What conditions and restrictions are imposed on keys and values in the TreeMap data structure?

In the data structure TreeMap in Java, keys and values have the following conditions and restrictions:

• Keys must be unique. If you add a key that already exists, the old value will be replaced with the new one.
• Keys must be comparable to each other. This is necessary to maintain order in the tree. Usually, keys should implement the Comparable interface or a Comparator should be provided when creating the TreeMap.
• null cannot be used as a key. Unlike HashMap, TreeMap does not allow null as a key, because comparison with null will throw a NullPointerException.
• Values can be anything, including null. There are no restrictions on values.

Example:

TreeMap<String, Integer> map = new TreeMap<>();
map.put("apple", 5);
map.put("banana", 3);
// map.put(null, 1); // will throw NullPointerException

Thus, keys in TreeMap must be orderable and not null to ensure the correct operation of the structure and to maintain sorting.