java - Use LinkedHashMap to implement LRU cache

Question

I was trying to implement a LRU cache using LinkedHashMap. In the documentation of LinkedHashMap (http://docs.oracle.com/javase/7/docs/api/java/util/LinkedHashMap.html), it says:

Note that insertion order is not affected if a key is re-inserted into the map.

But when I do the following puts

public class LRUCache<K, V> extends LinkedHashMap<K, V> {
    private int size;

    public static void main(String[] args) {
        LRUCache<Integer, Integer> cache = LRUCache.newInstance(2);
        cache.put(1, 1);
        cache.put(2, 2);
        cache.put(1, 1);
        cache.put(3, 3);

        System.out.println(cache);
    }

    private LRUCache(int size) {
        super(size, 0.75f, true);
        this.size = size;
    }

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

    public static <K, V> LRUCache<K, V> newInstance(int size) {
        return new LRUCache<K, V>(size);
    }

}

The output is

{1=1, 3=3}

Which indicates that the re-inserted did affected the order. Does anybody know any explanation?

Answer 1

As pointed out by Jeffrey, you are using accessOrder. When you created the LinkedHashMap, the third parameter specify how the order is changed.

"true for access-order, false for insertion-order"

For more detailed implementation of LRU, you can look at this http://www.programcreek.com/2013/03/leetcode-lru-cache-java/