What Does it Mean when a Directory's <list()> Returns <null>?

[Kevin Simonson]

I have some code I wrote quite a while ago that opens a <File> object, and that processes it differently depending on whether or not <isDirectory()> returns <true> or not. If it does return <true>, then it creates a <String[]> object named <children> and sets it equal to the value the <File> object's <list()> returns.

But then before my code actually does anything with <children>, it checks it to see if it equals <null>. What would it mean for the value returned by <list()> to be <null>? Is that an error condition I should check for?

The code I referred to is pretty complex, so I won't include it here, but if you need an example, here's a pretty simple one.

import java.io.File;

public class Dir
{
  public static void main ( String[] arguments)
  {
    if (arguments.length == 1)
    { File element = new File( arguments[ 0]);
      if      (! element.exists())
      { System.out.println( "File \"" + arguments[ 0] + "\" doesn't exist!");
      }
      else if (element.isDirectory())
      { String[] children = element.list();
        if (children == null)
        { System.out.println
            ( "Couldn't list children of \"" + arguments[ 0] + "\"!");
        }
        else
        { System.out.println( "Contents of \"" + arguments[ 0] + "\":");
          for (String child : children)
          { System.out.println( "  " + child);
          }
        }
      }
      else
      { System.out.println( "File \"" + arguments[ 0] + "\" exists.");
      }
    }
    else
    { System.out.println( "Usage is");
      System.out.println( "  java Dir <element>");
    }
  }
}

Kevin Simonson

[Jeff Verdegan]

Well, if you look at the docs, you'll see they say this:

Returns null if this abstract pathname does not denote a directory, or if an I/O error occurs.

[Rob Spoor]

Two common causes are a) the File object not being a directory (but that's not the case here), and b) the directory not being readable for you.