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