Create least recently used (LRU) cache in java using LinkedHashMap

• LRU is one of the most popular caching algorithm.
• The items, which are least recently used are removed from the cache, using eviction policy.

Types of iteration order in LinkedHashMap:

1. Insertion order: This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map
2. Access order:  Access order is the order of iteration in which its entries were last accessed, from least-recently accessed to most-recently .
   • LRU cache implementation will use access order of LinkedHashMap.
   • LinkedHashMap has removeEldestEntry method, which decides the eviction policy.

Program – Least recently used (LRU) cache in java using LinkedHashMap

package org.learn.collection.map.lhashmap;

import java.util.LinkedHashMap;
import java.util.Map;

class LRUCache<K, V> extends LinkedHashMap<K, V> {

	private static final long serialVersionUID = 1L;
	private int lruSize;

	public LRUCache(int lruSize) {
		super(16, 0.75f, true);
		this.lruSize = lruSize;
	}

	@Override
	protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
		return size() > lruSize;
	}	
}

public class LRUCacheClient {
	public static void main(String[] args) {
		int cacheSize = 5;
		Map<Integer, String> mapVehicleNoAndOwner = new LRUCache<>(cacheSize);

		mapVehicleNoAndOwner.put(1000, "Federer");
		mapVehicleNoAndOwner.put(2000, "Bradman");
		mapVehicleNoAndOwner.put(3000, "Jordan");
		mapVehicleNoAndOwner.put(4000, "Woods");
		mapVehicleNoAndOwner.put(5000, "Ali");

		System.out.println("1. Iterating initial cache of size = "+cacheSize);
		demoIterateCache(mapVehicleNoAndOwner);
		
		int key = 1000;
		System.out.printf("2. Accessting value at key: %d is %s\n",key,mapVehicleNoAndOwner.get(key));
		
		key = 3000;
		System.out.printf("3. Accessting value at key: %d is %s\n",key,mapVehicleNoAndOwner.get(key));
		
		System.out.println("4. Iterating cache after accessing its keys: ");
		demoIterateCache(mapVehicleNoAndOwner);
		
		key = 6000;
		String value = "Don";
		System.out.printf("5. Adding new entry to cache, key=%d, value=%s\n",key,value);
		mapVehicleNoAndOwner.put(6000, "Don");
		key = 7000;
		value = "Campbell";
		System.out.printf("6. Adding new entry to cache, key=%d, value=%s\n",key,value);
		mapVehicleNoAndOwner.put(7000, "Campbell");

		System.out.println("7. Iterating cache after adding entries beyond its size: ");
		demoIterateCache(mapVehicleNoAndOwner);
	}

	private static void demoIterateCache(Map<Integer, String> mapVehicleNoAndOwner) {

		mapVehicleNoAndOwner.forEach((key, value) -> {
			System.out.println("Key:" + key + ", Value:" + value);
		});
	}
}

Output – Least recently used (LRU) cache in java using LinkedhashMap

1. Iterating initial cache of size = 5
Key:1000, Value:Federer
Key:2000, Value:Bradman
Key:3000, Value:Jordan
Key:4000, Value:Woods
Key:5000, Value:Ali
2. Accessting value at key: 1000 is Federer
3. Accessting value at key: 3000 is Jordan
4. Iterating cache after accessing its keys: 
Key:2000, Value:Bradman
Key:4000, Value:Woods
Key:5000, Value:Ali
Key:1000, Value:Federer
Key:3000, Value:Jordan
5. Adding new entry to cache, key=6000, value=Don
6. Adding new entry to cache, key=7000, value=Campbell
7. Iterating cache after adding entries beyond its size: 
Key:5000, Value:Ali
Key:1000, Value:Federer
Key:3000, Value:Jordan
Key:6000, Value:Don
Key:7000, Value:Campbell