Question

 Creating a user defined Exception “FileFormatException” by extending IOException as following way

class FileFormatException extends IOException {

public FileFormatException ( ) { }

public FileFormatException ( String gripe ){

super (gripe);

}

} 

And using that custom exception as follows:

String readData( BufferReader in ) throws FileFormatException {

……. 

while ( ……) { 

if ( ch = = -1) // EOF encountered 

{

if ( n < len )

throw new FileFormatException ( ) ; 

}

………

}

return s ;

} 

i) What are the differences between throw and throws? ii) Analyze the above code and write down your comments on whether the procedure is correct ways to use exception or not.

Answer 1

i) Differences between throws and throw.

• Throws clause is applied when declaring exceptions. This means that throws clause functions similarly as a try-catch block while throw is useful when throwing an exception explicitly.
• Exception class names always follow the throws clause while while an object of Exception class follows the keyword throw.
• Throws clause is written in the method signature while throw keyword is placed within a method body.

ii) The use of exception in the above code is correct because of the following reasons:

•  FileFormatException follows the keyword throws, indicating exception declaration.
• new FileFormatException ( ), a instance of FileFormatException class follows the keyword throw.
• The throw keyword is also used inside the body of a function