20.10 Sets
A Set is an unordered Collection of unique elements (i.e., no duplicate elements). The collections framework contains several Set implementations, including HashSet and TreeSet. HashSet stores its elements in a hash table, and TreeSet stores its elements in a tree. Hash tables are presented in Section 20.11. Trees are discussed in Section 22.7.
Figure 20.16 uses a HashSet to remove duplicate strings from a List. Recall that both List and Collection are generic types, so line 16 creates a List that contains String objects, and line 20 passes a Collection of Strings to method printNonDuplicates.
Method printNonDuplicates (lines 24–35) takes a Collection argument. Line 27 constructs a HashSet<String> from the Collection<String> argument. ...
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