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Play m4a files in Java

I want to create a media player in Java. The mp3 support already works with the JLayer library but which library can play m4a files?
I read about vlcj here on stackoverflow, but this seems to depend on Swing/AWT which I wouldn't use because I want to port the application to Android later on.

Have you looked at JAAD? It's a Javasound SPI that decodes AAC audio, I've used it with success previously.
Note that m4a is a container format, and while it usually contains (in my experience) AAC audio, in theory it could contain other formats instead.
You can find some information about getting it working without Javasound (and a test case) here.

This answer is indirect. I don't really know anything about m4a files. But what I have found is an open source library that can stream them as a flash server named red5. It's written in Java so theoretically you should be able to browse their code to figure out how to do it.
Hopefully someone here can give a more direct answer, this is the best I can do.

If you have Java 7 or later, you have access to the Javafx library. You can also use your media player (like iTunes or Windows Media Player) to convert to the simpler mp3 version and run that. I wouldn't recommend .wav files as they have significantly more data usage than mp3s, (which condense the file size by compressing the .wav data and omitting inaudible and otherwise garbage-y data).
import javafx.scene.media.*;
String name = "song.mp3";
Media song = new Media(name);
MediaPlayer player = new MediaPlayer(song);
player.play();

Open source Java Library for guitar chords detection when you play an mp3 music file

I am looking for an alternative in java as most of the libraries I found were written in c++
E.x. the clam library. http://clam-project.org/wiki/Chordata_tutorial
Does any body know a good java library which does pitch synthesis from a raw mp3 file and does chord matching afterwards based on some predefined algorithms?
I want to use it in android.

You might want to try using Android NDK and build and use clam with it.
http://developer.android.com/sdk/ndk/index.html

MP4 container writer in Java

I would like to find a FREE MP4 (container) writer for Java. I do not need an encoder, only something which can write the correct atoms given their expected values. Bonus for such a library that also can write "valid" F4V.
I would prefer a pure Java solution rather than something using JNI or external executables.

Even though my answer comes very late you could have a look into my MP4 Parser/Unparser at Github. You can parse MP4 files, modify them and write the result. You can even start from scratch creating boxes programatically and write your object representation to some sink.

You can have a look at JCodec ( http://jcodec.org ). It contains an MP4 library and MP4 demuxer and muxer.

May be you are looking for something like StreamBaby.

Can't vouch for it, but red5 is an open source flash server written in Java, which claims support for streaming mp4 and has implementations of mp4 IO objects which may be able to create said format.
Also, IBM created their Toolkit For MPEG-4 a while back and though it's not free, it might help.

FFMPEG's java bindings?
http://fobs.sourceforge.net/f4jmf_first.html
Or simpler a JNA proxy over some C++ MP4 library.

I have implemented a QuickTimeWriter class which can write a QuickTime container in pure Java.
As far as I know, the QuickTime file format is structurally equivalent to MPEG-4 Part 14. There are only a few minor differences in the fields inside atoms/boxes.
So, with an MPEG-4 spec on your lap, and a few hours of work, you should be good.

The Java I've seen which modifies MP4 files would invoke Nero AAC Codec externally (a Windows native .exe which Nero does not supply the source for) to modify AAC files (which are Apple's audio-only MP4 files). It works for audio only, not to video.

Splitting/Merging 3gp files using a java library

I'm wondering if there's a java library out there that can manipulate 3gp files. Mostly I'm interested in splitting or merging existing video files.
I've looked at JMF (java media framework), but it doesn't support 3gp…and FFmpeg looks promising, but it's not clear that the library allows splitting/merging of existing files.
Does such a library exist?

Try Xuggler http://www.xuggle.com/ (its FREE btw)

How to create an excel file in google app engine (java)?

A question that seems to have quite a few options for Python, but none for Java after googling for two days. Really really could use some help all I have found so far is a recommendation to use gaeVFS to build an excel file from the xml components and then zip it all together which sounds like a slap in the face. Oh yes and if you were wondering I am questioning my use of Java rather than python but at 5,000 lines of code it would be insane to turn back now...
Other things you might find useful
Client: GWT
Server: Servlets running
on google app engine storing data
into the google data store
Excel file: mandatory, CSV isn't good
enough, no need to save the file just
to be able to "serve" it to the
client i.e. open a "Save As" box.

Have you checked out this api already: Java Excel API ?

You could also take a look at the Apache POI project. You can read and write MS Excel documents with this library.

Take a look at this post.
It's a step by step tutorial on how to generate excel files on google app engine.

Try this :
http://code.google.com/p/gwt-table-to-excel/

google app engine do not support input/output stream classes, you need to use google app engine virtual file system.