[Java] Book update

[ruiixu23]

In Section 1.2.3 of the book. The dictionary/map was compared to the USet interface. One subtle issue here that we won't be able to update the dictionary/map using the add(x) method of USet, since 1) according to the text, two pairs are treated as equal if their keys are equal and 2) the USet add(x) interface states that the element x will be added to the set if not already present.

To create a dictionary/map, one forms compound objects called Pairs, each of which contains a key and a value. Two Pairs are treated as equal if their keys are equal. If we store some pair (k,v) in a USet and then later call the find(x) method using the pair x = (k, null) the result will be y = (k, v). In other words, it is possible to recover the value, v, given only the key, k.