code: public DataLoader<K, V> prime(K key, V value) {
          Object cacheKey = getCacheKey(key);
          synchronized (this) {
              if (!futureCache.containsKey(cacheKey)) {
                  futureCache.set(cacheKey, CompletableFuture.completedFuture(value));
              }
          }
          return this;
      }
docs: Primes the cache with the given key and value.
  
  @param key   the key
  @param value the value
  
  @return the data loader for fluent coding