With TreeSets the key must be embedded in the object you store in the collection. Often you might have TreeSet of Strings. All you can do then is tell if a given String is in the Set. It won’t find you an associated object he way a Treemap will. With a TreeMap the keys and the objects they are associated with are separate.
TreeSet and its brother TreeMap oddly have nothing to do with representing trees. Internally they use a tree organisation to give you an alphabetically sorted Set/Map, but you have no control over links between parents and children.
Internally TreeSet uses red-black trees. There is no need to presort the data to get a well-balanced tree. On the other hand, if the data are sorted (ascending or descending), it won’t hurt as it does with some other types of tree.
If you don’t supply a Comparator to define the ordering you want, TreeSet requires a Comparable implementation on the item class to define the natural order.
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