Develop Reference

On Windows, file starting with / is not considered absolute

This surprised me, but apparently, a path on Windows that starts with a slash is not considered absolute. You need to specify c:. But surely, there is a difference between /foo and foo
System.out.println(new File(new File("/folder"), "abc"));
System.out.println(new File(new File("/folder"), "abc").isAbsolute());
\folder\abc
false
Is this correct behavior?
I want to be able to identify a relative path, such as foo, so I can prepend a default directory. But I would consider /foo to be an absolute path and would not prepend the default

You don't need to check for "isAbsolute" by string comparisons, just use Path and resolve using defaultDir.resolve(path)
Consider:
var rel = Path.of("relative");
var abs = Path.of("/absolute");
for (Path dir : List.of(Path.of("."), Path.of("abc"),Path.of("/abc"))) {
System.out.println("dir="+dir);
System.out.println("dir.resolve("+abs+")="+dir.resolve(abs));
System.out.println("dir.resolve("+rel+")="+dir.resolve(rel));
}
This handles resolving from the default directory path using another path which may be relative or absolute (in sense that it starts with path separator) on Windows or Linux.
Example run from Linux (Windows is same answer but with Windows path separator character).
dir=.
dir.resolve(/absolute)=/absolute
dir.resolve(relative)=./relative
dir=abc
dir.resolve(/absolute)=/absolute
dir.resolve(relative)=abc/relative
dir=/abc
dir.resolve(/absolute)=/absolute
dir.resolve(relative)=/abc/relative

You can check if the start of your file path begins with a "/" using Boolean isPathAbsolute = "/foo".startsWith("/") and use this variable to decide if you prepend the default or not!

Getting an element inside a directory using java.nio.file.Path's

I am currently getting to grips with file management in Java. As far as i've read, java.nio.file.Path is the preferred way of doing so.
Say I want to copy the contents of oldDir to the, currently empty, newDir. Every time I copy a file, I need this loong line just to get the Path of newFile:
Path newDir = FileSystems.getDefault().getPath("new");
Path oldDir = FileSystems.getDefault().getPath("old");
for (Path oldFile : oldDir) {
Path newFile = FileSystems.getDefault().getPath("new", oldFile.getFileName().toString()); // Why so complicated? :(
Files.copy(oldFile, newFile);
}
Is there something like newDir.getChild(oldFile.getFileName()) to do what I want, or is there really no shorter way of doing it?

There are a couple things you can do to make the code simpler:
Use Path#of(String,String...) or Paths#get(String,String...) to create your Path instances. Both methods delegate to the default FileSystem. The former was added in Java 11 and is now the preferred approach.
Use Path#resolve(Path) to append a relative path to some absolute path.
But there's also an error in your code. You are iterating over oldDir which works because Path implements Iterable<Path>. However, that iterates over the names of the path, not the children of the path. In other words, this:
Path path = Path.of("foo", "bar", "file.txt");
for (Path name : path) {
System.out.println(name);
}
Will output:
foo
bar
file.txt
If you want to iterate over the children of a directory you need to use something like Files#list(Path) or Files#newDirectoryStream(Path) (the latter has two overloads). Both those methods only return the immediate children of the directory. There are other methods available to recursively iterate a directory and its sub-directories; browse the Files documentation to see what's provided.
So your code should instead look something like:
Path oldDir = Path.of(...);
Path newDir = Path.of(...);
try (DirectoryStream<Path> dirStream = Files.newDirectoryStream(oldDir)) {
for (Path oldFile : dirStream) {
Path newFile = newDir.resolve(oldFile.getFileName());
Files.copy(oldFile, newFile);
}
}

Is there a cleaner way to represent a parent path from a filesystem URL?

