file name loop

[John Powell]

I'm trying to ask the user for a file to use. If the file is found then the program continues, but if the file is not found the user is asked to enter the filename again and again until it's found. I assume I could do it with a loop, but I'm unsure of the condition to test for the file not found.

BufferedReader stdin = new BufferedReader(new InputStreamReader
System.in));
String filename = null;

do{
System.out.print("Please enter filename: ");
filename = stdin.readLine();
}while (file is not found???)

Any ideas?

[Michael Dunn]

  ...
  filename = stdin.readLine();
  File file = new File(filename);
}while (!file.exists());

[John Powell]

Thanks for the info.

class test {
static File file;

public static void main(String[] args) throws IOException {
BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
String filename=null;

do{
System.out.print("Please enter filename: ");
filename = stdin.readLine();
File file = new File(filename);
}while (!file.exists());
}
}  

The problem I run into now is a Null pointer exception. If I surround the loop with a try/catch it works ok, but it only runs once. I'm not sure if I'm missing something or whether there's a problem with the logic.

[Joyce Lee]

Hi John,

static File file; // class variable
File file = new File(filename); // local variable
You declared the variable file in two locations. By doing so, the local one is used within the curl brackets of do...while loop. This is called shadowing. Once it is out of curl brackets, the class variable file referencing to a null object is used.

  
do {
... // within the curl brackets
} while (file.exists()); // this one is out of curl brackets. 
                         // Hence class variable file is used.

To resolve the NullPointerException, simply remove the declaration for the local variable file.

class test {
static File file;

public static void main(String[] args) throws IOException {
BufferedReader stdin = new BufferedReader(new          
                        InputStreamReader (System.in));
String filename=null;

do{
System.out.print("Please enter filename: ");
filename = stdin.readLine();
   [B]file = new File(filename); // corrected[/B]
}while (!file.exists());
}
}  

Joyce
[ November 13, 2004: Message edited by: Joyce Lee ]

[John Powell]

Thank you. It's finally starting to make sense.