Learn java

Classes in Java

2009-10-25T20:44:00.000-07:00

JAVA CLASS

A Java class is a group of Java methods and variables.

Each Java source code file can contain one public class. The name of this public class must match the name of the Java source code file. If the public class is called "ballistics", then the filename would be "ballistics.java".

A sample Java class

class Ammunition { int diameter; int length; String name; }

Learn JAVA

2009-10-12T01:39:00.001-07:00

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 Java Programming Language

The Java programming language is a high-level language that can be characterized by all of the following buzzwords:

Simple		Architecture neutral
Object oriented		Portable
Distributed		High performance
Multithreaded		Robust
Dynamic		Secure

In the Java programming language, all source code is first written in plain text files ending with the .java extension. Those source files are then compiled into .class files by the javac compiler. A.class file does not contain code that is native to your processor; it instead contains bytecodes — the machine language of the Java Virtual Machine(Java VM). The java launcher tool then runs your application with an instance of the Java Virtual Machine.

An overview of the software development process.

Java applet

2009-10-12T01:00:00.000-07:00

A Java applet is an applet delivered to the users in the form of Java bytecode. Java applets can run in a Web browser using a Java Virtual Machine(JVM), or in Sun's AppletViewer, a stand-alone tool for testing applets. Java applets were introduced in the first version of the Java language in 1995. Java applets are usually written in the Java programming language but they can also be written in other languages that compile to Java bytecode such as Jython or Eiffel

Applets Structure

- There is no main method.
- Two methods that are called automatically- init() and paint()
- The init method initializes variables and objects; if you don't have one you will inherit one from the JApplet class.
- Use paint to draw screen
- A lot of methods exist in JApplet class so the "extends" keyword inherits everything that the class has. In the above example JApplet is parent class and shellapplet is the subclass so use the keyword "extends" to create inheritance.
- You have to import JApplet and java.awt.Graphics (abstract windowing toolkit) to get Graphics to paint.
- All applets must inherit JApplet
- Use the "super" keyword in subclass to invoke method in the superclass.
- super.paint( g ); this says uses the paint method from JApplet in my paint class and I'm not adding anything to it.

Example of Applet

/* Java applet to demonstrate the use of classes. This is the exmample from Chapter 10 of Bell & Parr */ import java.awt.*; import java.applet.Applet; import java.awt.event.*; public class PlayBalloon extends Applet implements ActionListener { private Button grow, shrink, left, right; private ColoredBalloon myRedBalloon; public void init() { makeGui(); myRedBalloon = new ColoredBalloon(255, 0, 0); } // init public void paint(Graphics g) { myRedBalloon.display(g); } // paint public void actionPerformed(ActionEvent e) { if (e.getSource() == grow) myRedBalloon.grow(); if (e.getSource() == shrink) myRedBalloon.shrink(); if (e.getSource() == left) myRedBalloon.left(); if (e.getSource() == right) myRedBalloon.right(); repaint(); } // actionPerformed private void makeGui() { grow = new Button("Grow"); add(grow); grow.addActionListener(this); shrink = new Button("Shrink"); add(shrink); shrink.addActionListener(this); left = new Button("Left"); add(left); left.addActionListener(this); right = new Button("Right"); add(right); right.addActionListener(this); } // makeGui } // PlayBaloon //---contents of Balloon.java------ /* The Balloon class is defined here */ import java.awt.*; class Balloon { int diameter = 10; int xCoord = 20, yCoord = 50; public void display(Graphics g) { g.drawOval(xCoord, yCoord, diameter, diameter); } // display public void left() { xCoord = xCoord - 10; } // left public void right() { xCoord = xCoord + 10; } // right public void grow() { diameter = diameter + 5; } // grow public void shrink() { diameter = diameter - 5; } // shrink } // Balloon class ColoredBalloon extends Balloon { private Color color = new Color(200, 200, 200); // some default color public ColoredBalloon(int r, int g, int b) { color = new Color(r, g, b); } public void display(Graphics g) { g.setColor(color); g.fillOval(xCoord, yCoord, diameter, diameter); } // display } // ColoredBalloon

Advantages

A Java applet can have any or all of the following advantages:

