What does the default sorting in the TreeMap data structure mean?.

In the TreeMap data structure in Java, the default sorting means that the elements (keys) are ordered according to their natural ordering, as defined by the Comparable interface. If the keys implement Comparable, then TreeMap uses the compareTo method to compare and order the keys.

If the keys do not implement Comparable, then creating a TreeMap without an explicit comparator will throw an exception.

Thus, the default sorting is based on the natural order of the keys, for example, ascending for numbers, lexicographical for strings.

Example:

TreeMap<Integer, String> map = new TreeMap<>();
map.put(3, "three");
map.put(1, "one");
map.put(2, "two");

// Keys will be sorted: 1, 2, 3