Creating your own exception classes:

Even though the Java class library defines a large number of exceptions, there may be times when you want to define your own exceptions that correspond to error conditions that are specific to a particular application. Since exceptions are just objects, all you need do is create a new class that inherits from one of the exception classes that are provided as part of the Java class library (normally Exception or a checkable subclass of Exception).

For example, if we wanted to have a more specific exception type for the file format error above, we could define a new exception type like this:

class FileFormatException extends IOException
{
private int errorPosition;

public FileFormatException () {}

public FileFormatException (String message, int bytesRead)
{
super(message);
errorPosition = bytesRead;
}

public int getErrorPosition ()
{
return errorPosition;
}
}

Most exception classes have two constructors - a constructor with no arguments, and a one-argument constructor that allows for a more detailed error message. In this case, we have a constructor with no arguments and one with two arguments. This makes it possible to store and retrieve more detailed information on the location of the error within the file.

This exception can then be used within code in the same way that standard Java exceptions can be used. For example, in a method header:

public boolean checkFormat (String fileName)
throws FileFormatException
Our new exception can also be used in a try-catch statement:

try
{
// code to be checked
if (checkFormat(theNewFile))
{
...
}
}
catch (FileFormatException exception)
{
String reason = exception.getMessage();
int errLoc = exception.getErrorPosition(); System.out.println(reason+" at "+errLoc);
}

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