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Reputation: 553

How to return Array Values as a output to a Method in Java

I am new to Java.I am working on my previous post link.

I want to return specific extension(like .txt) file names under directory.

For this,I wrote the following method with 2 arguments.

  public void ListOfFileNames(String directoryPath,String fileType)
  {
    //Creating Object for File class
File fileObject=new File(directoryPath);
//Fetching all the FileNames under given Path
File[] listOfFiles=fileObject.listFiles();
//Creating another Array for saving fileNames, which are satisfying as far our requirments
String[] fileNames;
for (int fileIndex = 0; fileIndex < listOfFiles.length; fileIndex++) 
{
    if (listOfFiles[fileIndex].isFile())
    {
        //True condition,Array Index value is File
        if (listOfFiles[fileIndex].getName().endsWith(fileType)) 
        {
            //System.out.println(listOfFiles[fileIndex].getName());
        }
    }
}

}

Here I am facing 2 problems.

  1. function return type.

  2. Adding FileName into fileNames Array.

How can I fix this.

Thanks.

Upvotes: 2

Views: 3757

Answers (8)

Prabhaker A
Prabhaker A

Reputation: 8473

It is good to use List instead of array
Change your code like this 

public void FileMoving()   
{  
   //created object for Class   
   ExternalFileExecutions ExternalFileExecutionsObject=new ExternalFileExecutions();  
   //calling Method from class object
   List<String> fileNames = ExternalFileExecutionsObject.ListOfFileNames("C:/Documents and Settings/mahesh/Desktop/InputFiles",".txt");    
   //to find out the length of list
   System.out.println(fileNames.size());  
}
public List<String> ListOfFileNames(String directoryPath,String fileType)
{
    //Creating Object for File class
    File fileObject=new File(directoryPath);
    //Fetching all the FileNames under given Path
    File[] listOfFiles=fileObject.listFiles();
    //Creating another Array for saving fileNames, which are satisfying as far our requirments
    List<String> fileNames = new ArrayList<String>();
    for (int fileIndex = 0; fileIndex < listOfFiles.length; fileIndex++) 
    {
        if (listOfFiles[fileIndex].isFile())
        {
          //True condition,Array Index value is File
          if (listOfFiles[fileIndex].getName().endsWith(fileType)) 
          {
              //System.out.println(listOfFiles[fileIndex].getName());
              fileNames .add(listOfFiles[fileIndex].getName());
          }
       }
     }  
     return fileNames ;

  }

Upvotes: 1

Ortwin Angermeier
Ortwin Angermeier

Reputation: 6183

You could use something like the following:

    public List<String> ListOfFileNames1(final String directoryPath,
            final String fileType) {
        List<String> result = new ArrayList<String>();
        // Creating Object for File class
        File root = new File(directoryPath);
        for (File f : root.listFiles()) {
            if (f.isFile() && f.getName().endsWith(fileType)) {
                result.add(f.getName());
            } // if
        } // for
        return result;
    } // ListOfFileNames1

    public List<File> ListOfFileNames2(final String directoryPath,
            final String fileType) {
        // Creating Object for File class
        File root = new File(directoryPath);
        return Arrays.asList(root.listFiles(new FileFilter() {

            @Override
            public boolean accept(File pathname) {
                return pathname.isFile()
                        && pathname.getName().endsWith(fileType);
            }
        }));
    } // ListOfFileNames2

Upvotes: 0

zapl
zapl

Reputation: 63955

I would usually prefer to return a File instead of a String. That way you don't have to create a new file each time you want to work with it and you can access for example the file name (.getName() > foo.txt) or the whole path (getPath() > /dir/foo.txt) depending on what you need.

File already has a built-in filter functionality, either working on the file as String (FileNameFilter) or on the File itself (FileFilter). As above, I would prefer working on the File.

Implementing a FileFilter is pretty simple.

private static class ExtensionFilter implements FileFilter {
    private final String extension;

    public ExtensionFilter(String extension) {
        // store ".TXT"
        this.extension = "." + extension.toUpperCase(Locale.ROOT);
    }

    @Override
    public boolean accept(File pathname) {
        // return true if it is a file and it ends with .TXT
        return pathname.isFile() && pathname.getName().toUpperCase(Locale.ROOT).endsWith(extension);
    }
}

Now add some methods that use such a filter and you are pretty much done.

