Main working method. You may be surprised that here, just 'AudioSystem.write()' is called. But internally, it works like this: AudioSystem.write() contains a loop that is trying to read from the passed AudioInputStream. Since we have a special AudioInputStream that gets its data from a TargetDataLine, reading from the AudioInputStream leads to reading from the TargetDataLine. The data read this way is then written to the passed File. Before writing of audio data starts, a header is written according to the desired audio file type. Reading continues untill no more data can be read from the AudioInputStream. In our case, this happens if no more data can be read from the TargetDataLine. This, in turn, happens if the TargetDataLine is stopped or closed (which implies stopping). (Also see the comment above.) Then, the file is closed and 'AudioSystem.write()' returns.
Starts the recording. To accomplish this, (i) the line is started and (ii) the thread is started.
Stops the recording. Note that stopping the thread explicitely is not necessary. Once no more data can be read from the TargetDataLine, no more data be read from our AudioInputStream. And if there is no more data from the AudioInputStream, the method 'AudioSystem.write()' (called in 'run()' returns. Returning from 'AudioSystem.write()' is followed by returning from 'run()', and thus, the thread is terminated automatically. It's not a good idea to call this method just 'stop()' because stop() is a (deprecated) method of the class 'Thread'. And we don't want to override this method.