I have an source path which I receive as a filesystem URL from which I want to resolve to an absolute path two parents above; the final type as a Path object is desired. To do this I have to convert it to a URI, then to a Path, and then call getParent() on it twice.
Is there a cleaner way to perform this such that I can perform some transformation on either the URL, URI, or Path with a relative string ../..? I've always found that multiple invocations of getParent() to transverse a path to be less intuitive to read at a glance than to provide a relative path with ../, etc.
// Example file system path as a URL
URL u = new URL("file:///a/b/c");
// Must convert to URI and then call 'getParent()' twice
Path p = Paths.get(u.toURI()).getParent().getParent();

If you do not need the URL variable anywhere else, then you can skip the conversion like so:
p = Paths.get(new URI("file:///a/b/c")).getParent().getParent();
Now, this next part could be seen as more complicated than it needs to be, but if you just need the folder name, you could do a .getName(0) instead of the two .getParent()'s. However, this does not include the root when you print it out, and it might not meet your needs.
I hope this helps.

File.mkdir is not working and I can't understand why

I've this brief snippet:
String target = baseFolder.toString() + entryName;
target = target.substring(0, target.length() - 1);
File targetdir = new File(target);
if (!targetdir.mkdirs()) {
throw new Exception("Errore nell'estrazione del file zip");
}
doesn't mattere if I leave the last char (that is usually a slash). It's done this way to work on both unix and windows. The path is actually obtained from the URI of the base folder. As you can see from baseFolder.toString() (baseFolder is of type URI and is correct). The base folder actually exists. I can't debug this because all I get is true or false from mkdir, no other explanations.The weird thing is that baseFolder is created as well with mkdir and in that case it works.
Now I'm under windows.
the value of target just before the creation of targetdir is "file:/C:/Users/dario/jCommesse/jCommesseDB"
if I cut and paste it (without the last entry) in windows explore it works...

The path you provide is not a file path, but a URI.
I suggest you try the following :
URI uri = new URI("file://c:/foo/bar");
File f = new File(uri).

It looks, to me, as if the "file:/" at the beginning is the problem... Try getAbsolutePath() instead of toString().

The File constructor taking a String expects a path name. A path name is not an URI.
Remove the file:/ from the front of the String (or better yet, use getPath() instead of toString()) to get to the path you need.

How do I rename (not move) a file in Java 7?

I'm a bit confused with all these new File I/O classes in JDK7.
Let's say, I have a Path and want to rename the file it represents. How do I specify the new name, when again a Path is expected?
Path p = /* path to /home/me/file123 */;
Path name = p.getName(); /* gives me file123 */
name.moveTo(/* what now? */); /* how to rename file123 to file456? */
NOTE: Why do I need JDK7? Handling of symbolic links!
Problem is: I have to do it with files whose names and locations are known at runtime. So, what I need, is a safe method (without exceptional side-effects) to create a new name-Path of some old name-Path.
Path newName(Path oldName, String newNameString){
/* magic */
}

In JDK7, Files.move() provides a short and concise syntax for renaming files:
Path newName(Path oldName, String newNameString) {
return Files.move(oldName, oldName.resolveSibling(newNameString));
}
First we're getting the Path to the new file name using Path.resolveSibling()
and the we use Files.move() to do the actual renaming.

You have a path string and you need to create a Path instance. You can do this with the getPath method or resolve. Here's one way:
Path dir = oldFile.getParent();
Path fn = oldFile.getFileSystem().getPath(newNameString);
Path target = (dir == null) ? fn : dir.resolve(fn);
oldFile.moveTo(target);
Note that it checks if parent is null (looks like your solution don't do that).

OK, after trying everything out, it seems I found the right method:
// my helper method
Path newName(Path oldFile, String newNameString){
// the magic is done by Path.resolve(...)
return oldFile.getParent().resolve(newNameString);
}
// so, renaming is done by:
oldPath.moveTo(newName(oldFile, "newName"));

If you take a look at Apache Commons IO there's a class called FileNameUtils. This does a ton of stuff wrt. file path names and will (amongst other things) reliably split up path names etc. I think that should get you a long way towards what you want.

If the destination path is identical to the source path except for the name of the file, it will be renamed rather than moved.
So for your example, the moveto path should be
/home/me/file456

If you can't get Java to do what you want with Unix I recommend Python scripts (run by your Java program). Python has get support for Unix scripting and it's not Perl :) This might sound inelegant to you but really in a larger program you'll benefit from using the right tool for the job.