It is simple to make it work on Linux, Windows and Mac OS i.e. to make it cross platform. Applets are supported by most web browsers
The same applet can work on "all" installed versions of Java at the same time, rather than just the latest plug-in version only. However, if an applet requires a later version of the JRE the client will be forced to wait during the large download.
Most web browsers cache applets, so will be quick to load when returning to a web page. Applets also improve with use: after a first applet is run, the JVM is already running and starts quickly (JVM will need to restart each time the browser starts fresh).
It can move the work from the server to the client, making a web solution more scalable with the number of users/clients
If standalone program (like Google Earth) talks to the web server, that server normally needs to support also previous versions as the user may not keep it always updated. Differently, the browser updates the applet so there is no need to support the legacy versions. Only due configuration mistakes the applet may get stuck in the cache and have issues when new versions come out.
The applet naturally supports the changing user state like figure positions on the chessboard.
Developers can develop and debug an applet direct simply by creating a main routine (either in the applet's class or in a separate class) and call init() and start() on the applet, thus allowing for development in their favorite J2SE development environment. All one has to do after that is re-test the applet in the appletviewer program or a web browser to ensure it conforms to security restrictions.
An untrusted applet has no access to the local machine and can only access the server it came from. This makes such applet much safer to run than standalone executable that it could replace. However signed applet can have full access to the machine it is running on if the user agrees.

Disadvantages

A Java applet may have any of the following disadvantages:

It requires the Java plug-in that may not available on some less popular web browser or operating systems.
Some organizations only allow software installed by the administrators. As a result, some users can only view applets that are important enough to contact the administrator asking to install Java plug-in.
As with any client side scripting, security restrictions may make difficult or even impossible for untrusted applet to achieve the desired goals.
Some more badly designed code may require a specific JRE.

java.io.File

2009-10-01T10:14:00.000-07:00

java.io.File is the central class in working with files and directories.

Files and directories are both represented by File objects. When a File object is created, the system doesn't test to see if a corresponding file/directory actually exists; you must call exists() to check.

Constructors
`f =`	`new File(path);`	Create File object for default directory (usually where program is located).
`f =`	`new File(dirpath, fname);`	Create File object for directory path given as string.
`f =`	`new File(dir, fname);`	Create File object for directory.
public static constants
`s =`	`File.separator;`	Default path separator (eg, "/" in Unix, "\" in Windows).
Getting Attributes
`b =`	`f.exists();`	true if file exists.
`b =`	`f.isFile();`	true if this is a normal file.
`b =`	`f.isDirectory();`	true if f is a directory.
`s =`	`f.getName();`	name of file or directory.
`b =`	`f.canRead();`	true if can read file.
`b =`	`f.canWrite();`	true if can write file.
`b =`	`f.isHidden();`	true if file is hidden.
`l =`	`f.lastModified();`	Time of last modification.
`l =`	`f.length();`	Number of bytes in file.
Setting Attributes
	`f.setLastModified(t);`	Sets last modified time to long value t.
`b =`	`f.setReadOnly();`	Make file read only. Returns true if successful.
Paths
`s =`	`f.getPath();`	path name.
`s =`	`f.getAbsolutePath();`	path name (how is it different from above?).
`s =`	`f.getCanonicalPath();`	path name. May throw IOException.
`s =`	`f.toURL();`	path with "file:" prefix and /'s. Directory paths end with /.
`s =`	`f.toURI();`	path with "file:" prefix and /'s. Directory paths end with /.
Creating and deleting files and directories
`b =`	`f.delete();`	Deletes the file.
`b =`	`f.createNewFile();`	Create file, may throw IOException. true if OK; false if already exists.
`b =`	`f.renameTo(f2);`	Renames f to File f2. Returns true if successful.
`b =`	`f.mkdir();`	Creates a directory. Returns true if successful.
`b =`	`f.mkdirs();`	Creates directory and all dirs in path. Returns true if successful.
Parents and Children
`s =`	`f.getParent();`	Name of parent directory.
`dir =`	`f.getParentFile();`	File of parent.
`sa =`	`dir.list();`	Array of file/directory names in dir.
`fa =`	`dir.listFiles();`	Array of files/directories in dir.
`fa =`	`dir.listFiles(ff);`	As above after applying java.io.FileFilter ff.

