Logical thinking in mathematical problem solving

This example requires some logical thinking:

4 boys 


4 boys A,B,C,D sit in a straight line, facing 1 direction. They cannot turn their heads or leave their chairs. A wall seperates A and B. They are told that 2 black and 2 white hats are placed on their heads.

A and C are wearing black hats, while B and D are wearing white hats.

D can see B and C's hats, C can see B's hat. B is facing the wall, and A cannot see any hat, since he is on the other side of the wall.
They are only given 1 chance to get their own hats right by shouting out the answer. After 1 minute, someone calls out?

1) Who called out? How can he be 100% sure?
2) Explain how each boy managed to figure out their hat's color.

( Problem courtesy of Cassandra)