Tell me about the structure of HashMap.

HashMap in Java is a data structure that implements an associative array (dictionary), where keys are mapped to values using a hash function.

Main components of HashMap:

• Bucket array: each element of the array contains a linked list (or a tree in newer versions) of elements with the same hash.
• Hash function: computes the bucket index based on the key.
• Collision handling: when hash codes collide, elements are stored in a linked list or a tree within the bucket.

When inserting an element:

1. The hash of the key is computed.
2. The bucket index is determined (usually hash % capacity).
3. If the bucket is empty, the element is placed there.
4. If the bucket is occupied, the list/tree is traversed to find the key or add a new element.

Searching is similar: compute the index, then iterate through the elements in the bucket to find the desired key.

Example of simple usage:

Map<String, Integer> map = new HashMap<>();
map.put("apple", 3);
map.put("banana", 5);
int count = map.get("apple"); // 3

HashMap provides amortized constant time access O(1), but performance may degrade with a high number of collisions.