LRU cache in Java with Generics and O(1) operations

Question

This is a question that comes up a lot in job interviews. The idea is to define a data structure instead of using Java\'s built in LinkedHashMap.

An LRU cache delete

Answer 1

Here is the java implementation with Generics and LinkedHashMap, and complexity of O(1) on get() and put().

import java.util.LinkedHashMap;

public class LRUCache {

    private LinkedHashMap lruCacheMap;
    private final int capacity;
    private final boolean SORT_BY_ACCESS = true;
    private final float LOAD_FACTOR = 0.75F;

    public LRUCache(int capacity){
        this.capacity = capacity;
        this.lruCacheMap = new LinkedHashMap<>(capacity, LOAD_FACTOR, SORT_BY_ACCESS);
    }

    public V get(K k){
        return lruCacheMap.get(k);
    }

    public void put(K k, V v){
        if(lruCacheMap.containsKey(k)){
            lruCacheMap.remove(k);
        }else if(lruCacheMap.size() >= capacity){
            lruCacheMap.remove(lruCacheMap.keySet().iterator().next());
        }
        lruCacheMap.put(k, v);
    }

    public void printSequence(){
        System.out.println(lruCacheMap.keySet());
    }
}

This is the code for testing it:

import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

public class TestLruCache {

    static class MyHardDrive {
        Map resources = new HashMap<>();

        MyHardDrive(){
            for(Character i='A'; i<='Z'; i++){
                resources.put(i.toString(), new Object());
            }
        }

        Object loadResource(String name){
            return resources.get(name);
        }
    }

    public static void main(String[] args) {
        MyHardDrive hd = new MyHardDrive();
        LRUCache cache = new LRUCache<>(4);

        for(String key: Arrays.asList("A","B","C","A","D","E","F","E","F","G","A")){
            Object object = cache.get(key);
            if(object==null){
                object = hd.loadResource(key);
                cache.put(key, object);
            }
            cache.printSequence();
        }
    }
}