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Is there a way to get the path of a File with "/" instead of "\"?

I am making an HTTP Server in Java so that (on start) it finds all files in a directory (and it's sub-directories) and adds them to the server. But when getting the path of a file and trying to give it to HttpServer.createContext(), it throws a java.lang.IllegalArgumentException: Illegal value for path or protocol. (with the string argument, say "\folder/index.html"). To get this value, I used
file.getParent().substring(24) + "/" + file.getName()
I used substring because I had to exclude the folder the web server is in. The illegal character is the backslash. I have tried extending File to change separator and separatorChar, but that only created 2 new variables. While using String.replace() didn't seem to have any effect. Is there a different method than File.getParent or File.getPath that I can use, or is there a way to use String.replace that I am not seeing?
EDIT:
String.replace() seems to be the best answer... But I am not completely sure how to use it.
EDIT 2: For some reason the backslash isn't showing up, so I changed it.

You have to use the java System.getProperty.
Notice that, in this context, "file.separator" is a key which we are
using to get this property from current system executing the java VM.
Insteady of using a slash (/), you should choose a platform agnostic file separator, as an example it should be:
String separator = System.getProperty("file.separator");
System.out.println(separator);
// unix / , windows \

Have a look at Paths.get(...)
Try Paths.get(".") // current working directory.
Or tell it, on which path it should start:
Use System.getProperty("user.dir"), for current loged in user, home directory.
String pathStr = "/";
Path homeDir = Paths.get(System.getProperty("user.dir"))
Getting from the user directory into the data directory: homeDir.get("data")
Path dataPath = Paths.get(System.getProperty("user.dir"));
File dataFile = dataPath.toFile();
Now use operations on dataFile, to check what files and directories there are, on that location of the file system.

Identify extension of a file based on its stem name

I have a simple problem that I am quite struggling with. I have several files in a directory and I am reading them and passing processing them based on their type (extension). However, as an input, I receive a path to the file without extension so I have to identify the type myself.
example (files):
files/file1.txt
files/file1.txt
files/pic1.jpg
----------------
String path = "files/file1";
String ext = FilenameUtils.getExtension(path); // this returns null
Is there a way to identify the type of file when the extension is not included in the path?

Your best bet here is to "do it yourself" by implementing instances of FileTypeDetectors.
When you have this, you can then just use Files.probeContentType() to have a string returned which describes the file contents as a MIME type.
The JDK does provide a default implementation but it relies on file extensions, basically; if you have a PNG image named foo.txt, the default implementation will return text/plain where the file is really an image/png.
Which is of course wrong.
Final note: if all you really have is only part of the file name, then use Files.newDirectoryStream() and provide it with the appropriate DirectoryStream.Filter<Path>. Not sure yet why you only have part of it though.

Since you're only given part of the file name, you'll need to search for files that start with that prefix. Note that there could be multiple matches.
Using java.nio.file
Path prefix = Paths.get(path);
Path directory = prefix.getParent();
try (Stream<Path> stream = Files.list(directory)) {
stream.filter(p -> p.getFileName().startsWith(prefix.getFileName() + "."))
.forEach(p -> System.out.printf("Found %s%n", p));
}
Using java.io
File prefix = new File(path);
File directory = prefix.getParentFile();
List<File> matches = directory.listFiles((dir, name) ->
name.startsWith(prefix.getName() + "."));
for (File match: matches) {
System.out.printf("Found %s%n", match);
}

Files.probeContentType(Path) implements a basic MIME type inquiry you can use (or extend), the internal details of which are platform specific. You can also make a little utility method that walks a Set of extensions. A combination of the two approaches may be necessary, depending on your application.
The MIME type checker will give different results on different releases implementations of the JRE. So, always have a fail-over solution.
See: http://docs.oracle.com/javase/7/docs/api/java/nio/file/Files.html#probeContentType%28java.nio.file.Path
[EDIT]
This actually does not answer the question posited, as this method needs a full, legal Path object to work on. If you are given just the stem name, and the extension is missing, then you neither have an extension to work with nor a valid Path name for Files to work with [and probeContentType() may, in some implementations, just use the extension anyway.]
I'm not sure how you can do this without Path that refers to a real on-disk file that the JRE can access, or by hand if you don't have an extension. If you don't have a File of some sort, you can't even open it up yourself to attempt file type "magic".

Convert Windows style path into unix path in java code

I am working in a java code that was designed to run on windows and contains a lot of references to files using windows style paths "System.getProperty("user.dir")\trash\blah". I am in charge to adapt it and deploy in linux. Is there an efficient way to convert all those paths(\) to unix style (/) like in "System.getProperty("user.dir")/trash/blah". Maybe, some configuration in java or linux to use \ as /.

