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Add cache with FIFO replacement policy (#6337)
This commit is contained in:
KevinMwita7
@ -0,0 +1,549 @@
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package com.thealgorithms.datastructures.caches;
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import java.util.Iterator;
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import java.util.LinkedHashMap;
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import java.util.LinkedHashSet;
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import java.util.Map;
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import java.util.Set;
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import java.util.concurrent.atomic.AtomicInteger;
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import java.util.concurrent.locks.Lock;
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import java.util.concurrent.locks.ReentrantLock;
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import java.util.function.BiConsumer;
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/**
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* A thread-safe generic cache implementation using the First-In-First-Out eviction policy.
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* <p>
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* The cache holds a fixed number of entries, defined by its capacity. When the cache is full and a
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* new entry is added, the oldest entry in the cache is selected and evicted to make space.
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* <p>
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* Optionally, entries can have a time-to-live (TTL) in milliseconds. If a TTL is set, entries will
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* automatically expire and be removed upon access or insertion attempts.
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* <p>
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* Features:
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* <ul>
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* <li>Removes oldest entry when capacity is exceeded</li>
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* <li>Optional TTL (time-to-live in milliseconds) per entry or default TTL for all entries</li>
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* <li>Thread-safe access using locking</li>
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* <li>Hit and miss counters for cache statistics</li>
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* <li>Eviction listener callback support</li>
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* </ul>
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*
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* @param <K> the type of keys maintained by this cache
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* @param <V> the type of mapped values
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* See <a href="https://en.wikipedia.org/wiki/Cache_replacement_policies#First_in_first_out_(FIFO)">FIFO</a>
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* @author Kevin Babu (<a href="https://www.github.com/KevinMwita7">GitHub</a>)
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*/
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public final class FIFOCache<K, V> {
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private final int capacity;
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private final long defaultTTL;
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private final Map<K, CacheEntry<V>> cache;
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private final Lock lock;
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private long hits = 0;
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private long misses = 0;
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private final BiConsumer<K, V> evictionListener;
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private final EvictionStrategy<K, V> evictionStrategy;
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/**
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* Internal structure to store value + expiry timestamp.
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*
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* @param <V> the type of the value being cached
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*/
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private static class CacheEntry<V> {
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V value;
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long expiryTime;
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/**
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* Constructs a new {@code CacheEntry} with the specified value and time-to-live (TTL).
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* If TTL is 0, the entry is kept indefinitely, that is, unless it is the first value,
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* then it will be removed according to the FIFO principle
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*
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* @param value the value to cache
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* @param ttlMillis the time-to-live in milliseconds
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*/
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CacheEntry(V value, long ttlMillis) {
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this.value = value;
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if (ttlMillis == 0) {
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this.expiryTime = Long.MAX_VALUE;
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} else {
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this.expiryTime = System.currentTimeMillis() + ttlMillis;
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}
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}
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/**
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* Checks if the cache entry has expired.
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*
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* @return {@code true} if the current time is past the expiration time; {@code false} otherwise
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*/
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boolean isExpired() {
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return System.currentTimeMillis() > expiryTime;
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}
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}
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/**
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* Constructs a new {@code FIFOCache} instance using the provided {@link Builder}.
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*
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* <p>This constructor initializes the cache with the specified capacity and default TTL,
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* sets up internal data structures (a {@code LinkedHashMap} for cache entries and configures eviction.
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*
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* @param builder the {@code Builder} object containing configuration parameters
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*/
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private FIFOCache(Builder<K, V> builder) {
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this.capacity = builder.capacity;
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this.defaultTTL = builder.defaultTTL;
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this.cache = new LinkedHashMap<>();
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this.lock = new ReentrantLock();
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this.evictionListener = builder.evictionListener;
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this.evictionStrategy = builder.evictionStrategy;
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}
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/**
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* Retrieves the value associated with the specified key from the cache.
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*
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* <p>If the key is not present or the corresponding entry has expired, this method
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* returns {@code null}. If an expired entry is found, it will be removed and the
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* eviction listener (if any) will be notified. Cache hit-and-miss statistics are
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* also updated accordingly.
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*
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* @param key the key whose associated value is to be returned; must not be {@code null}
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* @return the cached value associated with the key, or {@code null} if not present or expired
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* @throws IllegalArgumentException if {@code key} is {@code null}
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*/
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public V get(K key) {
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if (key == null) {
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throw new IllegalArgumentException("Key must not be null");
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}
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lock.lock();
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try {
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evictionStrategy.onAccess(this);
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CacheEntry<V> entry = cache.get(key);
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if (entry == null || entry.isExpired()) {
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if (entry != null) {
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cache.remove(key);
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notifyEviction(key, entry.value);
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}
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misses++;
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return null;
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}
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hits++;
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return entry.value;
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} finally {
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lock.unlock();
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}
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}
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/**
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* Adds a key-value pair to the cache using the default time-to-live (TTL).
