Random Cache Implementation Java

RandomCache.java

package com.yazilimmimari.hackerrank;

import java.util.*;

public class RandomCache<S,T> {
    HashMap<S,T> cache;
    int capacity;
    RandomCache(int capacity) {
        cache = new HashMap<>(capacity);
        this.capacity = capacity;
    }

    T get(S key) {
        if(!cache.containsKey(key))
            return null;
        return cache.get(key);
    }

    void put(S key, T value) {
        if(cache.containsKey(key))
        {
            cache.replace(key, value);
            return;
        }
        else if(cache.size() == capacity)
        {
            //List<S> list = (List<S>)Arrays.asList(cache.keySet().toArray());
            //Collections.shuffle(list);
            //cache.remove(list.get(0));
            Random generator = new Random();
            T[] keys = (T [])cache.keySet().toArray();
            T randomKey = keys[generator.nextInt(keys.length)];
            cache.remove(randomKey);
        }
        cache.put(key,value);
    }
}