I made the following changes/admissions:

1) I added logic to the score manager that awards extra lives to the player each time the score reaches the increment specified in the newly added constant SIConstants.NEW_LIFE_SCORE.

2) I added logic to restrict the rate at which shots are fired when the space bar is pressed. Before a solid wall of shots would be released from the player entity when the space bar is pressed. Now the rate of fire is restricted by the value specified in the constant SIConstants.PLAYER_SHOT_TIME_INTERVAL, which is in milliseconds.

3) I added a boss that appears every certain number of levels based on the value that is specifed in SIConstants.BOSS_LEVEL_APPEARANCE.

4) I added additional methods to the PlayerEntity class to manipulate the number of lives.

5) I cleaned up some more code in the SquareInvaders class. There was additional logic to dislay the number of remaining lives to the screen. Some of this code was not needed so I removed it.

6) I corrected the spelling of the class SquareInvaders.java. Before it was named SqaureInvaders.java.