How to check if a given path is possible child of another path?

Question

I am trying to find if given path is possible child of another path using java. Both path may not exist.

Say c:\\Program Files\\My Company\\test\\My App

Answer 1

Be aware of relative paths! I think that simplest solution is something like this:

public boolean myCheck(File maybeChild, File possibleParent) {
  if (requestedFile.isAbsolute) {
    return possibleParent.resolve(maybeChild).normalize().toAbsolutePath.startsWith(possibleParent.normalize().toAbsolutePath)
  } else {
    return maybeChild.normalize().toAbsolutePath.startsWith(possibleParent.normalize().toAbsolutePath)
  }
}

In scala you can have similar approach:

val baseDir = Paths.get("/home/luvar/tmp")
val baseDirF = baseDir.toFile
//val requestedFile = Paths.get("file1")
val requestedFile = Paths.get("../.viminfo")
val fileToBeRead = if (requestedFile.isAbsolute) {
  requestedFile
} else {
  baseDir.resolve(requestedFile)
}
fileToBeRead.toAbsolutePath
baseDir.toAbsolutePath
fileToBeRead.normalize()
baseDir.normalize()
val isSubpath = fileToBeRead.normalize().toAbsolutePath.startsWith(baseDir.normalize().toAbsolutePath)

Answer 2

This will work for your example. It will also return true if the child is a relative path (which is often desirable.)

boolean myCheck(File maybeChild, File possibleParent)
{
    URI parentURI = possibleParent.toURI();
    URI childURI = maybeChild.toURI();
    return !parentURI.relativize(childURI).isAbsolute();
}

Answer 3

You can also use java.nio.file.Path to do this much more easily. The java.nio.file.Path.startsWith method seems to handle all possible cases.

Example:

private static void isChild(Path child, String parentText) {
    Path parent = Paths.get(parentText).toAbsolutePath();
    System.out.println(parentText + " = " + child.startsWith(parent));
}

public static void main(String[] args) {
    Path child = Paths.get("/FolderA/FolderB/File").toAbsolutePath();
    isChild(child, "/FolderA/FolderB/File");
    isChild(child, "/FolderA/FolderB/F");
    isChild(child, "/FolderA/FolderB");
    isChild(child, "/FolderA/Folder");
    isChild(child, "/FolderA");
    isChild(child, "/Folder");
    isChild(child, "/");
    isChild(child, "");
}

outputs

/FolderA/FolderB/File = true
/FolderA/FolderB/F = false
/FolderA/FolderB = true
/FolderA/Folder = false
/FolderA = true
/Folder = false
/ = true
 = false

If you need more reliability you can use "toRealPath" instead of "toAbsolutePath".