Using collections to get the max value from a hash map

Im trying to use Collections to get the maximum value of a hash map where the hash map is made up of strings for keys, and doubles for values like below
{wednesday=1.23483,...}

I am trying to use the method max() from Collections, but not sure how it should be used. This is what i am doing

Code:

Collection map = map1.values();
Collections col;
System.out.println(col.max(c));

I dont know what to initialise col to however, can anybody help me with this?
Thanks