Hi king000000 and welcome to DaniWeb

The play method should either:

Start playing a track if the player is not already playing;
OR
Do nothing if the player is already playing.

The stop method should either:

Stop playing the track if the player is playing;
OR
Do nothing if the player is not playing.

To do both of these methods (play and stop) you will need to maintain the state of the Player. This will involve a simple state machine - draw diagrams before you begin, it will be easier in the long run. You will need to play a Track when the state is playing and the GUI should probably display some way to inform the user that it is playing.

The setTrack method should:

Change the current track to the specified value - need another member variable for this.

These tips should give you something to go on. If you need more help, feel free to repost with another question.