Why Does a Non-existent File Have a Length of Zero?

[Kevin Simonson]

I realize that the Java documentation kind of hints that non-existent files might be considered to have a length of zero. In my own case I wrote the following code:

import java.io.File;
import java.io.FileInputStream;
import java.io.BufferedInputStream;
import java.io.PrintStream;
import java.io.FileNotFoundException;

public class Bug
{
  private static final PrintStream SY_O = System.out;

  public static void main ( String[] arguments)
  {
    if (arguments.length == 1)
    { String ntName = arguments[ 0];
      try
      { File ntFile = new File( ntName);
        SY_O.println
          ( "File \"" + ntName + "\" is of size " + ntFile.length() + '.');
        BufferedInputStream notThere
                      = new BufferedInputStream( new FileInputStream( ntFile));
      }
      catch (FileNotFoundException fnfExc)
      { SY_O.println( "Unable to open file \"" + ntName + "\" for input!");
      }
    }
    else
    { SY_O.println( "Usage is");
      SY_O.println( "  java Bug <file-name>");
    }
  }
}

and when I ran it on a non-existent file it did indeed give me a length of zero for the file, before the next line threw an exception, so my own application considers a non-existent file to have length zero. But does anybody know why it was chosen for such a file to have length zero? Why didn't my call to the {length()} method for object {ntFile} throw a {FileNotFoundException} itself?

KevinSim

[Jeff Verdegan]

I would guess they did it that way to avoid having to make extra calls in the more common cases. If we're calling length(), then it's one of the following cases:

We already know the file exists, so no point in forcing us to make an extra call to test for existence or to catch an exception.

We're not going to do anything with an empty file, so no point in forcing us to make an extra call to test for existence or to catch an exception.

We're going to try accessing the file, even if it's empty. We'll have to handle the exception at that point anyway, so nothing gained by throwing it at the length check.

In short, there's nothing to be gained by throwing an exception, and possibly some extra work to be added that we don't have to do now.

[Jesper de Jong]

Note that class File also has an exists() method, that may be an easier way to check if a file exists than trying to open it and catch FileNotFoundException.