Definition: The State pattern allows an object to alter its behavior when its internal state changes.

Usage: The State pattern is essentially an alternative to having a bunch of conditionals in some code. It moves the code for each conditional into a separate class. The resulting code is much cleaner and even marginally faster (one method call is faster than a ton of conditionals).

Example

enum AlienAi {
	MOVE_LEFT() {
		State move() {
			move left
			if has hit wall
				return MOVE_RIGHT
			else
				return MOVE_LEFT
		}
	}
	MOVE_RIGHT() {
		State move() {
			move right
			if has hit wall
				return MOVE_LEFT
			else
				return MOVE_RIGHT
		}
	}

	State move()
}

class Alien {
	move() {
		state = state.move()
	}

	State state = AlienAi.MOVE_LEFT
}

These classes model an alien that moves back and forth like the aliens in Space Invaders. The different states need not be stored in an enum as above.