Java-chess board

2009-09-11T20:21:00.000-07:00

import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;

public class chessboard {

public static void main(String[] args)
{
DrawFrame frame = new DrawFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.show();
}

}
/**
A frame that contains a panel with drawings
*/
class DrawFrame extends JFrame
{
static int one_for_all=700; //decides the width and heigth ofthe frame
Toolkit kit = Toolkit.getDefaultToolkit();
Dimension screenSize = kit.getScreenSize();
public int screenHeight = screenSize.height;
public int screenWidth = screenSize.width;

public DrawFrame()
{
setSize(FRAME_WIDTH, FRAME_HEIGHT);
// center frame in screen
setLocation(screenWidth / 4, screenHeight / 4);
setResizable(true);
setTitle("A Standard Chess Board");
setUndecorated(false);//try changing TRUE | FALSE
/*The title bar, title, frame border, close(X) button everything will be removed
What you will see will just be a plain frame,a plain area.
*/

// add panel to frame

DrawPanel panel = new DrawPanel();
Container contentPane = getContentPane();
contentPane.add(panel);
}
public static final int FRAME_WIDTH = one_for_all;
public static final int FRAME_HEIGHT = one_for_all;
}

/**
A panel that displays rectangles and ellipses.
*/
class DrawPanel extends JPanel
{
public void paintComponent(Graphics g)
{
DrawFrame frame = new DrawFrame();
int XXX=25;//x coordinate of the point where drawing starts
int YYY=25;//x coordinate of the point where drawing starts
System.out.println(XXX);
int co_x=XXX;
int co_y=YYY;
int no=8; // value of no should be at the most frameSize/10
int size=80; //size of each box
boolean flag=false;
setBackground(new Color(80,40,40));
super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g;
for(int j=0;j {
if(j%2==0)
{flag=false;}
else if(j%2==1)
{flag=true;}
co_x=XXX;
for(int i=0;i {
if(flag==true)
{
g2.setPaint(Color.WHITE);
g2.fill(new Rectangle2D.Double(co_x,co_y,size,size));
flag=false;
}
else
{
g2.setPaint(Color.BLACK);
g2.fill(new Rectangle2D.Double(co_x,co_y,size,size));
flag=true;
}
co_x+=size;
}
co_y+=size;
}
}
}

JAVA API

2009-09-01T20:19:00.000-07:00

There are three types of Java Programming Language Application Programming Interfaces (APIs) :the official core Java API, contained in the JDK or JRE, of one of the editions of the Java Platform. The three editions of the Java Platform are Java ME (Micro edition), Java SE (Standard edition), and Java EE (Enterprise edition).optional official APIs that can be downloaded separately. The specification of these APIs are defined according to a Java Specification Request (JSR), and sometimes some of these APIs are later included in the core APIs of the platform (the most notable example of this kind is Swing).unofficial APIs, developed by third parties, but not related to any JSRs.It must be noted that third-parties can freely implement any JSR specifications for an official API (even for the core API of the language), providing that they conform to the Technology Compatibility Kit (TCK) for this JSR (the TCK is a suite of tests that checks conformance of implementations for a JSR). The result of this freedom is that many official APIs have more implementations than the Sun's Reference implementation (RI).

2009-08-23T21:16:00.000-07:00

Servlets are Java programming language objects that dynamically process requests and construct responses. The Java Servlet API allows a software developer to add dynamic content to a Web serverusing the Java platform. The generated content is commonly HTML, but may be other data such as XML. Servlets are the Java counterpart to non-Java dynamic Web content technologies such as PHP, CGI andASP.NET, and as such some find it easier to think of them as 'Java scripts'. Servlets can maintain state across many server transactions by using HTTP cookies, session variables or URL rewriting.

The servlet API, contained in the Java package hierarchy javax.servlet, defines the expected interactions of a Web container and a servlet. A Web container is essentially the component of a Web server that interacts with the servlets. The Web container is responsible for managing the lifecycle of servlets, mapping a URL to a particular servlet and ensuring that the URL requester has the correct access rights.

