Better Understanding Java Concepts : File , Exception Handling

Question

I want to build a data structure where I would store all the files from the file-system. For that I have a class directoryNode:

class directoryNode{
    private String name;
    private String path;
    File file;

    //This List Stores the sub-directories of the given Directory.

    private List<directoryNode> subDirectories = new ArrayList<directoryNode>();

    //This List stores the simple files of the given Directory

    private List<String> fileNames = new ArrayList<String>();

    //The Default Constructor.
    directoryNode(File directoryName){
        this.name = directoryName.getName();
        this.path = directoryName.getPath();
        this.file = directoryName;
        //A Function to build this directory.
        buildDirectory();
    }

    File[] filesFromThisDirectory;

    private void buildDirectory(){
        //get All the files from this directory
        filesFromThisDirectory = file.listFiles();
        try{
            for(int i = 0 ; i < filesFromThisDirectory.length ; i++){
                if(filesFromThisDirectory[i].isFile()){
                    this.fileNames.add(filesFromThisDirectory[i].getName());
                } else if(filesFromThisDirectory[i].isDirectory()){
                    directoryNode Dir = new directoryNode(filesFromThisDirectory[i]);
                    this.subDirectories.add(Dir);
                }
            }
        }catch(Exception e){
            System.out.println(e.getMessage());
        }
    }
}

My Program Works fine but I get some weird behavior when I don't use the try- Catch block in the buildDirectory() Function. Build Function recursilvely builds the structure for the list of files as written in the code.

when I do:

directoryNode d1 = new directoryNode(new File("/"));

when try-Catch is there , program works fine but if I remove the try catch block : i get an error after executing it for some time: The error that I get is :

 Exception in thread "main" java.lang.NullPointerException
  at directoryNode.buildDirectory(myClass.java:47)
  at directoryNode.<init>(myClass.java:22)
  at directoryNode.buildDirectory(myClass.java:55)
  at directoryNode.<init>(myClass.java:22)
  at directoryNode.buildDirectory(myClass.java:55)
  at directoryNode.<init>(myClass.java:22)
  at myClass.main(myClass.java:75)

but when I run :

  directoryNode d1 = new directoryNode(new File("/home/neeraj"));

With or without try - catch block , I the program runs fine without any error. Why is this so ? Why do I get different results in these these scenarios?

@fge Just curious, what's the better alternative for Java 7/8? — awksp
– awksp, Commented May 17, 2014 at 7:49

fge · Accepted Answer · 2014-05-17 08:14:04Z

1

The problem is with this line:

    filesFromThisDirectory = file.listFiles();

This will return null if the File object is not a directory... OR if it is but you don't have the necessary privileges.

You must therefore check that filesFromThisDirectory is not null before entering your loop.

But do yourself a favour, drop File and use java.nio.file instead.

answered May 17, 2014 at 8:14

fge

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