FIle.list() for specific file types

[Paul Peterson]

Is it possible to use the list() method of the file class to create an array of only certain file types.  For example, say I wanted to search a given directory for files ending in .ser.  Or would I have to create the original array using the list command then create a new array searching for the correct file types?

[Jeanne Boyarsky]

Yes. Take a look at the version of the method that takes a parameter.

[Paul Peterson]

I found an example for filter filename and modified it to fit my application.  Obviously I missed something, probably something silly.  I am getting a null pointer exception.  This is what I am trying to use:

    public void buildMasterHashMap(){
 
        String fileExtension = ".ser";
        File [] oFiles ;
        File f = null;
        
        try{
            FilenameFilter filter = new FilenameFilter(){
                @Override
                public boolean accept(File dir, String name) {
                    //get last index of "."
                    int lastIndex = name.lastIndexOf(".");
                    
                    //get file extension
                    String str = name.substring(lastIndex);
                    
                    //compare file extensions
                    if (str.equalsIgnoreCase(fileExtension)){
                        return true;
                    }
                    return false;
                }
                
            };
            //return array of files
            oFiles = f.listFiles(filter);        //throwing null pointer exception
            System.out.println(oFiles.length);
            HashMap<String, Order> hm = new HashMap();
            //build HashMaps
            for (File orderFile : orderFiles) {
                String fName = orderFiles.toString();
                //deserialize
                ArrayList<Order> al = deserializeOrders(fName);
                for (Order tempOrder : al) {
                    String oDate = tempOrder.getDate();
                    String oNumber = tempOrder.getId();
                    hm.put(oNumber, tempOrder);
                    master.put(oDate, hm);
                }
            }
        }catch(Exception e){
         // if any error occurs
         e.printStackTrace();
      }
    }

[Paweł Baczyński]

First, you set f to null at line 5.
Then you call a method on f on line 26.
That's the reason of NPE.

Please provide SSCCE when asking questions.
I was not able to run your code because it missed declarations of master, Order, deserializeOrders, orderFiles, etc...

[Henry Wong]

Paul Peterson wrote:Obviously I missed something, probably something silly.  I am getting a null pointer exception.  This is what I am trying to use:

            oFiles = f.listFiles(filter);        //throwing null pointer exception

Well, you set the "f" reference variable to null, so, when you tried to deference it to call the listFiles() method, you will get a null pointer exception.

Henry

[Paul Peterson]

As I mentioned, I was following an example I found online.  And we know if it's on the internet it must be true...

This was the source I referenced...

package com.tutorialspoint;

import java.io.File;
import java.io.FilenameFilter;

public class FileDemo {
   public static void main(String[] args) {
      
      File f = null;
      File[] paths;
      
      try{      
         // create new file
         f = new File("c:/test");
         
         // create new filename filter
         FilenameFilter fileNameFilter = new FilenameFilter() {
   
            @Override
            public boolean accept(File dir, String name) {
               if(name.lastIndexOf('.')>0)
               {
                  // get last index for '.' char
                  int lastIndex = name.lastIndexOf('.');
                  
                  // get extension
                  String str = name.substring(lastIndex);
                  
                  // match path name extension
                  if(str.equals(".txt"))
                  {
                     return true;
                  }
               }
               return false;
            }
         };
         // returns pathnames for files and directory
         paths = f.listFiles(fileNameFilter);
         
         // for each pathname in pathname array
         for(File path:paths)
         {
            // prints file and directory paths
            System.out.println(path);
         }
      }catch(Exception e){
         // if any error occurs
         e.printStackTrace();
      }
   }
}

A nudge in the correct direction is much appreciated.

[Paweł Baczyński]

Please note line 14 of the "Internet code" and compare it with yours.

[Paul Peterson]

Thank you, that cleared the error.