Why does the Map class not inherit from the Collection interface?

Collections (Collection) represent groups of objects where each element has the same type or is a subtype. Operations on collections usually concern individual elements: adding, removing, checking for presence, and iterating.

Map stores key-value pairs. Keys must be unique, values can repeat. Main operations on Map include retrieving a value by key, setting a value for a key, and removing by key.

The Map API does not match the Collection API. For example, Collection has a method add(E element), but Map does not have such a method for adding a single element (a key-value pair is required). Iteration over Map is only possible over keys (keySet()), values (values()), or key-value pairs (entrySet()), not directly over all elements as in Collection.

Inheritance of Map from Collection would require implementing Collection methods that do not make sense for the Map structure (e.g., adding a single value without a key) or would lead to a confusing API.

// Example Collection
List<String> list = new ArrayList<>();
list.add("A");
list.add("B");
// Iteration over elements
for (String s : list) {
    // ...
}

// Example Map
Map<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);
// Iteration over keys (or values/entries)
for (String key : map.keySet()) {
    // ...
}

Thus, Map is a separate category of data structures due to its fundamentally different storage and data access model.