Servlets can be generated automatically by JavaServer Pages (JSP) compiler, or alternately use template engines such as WebMacro or Apache velocity to generate HTML. Often servlets are used in conjunction with JSPs in a pattern called "Model 2", which is a flavor of the model-view-controller pattern.

Lifecycle of a servlet

The servlet lifecycle consists of the following steps:

The servlet class is loaded by the container during start-up.
The container calls the init() method. This method initializes the servlet and must be called before the servlet can service any requests. In the entire life of a servlet, the init()method is called only once.
After initialization, the servlet can service client requests. Each request is serviced in its own separate thread. The container calls the service() method of the servlet for every request. The service() method determines the kind of request being made and dispatches it to an appropriate method to handle the request. The developer of the servlet must provide an implementation for these methods. If a request for a method that is not implemented by the servlet is made, the method of the parent class is called, typically resulting in an error being returned to the requester.
Finally, the container calls the destroy() method that takes the servlet out of service. The destroy() method like init() is called only once in the lifecycle of a servlet.

Some More Jave Interview Questions

2009-08-18T05:58:00.000-07:00

1. What is J2EE?
J2EE is an environment for developing and deploying enterprise applications. The J2EE platform consists of a set of services, application programming interfaces (APIs), and protocols that provide the functionality for developing multitiered, web-based applications.

2. What is the J2EE module?
A J2EE module consists of one or more J2EE components for the same container type and one component deployment descriptor of that type.

3. What are the components of J2EE application?
A J2EE component is a self-contained functional software unit that is assembled into a J2EE application with its related classes and files and communicates with other components. The J2EE specification defines the following J2EE components:
Application clients and applets are client components.
Java Servlet and JavaServer PagesTM (JSPTM) technology components are web components.
Enterprise JavaBeansTM (EJBTM) components (enterprise beans) are business components.
Resource adapter components provided by EIS and tool vendors.

4. What are the four types of J2EE modules?
1. Application client module
2. Web module
3. Enterprise JavaBeans module
4. Resource adapter module

5. What does application client module contain?
The application client module contains:
--class files,
--an application client deployment descriptoor.
Application client modules are packaged as JAR files with a .jar extension.

6. What does web module contain?
The web module contains:
--JSP files,
--class files for servlets,
--GIF and HTML files, and
--a Web deployment descriptor.
Web modules are packaged as JAR files with a .war (Web ARchive) extension.

7. What are the differences between Ear, Jar and War files? Under what circumstances should we use each one?
There are no structural differences between the files; they are all archived using zip-jar compression. However, they are intended for different purposes.
--Jar files (files with a .jar extension) arre intended to hold generic libraries of Java classes, resources, auxiliary files, etc.
--War files (files with a .war extension) arre intended to contain complete Web applications. In this context, a Web application is defined as a single group of files, classes, resources, .jar files that can be packaged and accessed as one servlet context.
--Ear files (files with a .ear extension) arre intended to contain complete enterprise applications. In this context, an enterprise application is defined as a collection of .jar files, resources, classes, and multiple Web applications.
Each type of file (.jar, .war, .ear) is processed uniquely by application servers, servlet containers, EJB containers, etc.

8. What is the difference between Session bean and Entity bean?one?
The Session bean and Entity bean are two main parts of EJB container.
Session Bean
--represents a workflow on behalf of a cliennt
--one-to-one logical mapping to a client. --created and destroyed by a client
--not permanent objects
--lives its EJB container(generally) does noot survive system shut down
--two types: stateless and stateful beans Entity Bean
--represents persistent data and behavior off this data
--can be shared among multiple clients
--persists across multiple invocations
--findable permanent objects
--outlives its EJB container, survives systeem shutdown
--two types: container managed persistence(CCMP) and bean managed persistence(BMP)

9. What is "applet" ?
A J2EE component that typically executes in a Web browser but can execute in a variety of other applications or devices that support the applet programming model.

10. What is "applet container" ?
A container that includes support for the applet programming model.

11. What is "application assembler" ?
A person who combines J2EE components and modules into deployable application units.

