Missing exception warning

exercise No. 246

Q:

According to the API documentation the Files.copy(...) method throws the following exceptions:

  • UnsupportedOperationException

  • FileAlreadyExistsException

  • DirectoryNotEmptyException

  • IOException

  • SecurityException

We consider the following code:

static void copy(final String sourceFileName,
                 final String destinationFileName) 
  throws FileAlreadyExistsException 
         /* Compile time warning: Exception 
                         'java.nio.file.FileAlreadyExistsException' 
               is never thrown in the method */ {
  try {
    Files.copy(Paths.get(sourceFileName),
               Paths.get(destinationFileName));
  } catch(final IOException e) {
    System.out.println("Error copying file");
  }
}

Though there is no ... catch (FileAlreadyExistsException e)... clause dealing with this checked exception we still get the indicated warning. Give an explanation.

A:

The exception class FileAlreadyExistsException inherits from FileSystemException which in turn inherits from IOException. Thus FileAlreadyExistsException being a subclass of IOException will be caught and can thus never be thrown. This contradicts the throws declaration giving rise to a compiler warning.

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