Is there a way to create a folder (with a specific name) in the main directory of my Java app. I have a user system in my game, and whenever a new user is created, I want it to create a folder that will store all there progress in (and the folder name should be the name they put in).
For example:
From this:
To this (just by entering a username in the game):
From this forum:
File f = new File("TestDir"); try{ if(f.mkdir()) System.out.println("Directory Created"); else System.out.println("Directory is not created"); }catch(Exception e){ e.printStacktrace(); } 
d in f.mkdir() should be lowercase.Establish a directory inside a sub-directory1 of a known and reproducible path2.
user.home - which should be a directory that the game can read from and write to.Define the "main directory of your Java app"? There is no such beast.
You want to be very careful when doing things like this, for security concerns. Best to define a directory in a property file and use that as a base directory for your application. Then, as already said, it is just a use of .mkdir{,s}() to achieve what you want.
And @C.Reed also rightly says that you should check for mkdir()'s return value: Java's File API is hugely flawed in that it will not throw an exception when directory/file creation/move will fail. Fortunately, Java 1.7 will cure that with its new APIs.
(an example I encountered is seeing code which would fail to .move() a file around: the problem is that it worked on the dev's machine, but on the production machine the directory was to be moved on another filesystem --> havoc)
Hint: use Apache's commons-io
If the main directory is the working direct of your application you can create a directory with
new File("new-dir").mkdir(); 