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

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".

File parent = maybeChild.getParentFile();
while ( parent != null ) {
  if ( parent.equals( possibleParent ) )
    return true;
  parent = parent.getParentFile();
}
return false;

Asides from the fact the paths may not exist (and the canonicalisation may not succeed), this looks like a reasonable approach that should work in the straightforward case.

You may want to look at calling getParentFile() on the "maybe child" in a loop, testing if it matches the parent at each step. You can also short-circuit the comparison if the parent isn't a (real) directory.

Perhaps something like the following:

boolean myCheck(File maybeChild, File possibleParent) throws IOException
{
    final File parent = possibleParent.getCanonicalFile();
    if (!parent.exists() || !parent.isDirectory()) {
        // this cannot possibly be the parent
        return false;
    }

    File child = maybeChild.getCanonicalFile();
    while (child != null) {
        if (child.equals(parent)) {
            return true;
        }
        child = child.getParentFile();
    }
    // No match found, and we've hit the root directory
    return false;
}

Note that if you want the child relationship to be strict (i.e. a directory is not a child of itself) you can change the initial child assignment on line 9 to be child.getParentFile() so the first check happens on the child's containing directory.