Get the file name without file extension in a Vim function

:help expand() should give you the answer, see expand().

You should use the r modifier for %, with %:r instead of % to get the file name without extension.

If you want to write functions to build and execute files, you should also have a look at the documentation for shellescape, in order to prevent problems with spaces in file name or path.

If you want to expand a filename (other than % etc) take a look at fnamemodify()

fnamemodify({fname}, {mods})                *fnamemodify()*
        Modify file name {fname} according to {mods}.  {mods} is a
        string of characters like it is used for file names on the
        command line.  See |filename-modifiers|.

fnamemodify("main.java", ":r") returns main.

I literally just read a similar question to this (in that someone else seemed to be trying to configure vim to build automagically for them with the F-key), and wrote an answer about how you can leverage the power of vim's :make command without even needing to write a Makefile. In your case, it's less directly related to the question, but I thought I'd mention it in case you were interested.

Furthermore, someone seems to have written something on Vim Tips Wiki about how to set up vim's :make command to specifically work with Java projects built with ant. I haven't worked with Java in a while myself, but in your case specifically it might be a good place to get started.

I came here looking for an answer for a similar question. I wanted to be able to extract the current class name from the java file being edited. I found a very neat way to do this in vim with an abbreviation:

ab xclass <C-R>=expand('%:t:r')<CR>

Place this line in your .vimrc (or similar) for this to work. An abbreviation will auto-trigger as soon as you press space, and so I usually prefix them with 'x' to avoid their accidental expansion.

The trick here is the combination of :t and :r in the argument to expand(). % is the "current file name", :t selects just the tail of the path ("last path component only") and :r selects just the root ("one extension removed"). (quoted parts are from the official expand() documentation.)

So when you are creating a new class in file /a/b/ClassIAmAboutToCreate.java you would type:

public class xclass {

the moment you press space after "xclass", the abbreviation will be expanded to public class ClassIAmAboutToCreate, which is exactly what you need.

Also, note that an abbreviation can be triggered by pressing Ctrl+] which avoids inserting a space after the class name.