Explain how a hash table works and when it is used.

A hash table is a data structure that stores key-value pairs and provides very fast access to data by key.

Working principle:

• The key is passed through a hash function, which computes an index in the array.
• At this index, a value or a list of values (in case of collisions) is stored.
• Collision resolution methods (such as chaining or open addressing) are used.

It is used when you need to quickly search, add, or delete elements by key, for example, for caching, implementing dictionaries, counting frequencies, etc.

Example in Java:

Map<String, Integer> map = new HashMap<>();
map.put("apple", 3);
int count = map.get("apple"); // quickly retrieve the value by key