Question

what are some of the predefined package in java that used to handle exceptions and give one implantation example How the predefined package handles exception?

Answer 1

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PREDEFINED PACKAGES

Predefined packages are those which are developed by SUN micro systems and supplied as a part of JDK (Java Development Kit) to simplify the task of java programmer.

NOTE:

Core packages of java starts with java. (For example: java.lang.*) and Advanced packages of java starts with javax. (For example: java.sql.*)

TYPES of predefined packages:

As a part of J2SE we have nine predefined packages which are given in the following table:

java.lang.*

This package is used for achieving the language functionalities such as conversion of data from string to fundamental data, displaying the result on to the console, obtaining the garbage collector. This is the package which is by default imported for each and every java program.

java.io.*

This package is used for developing file handling applications, such as, opening the file in read or write mode, reading or writing the data, etc.

java.awt.* (abstract windowing toolkit)

This package is used for developing GUI (Graphic Unit Interface) components such as buttons, check boxes, scroll boxes, etc.

java.awt.event.*

Event is the sub package of awt package. This package is used for providing the functionality to GUI components, such as, when button is clicked or when check box is checked, when scroll box is adjusted either vertically or horizontally.

java.applet.*

This package is used for developing browser oriented applications. In other words this package is used for developing distributed programs. An applet is a java program which runs in the context of www or browser.

java.net.*

This package is used for developing client server applications.

java.util.*

This package is used for developing quality or reliable applications in java or J2EE. This package contains various classes and interfaces which improves the performance of J2ME applications. This package is also known as collection framework (collection framework is the standardized mechanism of grouping of similar or different type of objects into single object. This single object is known as collection object).

java.text.*

This package is used for formatting date and time on day to day business operations.

java.lang.reflect.*

Reflect is the sub package of lang package. This package is basically used to study runtime information about the class or interface. Runtime information represents data members of the class or interface, Constructors of the class, types of methods of the class or interface.

java.sql.*

This package is used for retrieving the data from data base and performing various operations on data base.

The java.io Package

The java.io package contains a relatively large number of classes, but, as you can see from Figure 11-1 and Figure 11-2, the classes form a fairly structured hierarchy. Most of the package consists of byte streams--subclasses of InputStream or OutputStream and (in Java 1.1) character streams--subclasses of Reader or Writer. Each of these stream types has a specific purpose, and, despite its size, java.io is a straightforward package to understand and to use.

Before we consider the stream classes in the package, let's examine the important non-stream classes. File represents a file or directory name in a system-independent way and provides methods for listing directories, querying file attributes, and renaming and deleting files. FilenameFilter is an interface that defines a method that accepts or rejects specified filenames. It is used by java.awt.FileDialog and File to specify what types of files should be included in directory listings. RandomAccessFile allows you to read from or write to arbitrary locations of a file. Often, though, you'll prefer sequential access to a file and should use one of the stream classes.

InputStream and OutputStream are abstract classes that define methods for reading and writing bytes. Their subclasses allow bytes to be read from and written to a variety of sources and sinks. FileInputStream and FileOutputStream read from and write to files. ByteArrayInputStream and ByteArrayOutputStream read from and write to an array of bytes in memory. PipedInputStream reads bytes from a PipedOutputStream, and PipedOutputStream writes bytes to a PipedInputStream. These classes work together to implement a pipe for communication between threads.

FilterInputStream and FilterOutputStream are special; they filter input and output bytes. When you create a FilterInputStream, you specify an InputStream for it to filter. When you call the read() method of a FilterInputStream, it calls the read() method of its InputStream, processes the bytes it reads, and returns the filtered bytes. Similarly, when you create a FilterOutputStream, you specify an OutputStream to be filtered. Calling the write() method of a FilterOutputStream causes it to process your bytes in some way and then pass those filtered bytes to the write() method of its OutputStream.

Figure 11-1. The java.io package

Figure 11-2. The exception classes of the java.io package

FilterInputStream and FilterOutputStream do not perform any filtering themselves; this is done by their subclasses. BufferedInputStream and BufferedOutputStream provide input and output buffering and can increase I/O efficiency. DataInputStream reads raw bytes from a stream and interprets them in various binary formats. It has various methods to read primitive Java data types in their standard binary formats. DataOutputStream allows you to write Java primitive data types in binary format.

In Java 1.1 and later, the byte streams I just described are complemented by an analogous set of character input and output streams. Reader is the superclass of all character input streams, and Writer is the superclass of all character output streams. These character streams supersede the byte streams for all textual I/O. They are more efficient than the byte streams, and they correctly handle the conversion between local encodings and Unicode text, making them invaluable for internationalized programs. Most of the Reader and Writer streams have obvious byte-stream analogs. BufferedReader is a commonly used stream; it provides buffering for efficiency and also has a readLine() method to read a line of text at a time. PrintWriter is another very common stream; its methods allow output of a textual representation of any primitive Java type or of any object (via the object's toString() method).

The ObjectInputStream and ObjectOutputStream classes are special. These byte-stream classes are new as of Java 1.1 and are part of the Object Serialization API.

CharConversionException

Java 1.1

java.io

serializable checked PJ1.1

Signals an error when converting bytes to characters or vice versa.

public class CharConversionException extends IOException {
// Public Constructors
public CharConversionException();
public CharConversionException(String s);
}
Hierarchy: Object-->Throwable(Serializable)-->Exception-->IOException--CharConversionException

EOFException

java 1.0

java.io

serializable checked PJ1.1

An IOException that signals the end-of-file.

public class EOFException extends IOException {
// Public Constructors
public EOFException();
public EOFException(String s);
}
Hierarchy: Object-->Throwable(Serializable)-->Exception-->IOException--EOFException

FileNotFoundException

java 1.0

java.io

serializable checked PJ1.1(opt)

An IOException that signals that a specified file cannot be found.

public class FileNotFoundException extends IOException {
// Public Constructors
public FileNotFoundException();
public FileNotFoundException(String s);
}
Hierarchy: Object-->Throwable(Serializable)-->Exception-->IOException--FileNotFoundException
Throw By: FileInputStream.FileInputStream(), FileOutputStream.FileOutputStream(), FileReader.FileReader(), RandomAccessFile.RandomAccessFile()

InterruptedIOException

java 1.0

java.io

serializable checked PJ1.1

An IOException that signals that an input or output operation was interrupted.

The bytesTransferred field contains the number of bytes read or written before the operation was interrupted.

public class InterruptedIOException extends IOException {
// Public Constructors
public InterruptedIOException();
public InterruptedIOException(String s);
// Public Instance Fields
public int bytesTransferred ;
}
Hierarchy: Object-->Throwable(Serializable)-->Exception-->IOException-->InterruptedIOException

InvalidClassException

Java 1.1

java.io

serializable checked PJ1.1

Signals that the serialization mechanism has encountered one of several possible problems with the class of an object that is being serialized or deserialization. The classname field should contain the name of the class in question, and the getMessage() method is overridden to return this class name with the message.

public class InvalidClassException extends ObjectStreamException {
// Public Constructors
public InvalidClassException (String reason);
public InvalidClassException (String cname, String reason);
// Public Methods OverridingThrowable
public String getMessage();
// Public Instance Fields
public String classname ;
}
Hierarchy: Object-->Throwable(Serializable)-->Exception-->IOException-->ObjectStreamException-->InvalidClassException