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

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

Answer 2

maybeChild.getCanonicalPath().startsWith( possibleParent.getCanonicalPath() );

Answer 3

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.

Answer 4

That will probably work fine as it is, although I would use getCanonicalPath() rather than getAbsolutePath(). This should normalize any weird paths like x/../y/z which would otherwise screw up the matching.

Answer 5

Old question but a pre-1.7 solution:

public boolean startsWith(String possibleRoot, String possibleChildOrSame) {
        String[] possiblePath = new File(possibleRoot).getAbsolutePath().replace('\\', '/').split("/");
        String[] possibleChildOrSamePath = new File(possibleChildOrSame).getAbsolutePath().replace('\\', '/').split("/");

        if (possibleChildOrSamePath.length < possiblePath.length) {
            return false;
        }

        // not ignoring case
        for (int i = 0; i < possiblePath.length; i++) {
            if (!possiblePath[i].equals(possibleChildOrSamePath[i])) {
                return false;
            }
        }
        return true;
}

For completeness the java 1.7+ solution:

public boolean startsWith(String possibleRoot, String possibleChildOrSame) {
        Path p1 = Paths.get(possibleChildOrSame).toAbsolutePath();
        Path p2 = Paths.get(possibleRoot).toAbsolutePath();
        return p1.startsWith(p2);
}

Answer 6

Surprisingly there is no simple, yet functional solution.

The accepted answer does consider same directories as child, which is wrong.

Here is one using java.nio.file.Path API only:

static boolean isChildPath(Path parent, Path child){
      Path pn = parent.normalize();
      Path cn = child.normalize();
      return cn.getNameCount() > pn.getNameCount() && cn.startsWith(pn);
}

Test cases:

 @Test
public void testChildPath() {
      assertThat(isChildPath(Paths.get("/FolderA/FolderB/F"), Paths.get("/FolderA/FolderB/F"))).isFalse();
      assertThat(isChildPath(Paths.get("/FolderA/FolderB/F"), Paths.get("/FolderA/FolderB/F/A"))).isTrue();
      assertThat(isChildPath(Paths.get("/FolderA/FolderB/F"), Paths.get("/FolderA/FolderB/F/A.txt"))).isTrue();

      assertThat(isChildPath(Paths.get("/FolderA/FolderB/F"), Paths.get("/FolderA/FolderB/F/../A"))).isFalse();
      assertThat(isChildPath(Paths.get("/FolderA/FolderB/F"), Paths.get("/FolderA/FolderB/FA"))).isFalse();

      assertThat(isChildPath(Paths.get("FolderA"), Paths.get("FolderA"))).isFalse();
      assertThat(isChildPath(Paths.get("FolderA"), Paths.get("FolderA/B"))).isTrue();
      assertThat(isChildPath(Paths.get("FolderA"), Paths.get("FolderA/B"))).isTrue();
      assertThat(isChildPath(Paths.get("FolderA"), Paths.get("FolderAB"))).isFalse();
      assertThat(isChildPath(Paths.get("/FolderA/FolderB/F"), Paths.get("/FolderA/FolderB/F/Z/X/../A"))).isTrue();
}