File and directory

[Anders Kviback]

Hello!

I have a problem when I'm trying to create a path to a map and its files. I have a class Directory which implements the interface DirectoryListing.

public interface DirectoryListing
{
	void list();
	void setDirectory(String dir);
	void setDirectory(java.io.File dir);
	java.io.File[] getFiles();
	java.io.File[] getDirectories();
	String toString();
}

And class Directory

package lektion2.uppgift1;

import java.io.File;
public class Directory implements DirectoryListing{

		private File directory = new File("C:\\Users\\Anders\\Documents\\Notes\\dagbok.odt");  

	
	public Directory(){
		directory = new File(System.getProperty("user.home"));
	}
	
	public Directory(File directory){
		this.directory = new File(directory, "exercise2");
		if (!this.directory.exists())
			this.directory = new File(System.getProperty("user.home"));
	}
	
	public Directory(String  directory){
		this.directory = new File(directory); 
		if (!this.directory.exists())
			this.directory = new File(System.getProperty("user.home"));
	}
	
	public void setDirectory(String dir){
		this.directory = new File(dir);
		if (!this.directory.exists())
			this.directory = new File("C:\\Users\\Anders\\Documents\\Notes\\dagbok.odt");
	}
	
	public void setDirectory(java.io.File dir){
		this.directory = new File (dir, "sdvds");
		if (!this.directory.exists())
			this.directory = new File("C:\\Users\\Anders\\Documents\\Notes\\dagbok.odt");
	}
	//Returns an array of the class File and contains all the files in
	//the present directory 
	public java.io.File[] getFiles(){
		java.io.File[] filer = directory.listFiles();
		if (filer.length == 0)
			return filer;
		java.io.File temp[] = new java.io.File[filer.length];
		for (int i = 0; i < filer.length; i++){
		if (filer[i].isFile()){ 
			temp[i] = filer[i];
			filer = temp;
		}
		}
		return filer;
	}
	//Returns an array of the class File and contains all the directories in
	//the present directory 
	public java.io.File[] getDirectories(){
		java.io.File[] katalog = directory.listFiles();
		if (katalog.length == 0)
			return katalog;
		java.io.File temp[] = new java.io.File[katalog.length];
		for (int i = 0; i < katalog.length; i++){
		if (katalog[i].isDirectory()){ 
			temp[i] = katalog[i];
			katalog = temp;
		}
		}
		return katalog;
	}
//Lists the content of the present directory (directories and files)
public void list(){
		
		File[] dir = getDirectories();
		File[] file = getFiles();
		
		long t = directory.lastModified();
		for (int i = 0; i < directory.length(); i++){
			System.out.format("%-10s %10s filmapp %10d ",dir[i].getName(),t);
			
		}
		for(int i = 0; i < directory.length();i++){
			System.out.format("%-10s %10d %2d kB  %10d ",file[i].getName(),file[i].length(),t);
		}
	}
	
	//Returnerar the complete path to the present directory as a  string
	public String toString(){
		
		String name = System.getProperty("user.home");
		File f = new File(name);
		String s = f.getAbsolutePath();
	    return s;
	}
}

To run this code I have a class Uppgift1.

public class Uppgift1 {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		
		Directory dir = new Directory();
		dir.list();

	}

}

But when I run Uppgift1 I get a NullPointerException at line 59 in class Directory ie
if (katalog[i].isDirectory()){

So, the program can't find any objects here, but what am I doing wrong here?
As you understand this is an assignment but I don't feel so comfortable with the constructors and the set - methods. The arguments in the constructors and in set methods are fix.
Can anyone please help me here?

Anders

[Campbell Ritchie]

You have created a katalog[] array, but not initialised its members. Each array member defaults to null.

[Rob Spoor]

Anders Kviback wrote:

			temp[i] = filer[i];
			filer = temp;
...
			temp[i] = katalog[i];
			katalog = temp;

When you create temp in both occurrences, it will contain only null values. The first line in both occurrences is not a problem; the second though will overwrite the original array (filer, katalog) with temp. Since temp only has one element set so far, all other elements of filer / katalog are null. When accessing the second element you will get a NullPointerException.

[Rob Spoor]

Campbell Ritchie wrote:You have created a katalog[] array, but not initialised its members. Each array member defaults to null.

Did you miss line 54 which declares and initializes the array?

Anders, you should check out the overridden listFiles method that takes a FileFilter. For getting the files you use an implementation that accepts files if they are actual files (isFile() returns true), for getting the directories you use an implementation that accepts files if they are directories (isDirectory() returns true). That way you don't need to copy anything anymore, the API will do the hard work for you.