12. What is "application client" ?
A first-tier J2EE client component that executes in its own Java virtual machine. Application clients have access to some J2EE platform APIs.

13. What is "application client container" ?
A container that supports application client components.

14. What is "application client module" ?
A software unit that consists of one or more classes and an application client deployment descriptor.

15. What is "application component provider" ?
A vendor that provides the Java classes that implement components' methods, JSP page definitions, and any required deployment descriptors.

16. What is "application configuration resource file" ?
An XML file used to configure resources for a JavaServer Faces application, to define navigation rules for the application, and to register converters, validators, listeners, renderers, and components with the application.

17. What is "archiving" ?
The process of saving the state of an object and restoring it.

18. What is "asant" ?
A Java-based build tool that can be extended using Java classes. The configuration files are XML-based, calling out a target tree where various tasks get executed.

19. What is "attribute"What is "asant" ?
A qualifier on an XML tag that provides additional information.

20. What is authentication ?
The process that verifies the identity of a user, device, or other entity in a computer system, usually as a prerequisite to allowing access to resources in a system. The Java servlet specification requires three types of authentication-basic, form-based, and mutual-and supports digest authentication.

21. What is authorization?
The process by which access to a method or resource is determined. Authorization depends on the determination of whether the principal associated with a request through authentication is in a given security role. A security role is a logical grouping of users defined by the person who assembles the application. A deployer maps security roles to security identities. Security identities may be principals or groups in the operational environment.

22. What is authorization constraint ?
An authorization rule that determines who is permitted to access a Web resource collection.

23. What is B2B ?
B2B stands for Business-to-business.

24. What is backing bean ?
A JavaBeans component that corresponds to a JSP page that includes JavaServer Faces components. The backing bean defines properties for the components on the page and methods that perform processing for the component. This processing includes event handling, validation, and processing associated with navigation.

25. What is basic authentication ?
An authentication mechanism in which a Web server authenticates an entity via a user name and password obtained using the Web application's built-in authentication mechanism.

26. What is bean-managed persistence ?
The mechanism whereby data transfer between an entity bean's variables and a resource manager is managed by the entity bean.

27. What is bean-managed transaction ?
A transaction whose boundaries are defined by an enterprise bean.

28. What is binding (XML) ?
Generating the code needed to process a well-defined portion of XML data.

29. What is binding (JavaServer Faces technology) ?
Wiring UI components to back-end data sources such as backing bean properties.

30. What is build file ?
The XML file that contains one or more asant targets. A target is a set of tasks you want to be executed. When starting asant, you can select which targets you want to have executed. When no target is given, the project's default target is executed.

31. What is business logic ?
The code that implements the functionality of an application. In the Enterprise JavaBeans architecture, this logic is implemented by the methods of an enterprise bean.

32.What is business method ?
A method of an enterprise bean that implements the business logic or rules of an application.

33. What is callback methods ?
Component methods called by the container to notify the component of important events in its life cycle.

34. What is caller ?
Same as caller principal.

35. What is caller principal ?
The principal that identifies the invoker of the enterprise bean method.

36. What is cascade delete ?
A deletion that triggers another deletion. A cascade delete can be specified for an entity bean that has container-managed persistence.

37. What is CDATA ?
A predefined XML tag for character data that means "don't interpret these characters," as opposed to parsed character data (PCDATA), in which the normal rules of XML syntax apply. CDATA sections are typically used to show examples of XML syntax.

38. What is certificate authority ?
A trusted organization that issues public key certificates and provides identification to the bearer.

39. What is client-certificate authentication ?
An authentication mechanism that uses HTTP over SSL, in which the server and, optionally, the client authenticate each other with a public key certificate that conforms to a standard that is defined by X.509 Public Key Infrastructure.

40. What is comment ?
In an XML document, text that is ignored unless the parser is specifically told to recognize it.

JDBC - ODBC Questions......

