Bulls & Cows: guess the number

This is the numerical version of the game. The game is played with 3, 4 or 5 digits

Android writes a 4-digit secret number. The digits must be all different. Then, the player try to guess the number. Android gives the number of matches. If the matching digits are in their right positions, they are "bulls", if in different positions, they are "cows".
Example:
Secret number: 4271
Opponent's try: 1234
Answer: 1 bull and 2 cows. (The bull is "2", the cows are "4" and "1".)