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*
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* <p>The key may overwrite an existing entry. The actual insertion is delegated
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* to the overloaded {@link #put(K, V, long)} method.
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*
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* @param key the key to cache the value under
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* @param value the value to be cached
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*/
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public void put(K key, V value) {
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put(key, value, defaultTTL);
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}
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/**
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* Adds a key-value pair to the cache with a specified time-to-live (TTL).
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*
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* <p>If the key already exists, its value is removed, re-inserted at tail and its TTL is reset.
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* If the key does not exist and the cache is full, the oldest entry is evicted to make space.
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* Expired entries are also cleaned up prior to any eviction. The eviction listener
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* is notified when an entry gets evicted.
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*
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* @param key the key to associate with the cached value; must not be {@code null}
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* @param value the value to be cached; must not be {@code null}
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* @param ttlMillis the time-to-live for this entry in milliseconds; must be >= 0
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* @throws IllegalArgumentException if {@code key} or {@code value} is {@code null}, or if {@code ttlMillis} is negative
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*/
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public void put(K key, V value, long ttlMillis) {
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if (key == null || value == null) {
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throw new IllegalArgumentException("Key and value must not be null");
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}
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if (ttlMillis < 0) {
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throw new IllegalArgumentException("TTL must be >= 0");
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}
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lock.lock();
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try {
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// If key already exists, remove it
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CacheEntry<V> oldEntry = cache.remove(key);
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if (oldEntry != null && !oldEntry.isExpired()) {
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notifyEviction(key, oldEntry.value);
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}
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// Evict expired entries to make space for new entry
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evictExpired();
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// If no expired entry was removed, remove the oldest
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if (cache.size() >= capacity) {
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Iterator<Map.Entry<K, CacheEntry<V>>> it = cache.entrySet().iterator();
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if (it.hasNext()) {
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Map.Entry<K, CacheEntry<V>> eldest = it.next();
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it.remove();
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notifyEviction(eldest.getKey(), eldest.getValue().value);
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}
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}
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// Insert new entry at tail
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cache.put(key, new CacheEntry<>(value, ttlMillis));
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} finally {
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lock.unlock();
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}
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}
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/**
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* Removes all expired entries from the cache.
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*
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* <p>This method iterates through the list of cached keys and checks each associated
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* entry for expiration. Expired entries are removed the cache map. For each eviction,
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* the eviction listener is notified.
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*/
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private int evictExpired() {
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int count = 0;
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Iterator<Map.Entry<K, CacheEntry<V>>> it = cache.entrySet().iterator();
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while (it.hasNext()) {
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Map.Entry<K, CacheEntry<V>> entry = it.next();
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if (entry != null && entry.getValue().isExpired()) {
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it.remove();
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notifyEviction(entry.getKey(), entry.getValue().value);
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count++;
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}
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}
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return count;
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}
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/**
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* Removes the specified key and its associated entry from the cache.
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*
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* @param key the key to remove from the cache;
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* @return the value associated with the key; or {@code null} if no such key exists
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*/
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public V removeKey(K key) {
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if (key == null) {
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throw new IllegalArgumentException("Key cannot be null");
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}
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CacheEntry<V> entry = cache.remove(key);
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// No such key in cache
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if (entry == null) {
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return null;
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}
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notifyEviction(key, entry.value);
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return entry.value;
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}
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/**
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* Notifies the eviction listener, if one is registered, that a key-value pair has been evicted.
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*
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* <p>If the {@code evictionListener} is not {@code null}, it is invoked with the provided key
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* and value. Any exceptions thrown by the listener are caught and logged to standard error,
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* preventing them from disrupting cache operations.
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*
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* @param key the key that was evicted
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* @param value the value that was associated with the evicted key
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*/
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private void notifyEviction(K key, V value) {
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if (evictionListener != null) {
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try {
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evictionListener.accept(key, value);
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} catch (Exception e) {
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System.err.println("Eviction listener failed: " + e.getMessage());
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}
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}
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}
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/**
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* Returns the number of successful cache lookups (hits).
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*
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* @return the number of cache hits
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*/
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public long getHits() {
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lock.lock();
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try {
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return hits;
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} finally {
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lock.unlock();
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}
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}
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/**
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* Returns the number of failed cache lookups (misses), including expired entries.