2009-08-16T05:52:00.000-07:00

1. What's the JDBC 2.0 API?

The JDBC 2.0 API is the latest update of the JDBC API. It contains many new features, including scrollable result sets and the new SQL:1999 (formerly SQL 3) data types. There are two parts to the JDBC 2.0 API:

the JDBC 2.0 core API (the java.sql package), which is included in the JavaTM 2 SDK, Standard Edition

the JDBC 2.0 Optional Package API (the javax.sql package), which is available separately or as part of the Java 2 SDK, Enterprise Edition

2. Does the JDBC-ODBC Bridge support the new features in the JDBC 2.0 API?

No, the JDBC-ODBC Bridge that is included in the Java 2 Platform initial release does not support the new features in the JDBC 2.0 API. However, Sun and Merant are working to produce a new version of the Bridge that does support the new features. Note that we do not recommend using the Bridge except for experimental purposes or when you have no other driver available.

3. Can the JDBC-ODBC Bridge be used with applets?

Use of the JDBC-ODBC bridge from an untrusted applet running in a browser, such as Netscape Navigator, isn't allowed. The JDBC-ODBC bridge doesn't allow untrusted code to call it for security reasons. This is good because it means that an untrusted applet that is downloaded by the browser can't circumvent Java security by calling ODBC. Remember that ODBC is native code, so once ODBC is called, the Java programming language can't guarantee that a security violation won't occur. On the other hand, Pure Java JDBC drivers work well with applets. They are fully downloadable and do not require any client-side configuration.

Finally, we would like to note that it is possible to use the JDBC-ODBC bridge with applets that will be run in appletviewer since appletviewer assumes that applets are trusted. It is also possible to use the JDBC-ODBC bridge with applets that are run in the HotJavaTM browser (available from Java Software), since HotJava provides an option to turn off applet security. In general, it is dangerous to turn applet security off, but it may be appropriate in certain controlled situations, such as for applets that will only be used in a secure intranet environment. Remember to exercise caution if you choose this option, and use an all-Java JDBC driver whenever possible to avoid security problems.

4. How do I start debugging problems related to the JDBC API?

A good way to find out what JDBC calls are doing is to enable JDBC tracing. The JDBC trace contains a detailed listing of the activity occurring in the system that is related to JDBC operations.

If you use the DriverManager facility to establish your database connection, you use the DriverManager.setLogWriter method to enable tracing of JDBC operations. If you use a DataSource object to get a connection, you use the DataSource.setLogWriter method to enable tracing. (For pooled connections, you use the ConnectionPoolDataSource.setLogWriter method, and for connections that can participate in distributed transactions, you use the XADataSource.setLogWriter method.)

5. How can I use the JDBC API to access a desktop database like Microsoft Access over the network?

Most desktop databases currently require a JDBC solution that uses ODBC underneath. This is because the vendors of these database products haven't implemented all-Java JDBC drivers.

The best approach is to use a commercial JDBC driver that supports ODBC and the database you want to use. See the JDBC drivers page for a list of available JDBC drivers.

The JDBC-ODBC bridge from Sun's Java Software does not provide network access to desktop databases by itself. The JDBC-ODBC bridge loads ODBC as a local DLL, and typical ODBC drivers for desktop databases like Access aren't networked. The JDBC-ODBC bridge can be used together with the RMI-JDBC bridge , however, to access a desktop database like Access over the net. This RMI-JDBC-ODBC solution is free.

6. Does the JDK include the JDBC API and the JDBC-ODBC Bridge?

Yes, the JDK 1.1 and the Java 2 SDK, Standard Edition (formerly known as the JDK 1.2), contain both the JDBC API and the JDBC-ODBC Bridge. The Java 2 SDK, Standard Edition, contains the JDBC 2.0 core API, which is the latest version. It does not include the JDBC 2.0 Optional Package, which is part of the Java 2 SDK, Enterprise Edition, or which you can download separately.

Note that the version of the JDBC API and the JDBC-ODBC Bridge provided for separate download on the JDBC download page are only for use with the JDK 1.0.2.

7. What JDBC technology-enabled drivers are available?

See our web page on JDBC technology-enabled drivers for a current listing.

8. What documentation is available for the JDBC API?

See the JDBC technology home page for links to information about JDBC technology. This page links to information about features and benefits, a list of new features, a section on getting started, online tutorials, a section on driver requirements, and other information in addition to the specifications and javadoc documentation.

