Working with files in Java

1. Files class

To work with files, there is a slick utility class — java.nio.file.Files. It has methods for every occasion. All the methods of this class are static and operate on Path object. There are a lot of methods, so we will consider only the most important ones:

Path createFile(Path path)

Path createDirectory(Path path)

Path createDirectories(Path path)

Path createTempFile(prefix, suffix)

Path createTempDirectory(prefix)

void delete(Path path)

Path copy(Path src, Path dest)

Path move(Path src, Path dest)

boolean isDirectory(Path path)

boolean isRegularFile(Path path)

boolean exists(Path path)

long size(Path path)

byte[] readAllBytes(Path path)

String readString(Path path)

List<String> readAllLines(Path path)

Path write(Path path, byte[])

Path writeString(Path path, String str)

DirectoryStream<Path> newDirectoryStream(Path dir)

2. Creating files and directories

Files and directories are very easy to create. Let's convince ourselves with some examples:

Files.createFile(Path.of("c:\\readme.txt"));

Files.createDirectory(Path.of("c:\\test"));

Files.createDirectories(Path.of("c:\\test\\1\\2\\3"));

3. Copying, moving and deleting

Copying, moving and deleting files is just as easy. This also applies to directories, but they must be empty.

Path path1 = Path.of("c:\\readme.txt");
Path path2 = Path.of("c:\\readme-copy.txt");
Files.copy(path1, path2);

Path path1 = Path.of("c:\\readme.txt");
Path path2 = Path.of("d:\\readme-new.txt");
Files.move(path1, path2);

Path path = Path.of("d:\\readme-new.txt");
Files.delete(path);

4. Checking a file's type and existence

When you have a path provided by someone else, you want to know whether it is a file or a directory. And in general, does such a file/directory exist or not?

There are special methods for this as well. You can also easily find out the length of a file:

Files.isRegularFile(Path.of("c:\\readme.txt"));

true

Files.isDirectory(Path.of("c:\\test"));

true

 Files.exists(Path.of("c:\\test\\1\\2\\3"));

false

Files.size(Path.of("c:\\readme.txt"));

10112

5. Working with file contents

Finally, there are a whole series of methods that make it easy to read or write the contents of a file. Example:

Path path = Path.of("c:\\readme.txt");
List<String> list = Files.readAllLines(path);

for (String str : list)
   System.out.println(str);

6. Getting the contents of a directory

The most interesting method still remains. It is used to get files and subdirectories in a given directory.

We are talking about the newDirectoryStream() method, which returns a special DirectoryStream<Path> object. It has an iterator(!) that you can use to get all the files and subdirectories of a given directory.

This is easier than it sounds.

Path path = Path.of("c:\\windows");

try (DirectoryStream<Path> files = Files.newDirectoryStream(path)) {
   for (Path path : files)
      System.out.println(path);
}

The DirectoryStream<Path> object has two properties. First, it has an iterator that returns file paths, and we can use this object inside a for-each loop.

And second, this object is a data stream, so it must be explicitly closed using the close() method, or declared use inside a try-with-resources block.

7. Files.newInputStream method

Starting with Java 5, the FileInputStream and FileOutputStream classes have been deprecated. One of their disadvantages was that when objects of these classes are created, files are immediately created on disk. And all exceptions related to file creation could potentially be thrown.

Later this was recognized as not the best decision. Accordingly, it is recommended to use the methods of the java.nio.Files utility class to create file objects.

Here's a comparison between the old approach and the new approach to creating files:

String src = "c:\\projects\\log.txt";
InputStream input = new FileInputStream(src);

String src = "c:\\projects\\log.txt";
InputStream input = Files.newInputStream( Path.of(src) );

And there is a similar replacement for FileOutputStream:

String src = "c:\\projects\\log.txt";
OutputStream  output = new FileOutputStream( src );

String src = "c:\\projects\\log.txt";
OutputStream  output = Files.newOutputStream( Path.of( src ) );