How to assign a specific char to a randomly generated position in 2d array? (java)

This is the first time I'm using java and I'm supposed to create a game using this program. The 2d array's 10 by 10, 5 specific chars are C,M,W,F and E. There are 15 C,M,W,F each, the remaining 40 boxes are occupied by E. The positions are randomly generated. Any ideas?