Why and when to use TreeMap

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Let’s say you want to implement a dictionary and print it in alphabetical order, you can use a combination of a TreeMap and a TreeSet:

public static void main(String args[]) {
    Map<String, Set<String>> dictionary = new TreeMap<>();
    Set<String> a = new TreeSet<>(Arrays.asList("Actual", "Arrival", "Actuary"));
    Set<String> b = new TreeSet<>(Arrays.asList("Bump", "Bravo", "Basic"));

    dictionary.put("B", b);
    dictionary.put("A", a);

    System.out.println(dictionary);
}

All the sorting is done automatically and it prints:

{A=[Actual, Actuary, Arrival], B=[Basic, Bravo, Bump]}

You could have sorted the structures manually too of course but using TreeMap/Set can be more efficient, reduces the number of lines of code (= the number of bugs) and is more readable.

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