LRU cache design with DLL and HashMap

LRUCache.java

public class LRUCache  {
   
    //Keep track of least recently used elements
    class Node {
        String key_;
        String val_;
        Node next;
        Node prev;
        Node (String key,String val) {  
            this.key_ = key;
            this.val_ = val;
            this.next = null;
            this.prev = null;
        }
    } 

    private final int capacity;
    private Map<String,String> _lruMap;
    private Node front;
    private Node back;
    

    private void addToFront(String key,String val) {
        
        Node tmpNode = new Node(key,val);
        if (front == back && front == null) {
            front = tmpNode;
            back = front;
        } else {
            
            //Do not add duplicate key at front
            if (front.key_.equals(key) && front.val_.equals(val) ) {
                return;
            }
            front.prev = tmpNode;
            tmpNode.next = front;
            front = tmpNode;
        }
    }

    private String removeFromBack() {
        if (back == null) {
            return back.key_;
        }
        Node tmpNode = back;
        back = back.prev ;
        back.next = null;
        return tmpNode.key_;
    }


    
    LRUCache(int capacity) {
        this.capacity = capacity;
        front = null;
        back = null;
       _lruMap = new HashMap<String,String>();
    }

    public String get(String key,String val) {
        if (_lruMap.containsKey(key) ) {
            //update the ds
             put (key,val);
            _lruMap.get(key);
        }
        return null;
    }


    public void put(String key ,String val) {
        if (capacity == _lruMap.size()) {
            //Remove the entry from the map
            String rKey = removeFromBack();
            _lruMap.remove(rKey);
        }
        addToFront(key,val);  
        _lruMap.put(key,val);
    }

}