FileVisitor

FileVisitor is an interface in Java's NIO.2 API, introduced in Java 7, that defines a visitor pattern for traversing a file tree. It allows for custom actions to be performed when encountering different types of file system entries, such as files, directories, and symbolic links. The interface consists of several methods: `preVisitDirectory`, `visitFile`, `visitFileFailed`, and `postVisitDirectory`. Each of these methods is called by the `Files.walkFileTree` method at appropriate stages of the traversal.

The `preVisitDirectory` method is invoked before visiting the contents of a directory. The `visitFile` method is called when a regular file is encountered. The `visitFileFailed` method is invoked if an I/O error occurs while accessing a file or directory. Finally, `postVisitDirectory` is called after all entries within a directory have been visited.

Implementations of FileVisitor can be used for a variety of tasks, including searching for specific files, calculating directory sizes, copying entire directory structures, or performing batch operations on files. To use FileVisitor, one typically creates a concrete class that implements the interface and then passes an instance of this class to the `Files.walkFileTree` method, along with the starting path for the traversal. The `Files.walkFileTree` method then recursively visits each entry in the file tree and invokes the corresponding methods in the provided FileVisitor implementation.