[Anders Kviback]

Thank you for answering,

I've changed the code and it now looks like this in the two methods getDirectories() and getFiles().

	public java.io.File[] getFiles(){
		java.io.File[] filer = directory.listFiles();
		if (filer.length == 0)
			return filer;
		java.io.File temp[] = new java.io.File[filer.length];
		for (int i = 0; i < filer.length; i++){
		if (filer[i].isFile()){ 
			temp[i] = filer[i];
		}
		}
		return temp;
	}
	
	public java.io.File[] getDirectories(){
		java.io.File[] katalog = directory.listFiles();
		if (katalog.length == 0)
			return katalog;
		java.io.File temp[] = new java.io.File[katalog.length];
		for (int i = 0; i < katalog.length; i++){
		if (katalog[i].isDirectory()){ 
			temp[i] = katalog[i];
		}
		}
		return temp;
	}

But now I get NPE at method list instead. But I've changed that code as well :

public void list(){
		
		File[] dir = getDirectories();
		File[] file = getFiles();
		
		//long t = directory.lastModified();
		for (int i = 0; i < dir.length; i++){
			System.out.format("%-10s %10s filmapp %10d ",dir[i].getName(), dir[i].lastModified());
			
		}
		for(int i = 0; i < file.length;i++){
			System.out.format("%-10s %10d %2d kB  %10d ",file[i].getName(),file[i].length(),file[i].lastModified());
		}
	}
	

But now I get the NPE at line 8 in the list() method. I e when I try to print it out. Do you understand why?

Anders

[John Jai]

You are creating an array of files (not necessarily to be directories) in the getDirectories() method and assign it to katalog array.

You try to create a new array temp with the same size as the katalog.

Now when you put elements into the temp array, you check if it's a directory. Only if it's a directory, the array elements of temp are assigned a value.

There might not be any directory and hence the File references of temp array points to null and hence the Exception occurs when you try to access temp[i] and call a File method on a null reference.

[Anders Kviback]

You say :

You are creating an array of files (not necessarily to be directories) in the getDirectories() method and assign it to katalog array.

Why don't they turn to directories when I use the method isDirectory()?

So you mean that I should do it this way:

public java.io.File[] getDirectories(){
		int count = 0;
		java.io.File[] temp = directory.listFiles();
		if (temp.length == 0)
			return temp;
		
		for (int i = 0; i < temp.length; i++){
		if (temp[i].isDirectory()){ 
			count++;
		}
		}
		java.io.File katalog[] = new java.io.File[count];
		for(int i = 0; i < katalog.length; i++){
			if (temp[i].isDirectory()){ 
				katalog[i] = temp[i];
				}
		}
		return katalog;
	}

Now, katalog has the correct size or?
But I get an NPE in the list method. Here:

System.out.format("%-10s %10s filmapp %10d ",dir[i].getName(), dir[i].lastModified());

What is wrong here???

Rob Spoor suggested to use an accept method from FileFilter but I don't understand how to do that??

[Rob Spoor]

Let's first focus on your NPE.
Assume that listFiles() returns the following:
- folder A
- folder B
- file C
- folder D
- file E
- file F

Your code will create the following arrays:
filer:
- null
- null
- file C
- null
- file E
- file F

katalogs:
- folder A
- folder B
- null
- folder D
- null
- null

As you can see, each has a null where the other has a file. There are ways to work around that (e.g. use a List<File> or use a separate counter for the destination array, then use Arrays.copyOf to "shrink" the array), but I'd go for the FileFilter solution.

File.listFiles is overloaded to take an instance of a class that implements java.io.FileFilter. This interface has one single method: public boolean accept(File file). When used with File.listFiles, the returned File[] will include only those File objects for which this one method returns true.
So now all you need to do is create an implementation and use that. For directories:

private static class DirectoryFileFilter implements FileFilter
{
    public boolean accept(File file)
    {
        return file.isDirectory();
    }
}

public java.io.File[] getDirectories()
{  
    FileFilter directoryFilter = new DirectoryFileFilter();
    File[] directories = directory.listFiles(directoryFilter);
    return directories;
    // or, in one line:
    //return directory.listFiles(new DirectoryFileFilter());
}

For files it's nearly identical, you just need another FileFilter implementation with a different implementation of accept.

One final warning: if you call listFiles on a File object that does not represent an existing directory, or if that directory cannot be accessed, listFiles will return null. You need to check that before using the array. To make life easier for you, you can integrate that check in your getDirectories() method:

public java.io.File[] getDirectories()
{  
    File[] directories = directory.listFiles(new DirectoryFileFilter());
    return (directories == null ? new File[0] : directories);
}

If listFiles now returns null, you will return an empty array instead.

[Anders Kviback]

Thanks alot for putting your time and effort into my programmingproblem. It took some time for me to answer back but now I've managed to solve the whole assignment. I used the array solution for that part.
All the help that you guys give here on this forum is really helpful. Sometimes I just get stuck and then it is really helpful to ask someone here.

Thanks again, Anders

[Rob Spoor]

You're welcome