How Can I Sort a Java Map's Values by its Keys?

Sort Map Values by Key in Java

Given a Map with String keys and values, the objective is to sort the keys in ascending order. The sorted keys will then be used to create two strings: one containing the sorted questions and the other containing the corresponding answers.

Short Answer

To sort a Map by its keys, utilize a TreeMap, which is specifically designed for this purpose. If the Map type is unknown, an alternative method using SortedSet and TreeSet can be employed:

SortedSet<String> keys = new TreeSet<>(map.keySet());
for (String key : keys) {
    String value = map.get(key);
    // Perform desired operations
}

Longer Answer

Technically, any class implementing SortedMap can be used. However, TreeMap is the most prevalent choice. For more complex key types or custom sorting requirements, use the following constructor to provide a Comparator:

Comparator<Foo> comparator = (Foo o1, Foo o2) -> {
    // Define custom sort logic
};

SortedSet<Foo> keys = new TreeSet<>(comparator);
keys.addAll(map.keySet());

Performance Considerations:

Note that TreeMap and TreeSet have different performance characteristics compared to HashMap and HashSet. Lookup operations, which take O(1) time in HashMap, require O(Log(N)) time in TreeMap. As the map size increases, the lookup time will increase correspondingly.

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