The "Restart" button will give you a new starting configuration. Note: it is not possible to win from all possible starting configurations, but the program always picks a configuration that has a solution.
You may also set your own starting position. Click on the "set" button, then click on the squares to set a configuration, then click the "play" button. The program will tell you whether or not the configuration is solvable. If you insist on picking an unsolvable configuration, the "solve" button will highlight some moves without complaint, but of course they won't actually solve the puzzle.
Check out the Java Source.