EXCEPTION HANDLING IN JAVA

An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including the following:

• A user has entered invalid data.
• A file that needs to be opened cannot be found.
• A network connection has been lost in the middle of communications or the JVM has run out of memory.

the three categories of exceptions:

• Checked exceptions: A checked exception is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.
• Runtime exceptions: A runtime exception is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compilation.
• Errors: These are not exceptions at all, but problems that arise beyond the control of the user or the programmer. Errors are typically ignored in your code because you can rarely do anything about an error. For example, if a stack overflow occurs, an error will arise. They are also ignored at the time of compilation.

Catching Exceptions:

A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception

Multiple catch Blocks:

A try block can be followed by multiple catch blocks.

The throws/throw Keywords:

If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method's signature.

You can throw an exception, either a newly instantiated one or an exception that you just caught, by using the throw keyword. Try to understand the different in throws and throw keywords.

The finally Keyword

The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred.

Using a finally block allows you to run any cleanup-type statements that you want to execute, no matter what happens in the protected code.

Common Exceptions:

In Java, it is possible to define two catergories of Exceptions and Errors.

• JVM Exceptions: - These are exceptions/errors that are exclusively or logically thrown by the JVM. Examples : NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException,
• Programmatic exceptions: - These exceptions are thrown explicitly by the application or the API programmers Examples: IllegalArgumentException, IllegalStateException.
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JAVA- STREAMS & FILES

Java provides strong but flexible support for I/O related to Files and networks but this tutorial covers very basic functionality related to streams and I/O. We would see most commonly used example one by one:

Byte Streams

Java byte streams are used to perform input and output of 8-bit bytes. Though there are many classes related to byte streams but the most frequently used classes are , FileInputStream andFileOutputStream

Character Streams

Java Byte streams are used to perform input and output of 8-bit bytes, where as Java Characterstreams are used to perform input and output for 16-bit unicode. Though there are many classes related to character streams but the most frequently used classes are , FileReader and FileWriter.. Though internally FileReader uses FileInputStream and FileWriter uses FileOutputStream but here major difference is that FileReader reads two bytes at a time and FileWriter writes two bytes at a time.

Standard Streams

All the programming languages provide support for standard I/O where user's program can take input from a keyboard and then produce output on the computer screen. If you are aware if C or C++ programming languages, then you must be aware of three standard devices STDIN, STDOUT and STDERR. Similar way Java provides following three standard streams

• Standard Input: This is used to feed the data to user's program and usually a keyboard is used as standard input stream and represented as System.in.
• Standard Output: This is used to output the data produced by the user's program and usually a computer screen is used to standard output stream and represented as System.out.
• Standard Error: This is used to output the error data produced by the user's program and usually a computer screen is used to standard error stream and represented as System.err.

Following is a simple program which creates InputStreamReader to read standard input stream until the user types a "q":

import java.io.*;

public class ReadConsole {
   public static void main(String args[]) throws IOException
   {
      InputStreamReader cin = null;

      try {
         cin = new InputStreamReader(System.in);
         System.out.println("Enter characters, 'q' to quit.");
         char c;
         do {
            c = (char) cin.read();
            System.out.print(c);
         } while(c != 'q');
      }finally {
         if (cin != null) {
            cin.close();
         }
      }
   }
}

Reading and Writing Files:

As described earlier, A stream can be defined as a sequence of data. The InputStream is used to read data from a source and the OutputStream is used for writing data to a destination.

Here is a hierarchy of classes to deal with Input and Output streams.