Using File.Listfiles with Filenameextensionfilter

Can I use FileNameExtensionFilter to filter out a list of files?

If by practical you mean writeable in 5 lines in java yes it is. There is no cleaner alternatives when you want to filter a list (as opposed to languages with functional constructs like Scala and its filter method).

I personaly do not see how you could do it better, if your use case is really to filter out files with bad extensions I would do the same in Java.

If you are worried to use interfaces and classes from javax.swing you should not, both these classes depend only on java.io or java.util.Locale (for setting file name to lowercase according to english locale), so they are as clean as the FilenameFilter from java.io. If your deal is really to filter according to case insensitive extension you should definitely go this way, the only alternative my eclipse sees is com.google.gwt.thirdparty.guava.common.io.PatternFilenameFilter implementing java.io.FilenameFilter, but then you have a new dependency and you write a Regex pattern which is far less readable and maintainable than your extensions. So I would stay with FileNameExtensionFilter if this is really your use case.

FilenameFilter to show list files with certain extension and directories

You have many ways of doing this. In particular, a java.io.FileFilter gives you more flexibility than a FilenameFilter, allowing you to test for directories using isDirectory() :

File[] listOfFiles = folder.listFiles(new FileFilter() {
            @Override
            public boolean accept(File pathname) {
                return pathname.getName().toLowerCase().endsWith(".txt") 
                    || pathname.isDirectory();
            }
        });

Is there any better way to do filefilter for many ext?

With Java 6 or above, this is a perfect case for a FileNameExtensionFilter... except that it extends javax.swing.filechooser.FileFilter instead of implementing java.io.FileFilter.

But it is trivial to write a wrapper for it:

File[] files = rootDir.listFiles(new FileFilter() {
    private final FileNameExtensionFilter filter =
        new FileNameExtensionFilter("Compressed files",
            "zip", "jar", "z", "gz", "tar", "bz2", "bz");
    public boolean accept(File file) {
        return filter.accept(file);
    }
});

File. ListFiles with FileNameFilter returning null instead of list of files with matching file name

Since you are using recursion, you should pass the list (of files) as a parameter to method find and not create a new list on each invocation. Hence the method find does not need to return a value.

public static void find(String path, String fName, List<File> fileList) {
    File dir = new File(path);
    if (dir.isDirectory()) {
        for (File aChild : dir.listFiles()) {
            if (aChild.isDirectory()) {
                find(aChild.getAbsolutePath(), fName, fileList);
            }
            else {
                String name = aChild.getName();
                if (name.startsWith(fName)  &&  name.endsWith(".txt")) {
                    fileList.add(aChild);
                }
            }
        }
    }
    else {
        String name = aChild.getName();
        if (name.startsWith(fName)  &&  name.endsWith(".txt")) {
            fileList.add(aChild);
        }
    }
}

If method parameter path indicates a directory, then list the files in that directory. For each file in the directory, check whether the name of the file matches your search criteria and if it does then add it to the list. If it doesn't then check if it is itself a directory and if it is then recursively call method find with the new directory.

Initially call method find like so

List<File> list = new ArrayList<File>();
find("C:\\Salary\\", "601246", list);

Now list should contain the list of relevant files. So the following line will print the contents of list

System.out.println(list);

How to get a file with exact extension from ArrayList of files

You can use a FilenameFilter to filter the files. The name of the file is passed a a String, to prevent the creation of large amounts of File objects when listing the files.

This class exists since JDK 1.0.

Single extension filter

Java 7 and lower example:

List<File> files = new ArrayList<File>(Arrays.asList(f.listFiles(
    new FilenameFilter() {
        @Override 
        public boolean accept(File dir, String name) {
            return name.endsWith(".ext");
        }
    }
)));

Java 8 example:

List<File> files = new ArrayList<File>(Arrays.asList(f.listFiles(
    (File dir, String name) ->  name.endsWith(".ext")
)));

Multiple extension filter

When filtering multiple extensions, it will become harder. We first store the allowed extensions in a Set<>, then use its contains() method to compare every entry.

Set<String> allowed = new HashSet<>();
allowed.add("txt");
allowed.add("exe");
allowed.add("sh");

Then we split on dot in the file filter and compare the last part:

List<File> files = new ArrayList<File>(Arrays.asList(f.listFiles(
    new FilenameFilter() {
        @Override 
        public boolean accept(File dir, String name) {
            String[] split = name.split("\.");
            if(split.length == 1)
                return allowed.contains("");
            return allowed.contains(split[split.length - 1]);
        }
    }
)));

Method references of listFiles

There are actually several interesting parts to this question:

• Q: What is isHidden, and why is it a permissible argument to File.listFiles()?

  https://www.geeksforgeeks.org/file-ishidden-method-in-java-with-examples

  The isHidden() function is a part of File class in Java . This
  function determines whether the is a file or Directory denoted by the
  abstract filename is Hidden or not.The function returns true if the
  abstract file path is Hidden else return false.

• Q: What is the File::isHidden syntax?

  The "double colon" is a method reference. It's new with Java 8 and higher.

• Q: So why is isHidden() an acceptable FileFilter parameter?

  https://docs.oracle.com/javase/8/docs/api/java/io/FileFilter.html

  This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

  File::isHidden is a lambda expression that returns "true" or "false".

listFiles(FileFilter) instead of listFiles(FileNameFilter)

Looks like you've imported

javax.swing.filechooser.FileFilter

instead of

java.io.FileFilter

Probably occurred when selecting the import from the IDE

If the correct import is used, the code should not compile due to the existence of the getDescription method which is only found in the former. The description is used for display purposes on JFileChooser dialogs and doesnt apply here.

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