// internal method that lists files, converts them into a List and makes sure it does not return null
private static List<File> listFilesWithExtension(File directory, ExtensionFilter filter) {
    File[] files = directory.listFiles(filter);
    return files != null ? Arrays.asList(files) : Collections.<File>emptyList();
}

/** use like listFilesWithExtension(directory, "txt") */    
public static List<File> listFilesWithExtension(File directory, String extension) {
    return listFilesWithExtension(directory, new ExtensionFilter(extension));
}

/** lists only .txt files */
private static final ExtensionFilter TXT_FILES = new ExtensionFilter("txt");
public static List<File> listTxtFiles(File directory) {
    return listFilesWithExtension(directory, TXT_FILES);
}

If you actually want a list / array of Strings you can convert it later via methods like

public static String[] getFileNamesFromFiles(List<File> files) {
    String[] result = new String[files.size()];
    for (int i = 0; i < files.size(); i++) {
        File file = files.get(i);
        result[i] = file.getName();
    }
    return result;
}

Upvotes: 0

JB Nizet
JB Nizet

Reputation: 691685

You want to return several String from your method. So it should return a String[] or a List<String>, for example.

You also want to respect the Java naming conventions, so the method should be declared as:

public String[] listOfFileNames(String directoryPath, String fileType)

If you read the javadoc of java.io.File, you'll notice that there is a method which does exactly that. You just need to pass a FilenameFilter, which tells the methof which file names should be returned:

public String[] listOfFileNames(String directoryPath, final String fileType) {
    return new File(directoryPath).list(new FilenameFilter() {
        @Override
        public boolean accept(File dir, String name) {
            return name.endsWith(fileType);
        } 
    });
}

This code above uses an anonymous inner class, which implements the FilenameFilter interface. Read the Java tutorial to learn about them.

Upvotes: 0

Xavier DSouza
Xavier DSouza

Reputation: 2931

public List<String> ListOfFileNames(String directoryPath, String fileType) {
    //Creating Object for File class
    File fileObject = new File(directoryPath);

    //Fetching all the FileNames under given Path
    File[] listOfFiles = fileObject.listFiles();

    //Creating another Array for saving fileNames, which are satisfying as far our requirments      
    List<String> fileNames = new ArrayList<String>();
    for (int fileIndex = 0; fileIndex < listOfFiles.length; fileIndex++) {
        if (listOfFiles[fileIndex].isFile()) {
            //True condition,Array Index value is File
            if (listOfFiles[fileIndex].getName().endsWith(fileType)) {
                //System.out.println(listOfFiles[fileIndex].getName());
                fileNames.add(listOfFiles[fileIndex].getName());
            }
        }
    }
    return fileNames;
}

Upvotes: 0

Mark Nenadov
Mark Nenadov

Reputation: 6917

For return type change your first line to

public String[] ListOfFileNames(String directoryPath,String fileType)

As for adding an item in array, you might want to consider using ArrayList instead. Like so:

List<String> stringList = new ArrayList<String>();
stringList.add("string")

Upvotes: 0

AurA
AurA

Reputation: 12363

The return type should be String[] or List<String> you need to put every file name you encounter that fulfils your criteria into the list or array and return the same in the end.

In this part of the code

If you are using Arrays 

String[] fileNames;
int count = 0;

if (listOfFiles[fileIndex].getName().endsWith(fileType)) 
{
     fileNames[count] = listOfFiles[fileIndex].getName();
     count++;
}

return fileNames;

The rest of the code remains same....

In case you want to do it Using List

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

if (listOfFiles[fileIndex].getName().endsWith(fileType)) 
{
     fileNames.add(listOfFiles[fileIndex].getName());
}

return fileNames;

Upvotes: 1

duffymo
duffymo

Reputation: 308753

Try this: 

public List<String> getListOfFileNames(String directoryPath, String fileType) {
    List<String> result = new ArrayList<String>();
    // add the code to loop over the director list and add them to the result
    return result;
}

An even better solution would be to use the list() method that's build into the File class. Pass it a FilenameFilter.

http://docs.oracle.com/javase/7/docs/api/

The best thing to do is reuse code rather than write it again.

Upvotes: 0

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