您如何在Java中以递归方式列出与扩展名数组不匹配的文件夹中的所有文件？

Question

I have a simple program that I am making that looks through a directory recursively and removes all files that do not match the files extensions chosen by the user. (essentially I am remaking a script that I wrote in PHP to help teach myself java)

The method works fine except that it returns an array of files that match the array of extensions. This would remove the wrong files. How can I return the files that do not match?

I am using org.apache.commons.io.FileUtils with a string array to get the files:

import java.io.File;
import java.util.List;
import java.io.IOException;
import org.apache.commons.io.FileUtils;

public class Recurse {
// The method that takes a directory path (file object called folder) and a string of extensions
    public static List<File> getFiles(final File folder, String[] extensions) throws IOException{

    List<File> fileList = (List<File>) FileUtils.listFiles(folder, extensions, true);

    return fileList;
    }
}

I have found examples that use just one file type eg ".txt" but cannot find one for an array. I would prefer to stick with org.apache.commons.io.FileUtils but I am willing to change my method.

Answer 1

Use File.listFiles(), and use a java.io.FileFilter. You write the filter to examine the filename and determine whether that file 'passes through' the filter. Instantiate your filter with the array of extensions you don't want or whatever. You aren't using FileUtils, but you don't need anything outside of standard Java libraries.

--

Edit after request for example

public class NotTheseExtensions implements java.io.FileFilter
{
  private ArrayList<String> extensions;

  public NotTheseExtensions(ArrayList<String> extensions)
  {
    this.extensions = extensions;
  }

  public boolean accept(String filename)
  {
    boolean result = true;
    boolean inList = false;  // set this in your own logic

    // logic to get the extension from the filename and look to see if it is
    // in your list -- hopefully I don't have to write that for you as well.
    if (inList) { result = false; }

    return result;
  }
}

Now, look at the (various) signatures for listFiles in java.io.File. One of them takes such a file filter. Only things that pass your file filter, ie, return true in the accept() method above, will be returned by listFiles() when given this filter.

Answer 2

I agree with rcook's answer, but if you REALLY want to stick with FileUtils , here's an alternative: get all the files, then remove the ones that match the extensions.

List<File> all = (List<File>) FileUtils.listFiles(folder, null, true);
List<File> unwanted = (List<File>) FileUtils.listFiles(folder, extensions, true);
all.removeAll(unwanted);
return all;

Note this is a bit wasteful, involving two recursions through the filesystem.

Answer 3

You can get all the files in the directory and then just remove the files that you get for extensions that you dont want.

I use this to get files with particular extension:

File dir = new File(fileDir);
FileFilter fileFilter = new WildcardFileFilter("*.txt");
File[] files = dir.listFiles(fileFilter);

Answer 4

Here's the pure java alternative to using FileUtils , avoiding the efficiency problem of my other answer and supporting recursion. This involves so much code I think I would favour the FileUtils solution.

public void myMethod() {
    List<File> files = recursiveList(folder, new String[] { "jpg", "png" });
    // ... do something ...
}

private List<File> recursiveList(File folder, String[] rejectedExtensions) {
    List<File> files = new ArrayList<File>();
    for (File subfolder : folder.listFiles(new DirectoryFilter())) {
        files.addAll(recursiveList(subfolder, rejectedExtensions));
    }
    files.addAll(Arrays.asList(folder.listFiles(new ExtensionFilter(rejectedExtensions))));
    return files;
}

private class DirectoryFilter implements FileFilter {
    @Override
    public boolean accept(File file) {
        return file.isDirectory();
    }
}

private class ExtensionFilter implements FileFilter {

    private final String[] rejectedExtensions;

    public ExtensionFilter(String[] rejectedExtensions) {
        this.rejectedExtensions = rejectedExtensions;
    }

    @Override
    public boolean accept(File file) {
        if (file.isDirectory()) {
            return false;
        }
        String name = file.getName();
        for (String rejectedExtension : rejectedExtensions) {
            if (name.endsWith(rejectedExtension)) {
                return false;
            }
        }
        return true;
    }

}