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*
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* @return the number of cache misses
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*/
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public long getMisses() {
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lock.lock();
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try {
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return misses;
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} finally {
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lock.unlock();
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}
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}
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/**
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* Returns the current number of entries in the cache, excluding expired ones.
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*
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* @return the current cache size
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*/
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public int size() {
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lock.lock();
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try {
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evictionStrategy.onAccess(this);
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int count = 0;
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for (CacheEntry<V> entry : cache.values()) {
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if (!entry.isExpired()) {
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++count;
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}
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}
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return count;
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} finally {
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lock.unlock();
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}
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}
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/**
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* Removes all entries from the cache, regardless of their expiration status.
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*
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* <p>This method clears the internal cache map entirely, resets the hit-and-miss counters,
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* and notifies the eviction listener (if any) for each removed entry.
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* Note that expired entries are treated the same as active ones for the purpose of clearing.
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*
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* <p>This operation acquires the internal lock to ensure thread safety.
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*/
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public void clear() {
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lock.lock();
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try {
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for (Map.Entry<K, CacheEntry<V>> entry : cache.entrySet()) {
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notifyEviction(entry.getKey(), entry.getValue().value);
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}
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cache.clear();
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hits = 0;
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misses = 0;
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} finally {
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lock.unlock();
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}
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}
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/**
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* Returns a set of all keys currently stored in the cache that have not expired.
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*
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* <p>This method iterates through the cache and collects the keys of all non-expired entries.
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* Expired entries are ignored but not removed. If you want to ensure expired entries are cleaned up,
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* consider invoking {@link EvictionStrategy#onAccess(FIFOCache)} or calling {@link #evictExpired()} manually.
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*
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* <p>This operation acquires the internal lock to ensure thread safety.
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*
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* @return a set containing all non-expired keys currently in the cache
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*/
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public Set<K> getAllKeys() {
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lock.lock();
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try {
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Set<K> keys = new LinkedHashSet<>();
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for (Map.Entry<K, CacheEntry<V>> entry : cache.entrySet()) {
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if (!entry.getValue().isExpired()) {
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keys.add(entry.getKey());
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}
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}
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return keys;
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} finally {
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lock.unlock();
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}
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}
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/**
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* Returns the current {@link EvictionStrategy} used by this cache instance.
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* @return the eviction strategy currently assigned to this cache
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*/
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public EvictionStrategy<K, V> getEvictionStrategy() {
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return evictionStrategy;
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}
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/**
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* Returns a string representation of the cache, including metadata and current non-expired entries.
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*
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* <p>The returned string includes the cache's capacity, current size (excluding expired entries),
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* hit-and-miss counts, and a map of all non-expired key-value pairs. This method acquires a lock
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* to ensure thread-safe access.
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*
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* @return a string summarizing the state of the cache
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*/
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@Override
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public String toString() {
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lock.lock();
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try {
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Map<K, V> visible = new LinkedHashMap<>();
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for (Map.Entry<K, CacheEntry<V>> entry : cache.entrySet()) {
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if (!entry.getValue().isExpired()) {
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visible.put(entry.getKey(), entry.getValue().value);
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}
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}
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return String.format("Cache(capacity=%d, size=%d, hits=%d, misses=%d, entries=%s)", capacity, visible.size(), hits, misses, visible);
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} finally {
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lock.unlock();
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}
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}
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/**
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* A strategy interface for controlling when expired entries are evicted from the cache.
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*
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* <p>Implementations decide whether and when to trigger {@link FIFOCache#evictExpired()} based
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* on cache usage patterns. This allows for flexible eviction behaviour such as periodic cleanup,
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* or no automatic cleanup.
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*
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* @param <K> the type of keys maintained by the cache
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* @param <V> the type of cached values
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*/
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public interface EvictionStrategy<K, V> {
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/**
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* Called on each cache access (e.g., {@link FIFOCache#get(Object)}) to optionally trigger eviction.
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*
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* @param cache the cache instance on which this strategy is applied
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* @return the number of expired entries evicted during this access
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*/
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int onAccess(FIFOCache<K, V> cache);
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}
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/**
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* An eviction strategy that performs eviction of expired entries on each call.
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*
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* @param <K> the type of keys
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* @param <V> the type of values
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*/
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public static class ImmediateEvictionStrategy<K, V> implements EvictionStrategy<K, V> {
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@Override
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public int onAccess(FIFOCache<K, V> cache) {
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return cache.evictExpired();
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}
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}
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||||
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/**
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* An eviction strategy that triggers eviction on every fixed number of accesses.
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*
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* <p>This deterministic strategy ensures cleanup occurs at predictable intervals,
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* ideal for moderately active caches where memory usage is a concern.
