MOOCs: Solved 8 Puzzle assignment

I've just solved the fourth assignment from Princeton's Algorithms I course in Coursera.

This time we had to write a program to solve the 8-puzzle problem (and its natural generalizations) using the A* search algorithm.

Although I didn't understand well the problem description the first time I read it, in the end, this has been the easiest assignment so far.
I used TDD to code the Board class but only tested afterwards the Solver class.

These were the assessment tests result:

Compilation:  PASSED
Style:        FAILED
Findbugs:     No potential bugs found.
API:          PASSED

Correctness:  24/24 tests passed
Memory:       8/8 tests passed
Timing:       17/17 tests passed

Raw score: 100.00% [Correctness: 65%, Memory: 10%, Timing: 25%, Style: 0%]

I was a bit tired so I decided not to fix the style.