Design a custom policy extension for an LRU Cache.

To design a custom policy extension for an LRU cache, you would typically:

1. Define the Policy Interface: Create an abstract class or interface that outlines the methods required for cache eviction. This might include methods like onAccess(key), onInsert(key, value), and onEvict(key, value).

2. Implement the LRU Policy: Create a concrete class that implements the policy interface. For LRU, this would involve using a doubly linked list to maintain the order of elements. When an element is accessed, it's moved to the front (most recently used). When the cache is full and a new element needs to be inserted, the element at the back (least recently used) is evicted.

3. Integrate with Cache Core: The cache core (which manages the key-value storage, e.g., using a HashMap) would hold an instance of the policy. When cache operations occur (get, put), the cache core would call the corresponding methods on the policy object to update its state.

4. Develop Custom Policies: To extend, create new classes implementing the policy interface. For example, a Least Frequently Used (LFU) policy would track access counts, or a Time-To-Live (TTL) policy would manage expiration based on timestamps. The cache core can then be configured to use any of these implemented policies.