9. Are there any ODBC drivers that do not work with the JDBC-ODBC Bridge?

Most ODBC 2.0 drivers should work with the Bridge. Since there is some variation in functionality between ODBC drivers, the functionality of the bridge may be affected. The bridge works with popular PC databases, such as Microsoft Access and FoxPro.

10. Does the JDBC-ODBC Bridge work with Microsoft J++?

No, J++ does not support the JDBC-ODBC bridge since it doesn't implement the Java Native Interface (JNI). Any all-Java JDBC driver should work with J++, however.

11. What causes the "No suitable driver" error?

"No suitable driver" is an error that usually occurs during a call to the DriverManager.getConnection method. The cause can be failing to load the appropriate JDBC drivers before calling the getConnection method, or it can be specifying an invalid JDBC URL--one that isn't recognized by your JDBC driver. Your best bet is to check the documentation for your JDBC driver or contact your JDBC driver vendor if you suspect that the URL you are specifying is not being recognized by your JDBC driver.

In addition, when you are using the JDBC-ODBC Bridge, this error can occur if one or more the the shared libraries needed by the Bridge cannot be loaded. If you think this is the cause, check your configuration to be sure that the shared libraries are accessible to the Bridge.

12. Why isn't the java.sql.DriverManager class being found?

This problem can be caused by running a JDBC applet in a browser that supports the JDK 1.0.2, such as Netscape Navigator 3.0. The JDK 1.0.2 does not contain the JDBC API, so the DriverManager class typically isn't found by the Java virtual machine running in the browser.

Here's a solution that doesn't require any additional configuration of your web clients. Remember that classes in the java.* packages cannot be downloaded by most browsers for security reasons. Because of this, many vendors of all-Java JDBC drivers supply versions of the java.sql.* classes that have been renamed to jdbc.sql.*, along with a version of their driver that uses these modified classes. If you import jdbc.sql.* in your applet code instead of java.sql.*, and add the jdbc.sql.* classes provided by your JDBC driver vendor to your applet's codebase, then all of the JDBC classes needed by the applet can be downloaded by the browser at run time, including the DriverManager class.

This solution will allow your applet to work in any client browser that supports the JDK 1.0.2. Your applet will also work in browsers that support the JDK 1.1, although you may want to switch to the JDK 1.1 classes for performance reasons. Also, keep in mind that the solution outlined here is just an example and that other solutions are possible.

13. Why doesn't calling the method Class.forName load my JDBC driver?

There is a bug in the JDK 1.1.x that can cause the method Class.forName to fail. A workaround is to explicitly call the method DriverManager.registerDriver(new YourDriverClass()). The exact problem in the JDK is a race condition in the class loader that prevents the static section of code in the driver class from executing and registering the driver with the DriverManager.

14. Why do the java.sql and java.math packages fail to download java.* packages? Is there a workaround?

For security reasons, browsers will not download java.* packages. In order to use the JDBC API with browsers that have not been upgraded to JDK1.1 or beyond, we recommend that the java.sql and java.math packages be renamed jdbc.sql and jdbc.math. Most vendors supplying JDBC technology-enabled drivers that are written purely in the Java programming language already provide versions of these renamed packages. When JDK 1.1 support has been added to your browser, you should convert your applets back to the java.* package names.

15. Why is the precision of java.math.BigDecimal limited to 18 digits in the JDK 1.0.2 add-on version of the JDBC API?

In JDK 1.1, java.math.BigInteger is implemented in C. It supports a precision of thousands of digits. The same is true for BigDecigmal.

The version of BigInteger provided with the JDK 1.0.2 add-on version of the JDBC API is a simplified version written in the Java programming language, and it is limited to 18 digits. Because the implementation of BigDecimal is based on BigInteger, it also is limited to this precision.

In the JDBC 2.0 API, you can use a new version of the method ResultSet.getBigDecimal that does not take a scale parameter and returns a BigDecimal with full precision.

16. Can the JDBC API be added to JDK 1.0.2?

Yes. Download the JDBC 1.22 API from the JDBC download page and follow the installation instructions in the release notes.

If you are using any version of the JDK from 1.1 on, the JDBC API is already included, and you should not download the JDBC 1.22 API.

