of Assembler

Algorithm of First pass of Assembler
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Step 1 : Set location counter to beginning

Step 2: Scan next line of code, if label present then go to step 3 , else go to step 5.

Step 3: Store symbol in address symbol table together with value of LC

Step 4: Increment Location Counter and go to Step 2.

Step 5: Check whether Origin or not, if origin then go to Step 6,else go to step 7.

Step 6:Set LC and go to Step 2

Step 7: Check whether end occur ,if yes then go to step 8,else go to step 4.

Step 8: Go to Second Pass.

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Monday, March 9, 2009



Welcome

I started this blog in the favour of mca students affiliated to RGPV for their better performance , material avabability and to solving other problems related to their performance.I hope Everyone facilitate from this blog.


Thank you very much.