LRU Cache

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
set(key, value) - Set or insert the value if the key is not already present. When the cache reached its capacity, it should invalidate the least recently used item before inserting a new item.

import java.util.HashMap;

public class LRUCache {

    private class CacheObject {
        public CacheObject pre;
        public CacheObject next;
        public int key;
        public int value;
        public CacheObject(int key,int value) {
            this.key = key;
            this.value = value;
            this.next = null;
            this.pre = null;
        }
    }

    public HashMap table = new HashMap ();

    public CacheObject head;

    public CacheObject tail;

    public int capacity;

    private boolean isTail(CacheObject co){
        if (tail==null) {
            return false;
        }
        return tail==co;
    }

    private void fixTail(CacheObject co) {
        if (tail==head){
            return;
        }
        if (isTail(co)){
            tail = co.pre;
            tail.next = null;
        }
    }

    private void move2head(CacheObject co){
        if (head==co){
            return;
        }
        if (tail==head){
            CacheObject oldHead = head;
            head = co;
            head.next = oldHead;
            head.pre = null;
            oldHead.pre = co;
            oldHead.next = null;
            return;
        }
        if (head != null){
            CacheObject pre = co.pre;
            CacheObject next = co.next;
            if (pre!=null && next !=null){
                pre.next = next;
                next.pre = pre;
            }
            head.pre = co;
            co.next = head;
            co.pre = null;
        }
        head = co;
    }

    public LRUCache(int capacity) {
        this.capacity = capacity;
    }

    public int get(int key) {
        if (table.containsKey(key)) {
            CacheObject added = (CacheObject)table.get(key);
            if (head!=added){
                fixTail(added);
                move2head(added);
            }
            return added.value;
        }
        return -1;
    }


    public void set(int key, int value) {
        if (table.containsKey(key)){
            CacheObject tmp = (CacheObject) table.get(key);
            fixTail(tmp);
            move2head(tmp);
            tmp.value = value;
            return;
        }
        CacheObject added = new CacheObject(key, value);
        if (this.capacity == table.size()) {
            table.remove(tail.key);
            fixTail(tail);
        }
        if (table.size()==0) {
            tail = added;
            head = added;
        } else {
            move2head(added);
        }
        table.put(key,added);
    }
}