17. How do I retrieve a whole row of data at once, instead of calling an individual ResultSet.getXXX method for each column?

The ResultSet.getXXX methods are the only way to retrieve data from a ResultSet object, which means that you have to make a method call for each column of a row. It is unlikely that this is the cause of a performance problem, however, because it is difficult to see how a column could be fetched without at least the cost of a function call in any scenario. We welcome input from developers on this issue.

18. Why does the ODBC driver manager return 'Data source name not found and no default driver specified Vendor: 0'

This type of error occurs during an attempt to connect to a database with the bridge. First, note that the error is coming from the ODBC driver manager. This indicates that the bridge-which is a normal ODBC client-has successfully called ODBC, so the problem isn't due to native libraries not being present. In this case, it appears that the error is due to the fact that an ODBC DSN (data source name) needs to be configured on the client machine. Developers often forget to do this, thinking that the bridge will magically find the DSN they configured on their remote server machine

19. Are all the required JDBC drivers to establish connectivity to my database part of the JDK?

No. There aren't any JDBC technology-enabled drivers bundled with the JDK 1.1.x or Java 2 Platform releases other than the JDBC-ODBC Bridge. So, developers need to get a driver and install it before they can connect to a database. We are considering bundling JDBC technology- enabled drivers in the future.

20. 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. In addition, deadlocks can occur between locks held in the database and the semaphore used by the Bridge. We are thinking about removing the synchronized methods in the future. They were added originally to make things simple for folks writing Java programs that use a single-threaded ODBC driver.

21. 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.

22. Does the JDBC-ODBC Bridge developed by Merant and Sun support result sets that contain Japanese Characters (DBCS)?

Yes, but we haven't tested this ourselves. The version of the Bridge in the Java 2 SDK, Standard Edition, and Java 2 SDK, Enterprise Edition, also supports a new charSet Connection property for specifying the character encoding used by the underlying DBMS.

23. Why can't I invoke the ResultSet methods afterLast and beforeFirst when the method next works?

You are probably using a driver implemented for the JDBC 1.0 API. You need to upgrade to a JDBC 2.0 driver that implements scrollable result sets. Also be sure that your code has created scrollable result sets and that the DBMS you are using supports them.

24. How can I retrieve a String or other object type without creating a new object each time?

Creating and garbage collecting potentially large numbers of objects (millions) unnecessarily can really hurt performance. It may be better to provide a way to retrieve data like strings using the JDBC API without always allocating a new object.

We are studying this issue to see if it is an area in which the JDBC API should be improved. Stay tuned, and please send us any comments you have on this question.

25. There is a method getColumnCount in the JDBC API. Is there a similar method to find the number of rows in a result set?

No, but it is easy to find the number of rows. If you are using a scrollable result set, rs, you can call the methods rs.last and then rs.getRow to find out how many rows rs has. If the result is not scrollable, you can either count the rows by iterating through the result set or get the number of rows by submitting a query with a COUNT column in the SELECT clause.

26. I would like to download the JDBC-ODBC Bridge for the Java 2 SDK, Standard Edition (formerly JDK 1.2). I'm a beginner with the JDBC API, and I would like to start with the Bridge. How do I do it?

The JDBC-ODBC Bridge is bundled with the Java 2 SDK, Standard Edition, so there is no need to download it separately.

27. If I use the JDBC API, do I have to use ODBC underneath?

No, this is just one of many possible solutions. We recommend using a pure Java JDBC technology-enabled driver, type 3 or 4, in order to get all of the benefits of the Java programming language and the JDBC API.

28. Once I have the Java 2 SDK, Standard Edition, from Sun, what else do I need to connect to a database?

You still need to get and install a JDBC technology-enabled driver that supports the database that you are using. There are many drivers available from a variety of sources. You can also try using the JDBC-ODBC Bridge if you have ODBC connectivity set up already. The Bridge comes with the Java 2 SDK, Standard Edition, and Enterprise Edition, and it doesn't require any extra setup itself. The Bridge is a normal ODBC client. Note, however, that you should use the JDBC-ODBC Bridge only for experimental prototyping or when you have no other driver available.