My approach is to use the Path object to hold the path information, handle concatenate and relative path. Then, call Path's toString() to get the path String.
For converting the path separator, I preferred to use the apache common io library's FilenameUtils. It provides the three usefule functions:
String separatorsToSystem(String path);
String separatorsToUnix(String path);
String separatorsToWindows(String path)
Please look the code snippet, for relative path, toString, and separator changes:
private String getRelativePathString(String volume, Path path) {
Path volumePath = Paths.get(configuration.getPathForVolume(volume));
Path relativePath = volumePath.relativize(path);
return FilenameUtils.separatorsToUnix(relativePath.toString());
}

I reread your question and realize you likely don't need help writing paths. For what you're trying to do I am not able to find a solution. When I did this in a project recently I had to take time to convert all paths. Further, I made the assumption that working out of the "user.home" as a root directory was relatively sure to include write access for that user running my application. In any case, here are some path problems I addressed.
I rewrote the original Windows code like so:
String windowsPath = "C:\temp\directory"; //no permission or non-existing in osx or linux
String otherWindowsPath = System.getProperty("user.home") + "\Documents\AppFolder";
String multiPlatformPath = System.getProperty("user.home") + File.separator + "Documents" + File.separator + "AppFolder";
If you're going to be doing this in a lot of different places, perhaps write a utility class and override the toString() method to give you your unix path over and over again.
String otherWindowsPath = System.getProperty("user.home") + "\Documents\AppFolder";
otherWindowsPath.replace("\\", File.separator);

Write a script, replace all "\\" with a single forward slash, which Java will convert to the respected OS path.

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.

Is there a Java utility which will convert a String path to use the correct File separator char?

I have developed a number of classes which manipulate files in Java. I am working on a Linux box, and have been blissfully typing new File("path/to/some/file");. When it came time to commit I realised some of the other developers on the project are using Windows. I would now like to call a method which can take in a String of the form "/path/to/some/file" and, depending on the OS, return a correctly separated path.
For example:
"path/to/some/file" becomes "path\\to\\some\\file" on Windows.
On Linux it just returns the given String.
I realise it wouldn't take long to knock up a regular expression that could do this, but I'm not looking to reinvent the wheel, and would prefer a properly tested solution. It would be nice if it was built in to the JDK, but if it's part of some small F/OSS library that's fine too.
So is there a Java utility which will convert a String path to use the correct File separator char?

Apache Commons comes to the rescue (again). The Commons IO method FilenameUtils.separatorsToSystem(String path) will do what you want.
Needless to say, Apache Commons IO will do a lot more besides and is worth looking at.

A "/path/to/some/file" actually works under Windows Vista and XP.
new java.io.File("/path/to/some/file").getAbsoluteFile()
> C:\path\to\some\file
But it is still not portable as Windows has multiple roots. So the root directory has to be selected in some way. There should be no problem with relative paths.
Edit:
Apache commons io does not help with envs other than unix & windows. Apache io source code:
public static String separatorsToSystem(String path) {
if (path == null) {
return null;
}
if (isSystemWindows()) {
return separatorsToWindows(path);
} else {
return separatorsToUnix(path);
}
}

This is what Apache commons-io does, unrolled into a couple of lines of code:
String separatorsToSystem(String res) {
if (res==null) return null;
if (File.separatorChar=='\\') {
// From Windows to Linux/Mac
return res.replace('/', File.separatorChar);
} else {
// From Linux/Mac to Windows
return res.replace('\\', File.separatorChar);
}
}
So if you want to avoid the extra dependency, just use that.

With the new Java 7 they have included a class called Paths this allows you to do exactly what you want (see http://docs.oracle.com/javase/tutorial/essential/io/pathOps.html)
here is an example:
String rootStorePath = Paths.get("c:/projects/mystuff/").toString();

Do you have the option of using
System.getProperty("file.separator")
to build the string that represents the path?

For anyone trying to do this 7 years later, the apache commons separatorsToSystem method has been moved to the FilenameUtils class:
FilenameUtils.separatorsToSystem(String path)

I create this function to check if a String contain a \ character then convert them to /
public static String toUrlPath(String path) {
return path.indexOf('\\') < 0 ? path : path.replace('\\', '/');
}
public static String toUrlPath(Path path) {
return toUrlPath(path.toString());
}

String fileName = Paths.get(fileName).toString();
Works perfectly with Windows at least even with mixed paths, for example
c:\users\username/myproject\myfiles/myfolder
becomes
c:\users\username\myproject\myfiles\myfolder
Sorry haven't check what Linux would make of the above but there again Linux file structure is different so you wouldn't search for such a directory

I think there is this hole in Java Paths.
String rootStorePath = Paths.get("c:/projects/mystuff/").toString();
works if you are running it on a system that has the file system you need to use. As pointed out, it used the current OS file system.
I need to work with paths between windows and linux, say to copy a file from one to another. While using "/" every works I guess if you are using all Java commands, but I need to make an sftp call so using / or file.separator etc... does not help me. I cannot use Path() because it converts mine to the default file system I am running on "now".
What Java needs is:
on windows system:
Path posixPath = Paths.get("/home/mystuff", FileSystem.Posix );
stays /home/mystuff/ and does not get converted to \\home\\mystuff
on linux system:
String winPath = Paths.get("c:\home\mystuff", FileSystem.Windows).toString();
stays c:\home\mystuff and does not get converted to /c:/home/mystuff
similar to working with character sets:
URLEncoder.encode( "whatever here", "UTF-8" ).getBytes();
P.S. I also do not want to load a whole apache io jar file to do something simple either. In this case they do not have what I propose anyways.

Shouldn't it be enough to say:
"path"+File.Seperator+"to"+File.Seperator+"some"+File.Seperator+"file"