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*
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* @param <K> the type of keys
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* @param <V> the type of values
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*/
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public static class PeriodicEvictionStrategy<K, V> implements EvictionStrategy<K, V> {
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private final int interval;
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private final AtomicInteger counter = new AtomicInteger();
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/**
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* Constructs a periodic eviction strategy.
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||||
*
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* @param interval the number of accesses between evictions; must be > 0
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* @throws IllegalArgumentException if {@code interval} is less than or equal to 0
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*/
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public PeriodicEvictionStrategy(int interval) {
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if (interval <= 0) {
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throw new IllegalArgumentException("Interval must be > 0");
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}
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this.interval = interval;
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}
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||||
@Override
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public int onAccess(FIFOCache<K, V> cache) {
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if (counter.incrementAndGet() % interval == 0) {
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return cache.evictExpired();
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}
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return 0;
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||||
}
|
||||
}
|
||||
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||||
/**
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* A builder for constructing a {@link FIFOCache} instance with customizable settings.
|
||||
*
|
||||
* <p>Allows configuring capacity, default TTL, eviction listener, and a pluggable eviction
|
||||
* strategy. Call {@link #build()} to create the configured cache instance.
|
||||
*
|
||||
* @param <K> the type of keys maintained by the cache
|
||||
* @param <V> the type of values stored in the cache
|
||||
*/
|
||||
public static class Builder<K, V> {
|
||||
private final int capacity;
|
||||
private long defaultTTL = 0;
|
||||
private BiConsumer<K, V> evictionListener;
|
||||
private EvictionStrategy<K, V> evictionStrategy = new FIFOCache.ImmediateEvictionStrategy<>();
|
||||
/**
|
||||
* Creates a new {@code Builder} with the specified cache capacity.
|
||||
*
|
||||
* @param capacity the maximum number of entries the cache can hold; must be > 0
|
||||
* @throws IllegalArgumentException if {@code capacity} is less than or equal to 0
|
||||
*/
|
||||
public Builder(int capacity) {
|
||||
if (capacity <= 0) {
|
||||
throw new IllegalArgumentException("Capacity must be > 0");
|
||||
}
|
||||
this.capacity = capacity;
|
||||
}
|
||||
|
||||
/**
|
||||
* Sets the default time-to-live (TTL) in milliseconds for cache entries.
|
||||
*
|
||||
* @param ttlMillis the TTL duration in milliseconds; must be >= 0
|
||||
* @return this builder instance for chaining
|
||||
* @throws IllegalArgumentException if {@code ttlMillis} is negative
|
||||
*/
|
||||
public Builder<K, V> defaultTTL(long ttlMillis) {
|
||||
if (ttlMillis < 0) {
|
||||
throw new IllegalArgumentException("Default TTL must be >= 0");
|
||||
}
|
||||
this.defaultTTL = ttlMillis;
|
||||
return this;
|
||||
}
|
||||
|
||||
/**
|
||||
* Sets an eviction listener to be notified when entries are evicted from the cache.
|
||||
*
|
||||
* @param listener a {@link BiConsumer} that accepts evicted keys and values; must not be {@code null}
|
||||
* @return this builder instance for chaining
|
||||
* @throws IllegalArgumentException if {@code listener} is {@code null}
|
||||
*/
|
||||
public Builder<K, V> evictionListener(BiConsumer<K, V> listener) {
|
||||
if (listener == null) {
|
||||
throw new IllegalArgumentException("Listener must not be null");
|
||||
}
|
||||
this.evictionListener = listener;
|
||||
return this;
|
||||
}
|
||||
|
||||
/**
|
||||
* Builds and returns a new {@link FIFOCache} instance with the configured parameters.
|
||||
*
|
||||
* @return a fully configured {@code FIFOCache} instance
|
||||
*/
|
||||
public FIFOCache<K, V> build() {
|
||||
return new FIFOCache<>(this);
|
||||
}
|
||||
|
||||
/**
|
||||
* Sets the eviction strategy used to determine when to clean up expired entries.
|
||||
*
|
||||
* @param strategy an {@link EvictionStrategy} implementation; must not be {@code null}
|
||||
* @return this builder instance
|
||||
* @throws IllegalArgumentException if {@code strategy} is {@code null}
|
||||
*/
|
||||
public Builder<K, V> evictionStrategy(EvictionStrategy<K, V> strategy) {
|
||||
if (strategy == null) {
|
||||
throw new IllegalArgumentException("Eviction strategy must not be null");
|
||||
}
|
||||
this.evictionStrategy = strategy;
|
||||
return this;
|
||||
}
|
||||
}
|
||||
}
|
||||
Reference in New Issue
Block a user