File hierarchies

To structure the data you store on a disk, you can arrange files into directories. A parent directory can include other directories or subdirectories, resulting in a file hierarchy. For instance, if you look at the file system hierarchy in Linux, the root directory / includes all other files and directories, regardless of their storage on different physical devices. For simplicity, we're not focusing on null safety in this context.

File Hierarchies

You can organize the data you store on a disk by arranging files into directories. A parent directory can include other directories, also known as subdirectories, resulting in a file hierarchy. For instance, consider the file system hierarchy in Linux: the root directory / includes all other files and directories, even those stored on different physical devices. Please note, for brevity and clarity, we're omitting null safety details.

Files and Directories

A file is a named data section on a storage medium. It's the fundamental unit of data interaction in operating systems.

A directory is an entity in a file system that helps organize files. Given that a typical file system contains a massive number of files, directories assist in organization by grouping them. You can also interact with them as File entities.

Kotlin offers various methods to work with directories and files. For instance, you might use the java.io library. Let's review some of the most popular methods:

• File.isDirectory checks if the File is a directory.

• File.isFile verifies if the File is indeed a file.

• File.exists() checks for the file's existence.

• File.resolve(String) returns a file with the name String in a directory.

• File.resolve(File) returns a file File from a directory.

• File.createNewFile() creates a new file.

• File.mkdir() crafts a new directory.

Here's an example to illustrate:

val outDir = File("outDir")
println(outDir.exists())    // false
outDir.mkdir()
println(outDir.exists())    // true
println(outDir.isDirectory) // true

val file = outDir.resolve("newFile.txt") // outDir/newFile.txt
println(file.exists())      // false
file.createNewFile()        
println(file.exists())      // true
println(file.isFile)        // true

Methods for Iterating Through File Hierarchies

You can navigate through file hierarchy using the java.io.File methods:

• File.getParentFile() returns an instance of java.io.File that represents the parent directory of a file, or null if the file doesn't have a parent (indicating it is the root).

• File.getParent() returns a string representation of the parent directory of a file, or null if the file doesn't have a parent.

• File.listFiles() returns an array of files located in a given directory, or null if the instance is not a directory.

• File.list() returns an array of files and directories in the directory specified by this abstract pathname.

The kotlin.io provides special methods that enable you to go through the entire file hierarchy. Let's examine three fundamental methods:

• File.walk(direction): FileTreeWalk provides the files and directories in this directory that you can visit; you need to specify the way you'll iterate (up or down the hierarchy structure).

• File.walkBottomUp(): FileTreeWalk offers the directories and files in this directory that you can visit. It employs a depth-first search, visiting directories after all their files have been visited.

• File.walkTopDown(): FileTreeWalk offers the directories and files in this directory for you to visit. A depth-first search is used, and the directories are visited before all their files.

The FileTreeWalk class is designed to iterate through a given file system. It lets you traverse all the files within a directory. The iterator method returns an iterator that traverses the files. You can iterate over this structure or convert it to a list with the toList() function.

In the following section, we'll showcase these methods using a simple file hierarchy example.

A Hierarchy Example

Consider a file hierarchy with the root directory named Files. It contains two subdirectories: CompletedProjects and Music. They also contain subdirectories: the HelloWorld directory houses two project-related files, the JCalculator directory contains just one file, and the Soundtracks directory is empty.

Here's a diagram representing this hierarchy:

In the following segments, we'll employ this file hierarchy as an example to illustrate how to maneuver the file system.

Path

In Kotlin Path converts the name sequence specified with the base path string and a number of subpaths additional names to a Path object of the default filesystem to perform basic operations on files and directories easily. We can use:

• Path.listDirectoryEntries(): we can obtain the list of files in a given directory.

• Path.copyTo(): copy a source to a destination.

• Path.moveTo(): move a file to a new destination.

• Path.writeText(): create a new text file.

• Path.readText(): read the full file as string.

• Path.readLines(): read a full file as a collection of lines.

• Path.walk(): for visiting directories recursively in an efficient manner.

• Path.copyToRecursively(): for copying a whole directory and its content to another path.

fun main() {
    val sourcePath = Path("sourceDir/file.txt")
    val destinationPath = Path("destDir/file.txt")

    // List all files in the 'sourceDir' directory
    val filesInDir = Path("sourceDir").listDirectoryEntries()
    println("Files in sourceDir: $filesInDir")

    // Copy the file from 'sourceDir' to 'destDir'
    sourcePath.copyTo(destinationPath, overwrite = true)
    println("File copied from sourceDir to destDir.")

    // Move the file within 'destDir' to a new file
    val movedPath = destinationPath.moveTo(Paths.get("destDir/movedFile.txt"), overwrite = true)
    println("File moved to: $movedPath")

    // Write text to the moved file
    movedPath.writeText("Hello, world!")
    println("Text written to file.")

    // Read the text from the file
    val text = movedPath.readText()
    println("Text read from file: $text")

    // Read the lines from the file as a collection
    val lines = movedPath.readLines()
    println("Lines read from file: $lines")

    // Walk all files and directories within 'destDir'
    val walkedPaths = Path("destDir").walk().toList()
    println("Walked paths: $walkedPaths")

    // Copy 'sourceDir' to 'copyDir' recursively
    Path("sourceDir").copyToRecursively(
        target = Paths.get("copyDir"),
        overwrite = true,
        followLinks = true,
    )
    println("Directory copied recursively.")
}

Files in sourceDir: 
File copied from sourceDir to destDir.
File moved to: destDir/movedFile.txt
Text written to file.
Text read from file: Hello, world!
Lines read from file: [Hello, world!]
Walked paths: [destDir, destDir/movedFile.txt]
Directory copied recursively.

Conclusion

Proficiency in handling file hierarchies is crucial for Kotlin developers. It boosts your ability to organize, access, and modify data productively, enhancing the overall efficiency and functionality of Kotlin-based applications. Remember, consistent practice and application of these concepts will lead to mastery in managing file systems in Kotlin.