Using Google Guava to get List

Question

I am new to Guava and I want to return comma separated list of users essentially a String. I am using one third party API to fetch the list. I want to cache that list and return that entire list if user queries.

I looked at few examples online and they use LoadingCache<k, v> and CacheLoader<k,v>. I do not have any second argument and usernames are unique. Our application is not going to support individual querying on the user

Is there any flavor of / I can twik LoadingCache which will allow me to do that? Something like

LoadingCache<String> 
.. some code .. 
CacheLoader<String> { 
/*populate comma separated list_of_users if not in cache*/ 
return list_of_users
}

Answer 1

As you've no doubt seen, the pattern for a LoadingCache is:

 LoadingCache<Key, Graph> graphs = CacheBuilder.newBuilder()
   .maximumSize(1000)
   .expireAfterWrite(10, TimeUnit.MINUTES)
   // ... other configuration builder methods ...
   .build(
       new CacheLoader<Key, Graph>() {
         public Graph load(Key key) throws AnyException {
           return createExpensiveGraph(key);
         }
       });

If your service doesn't take a key, then you could just ignore it, or use a constant.

 LoadingCache<String, String> userListSource = CacheBuilder.newBuilder()
   .maximumSize(1)
   .expireAfterWrite(10, TimeUnit.MINUTES)
   // ... other configuration builder methods ...
   .build(
       new CacheLoader<String, String>() {
         public Graph load(Key key) {
           return callToYourThirdPartyLibrary();
         }
       });

You can hide the fact that the ignored key exists at all, by wrapping it in another method:

  public String userList() {
        return userListSource.get("key is irrelevant");
  }

It doesn't feel as if you need all the power of a Guava cache in your use case. It expires the cache after a time period, and supports removal listeners. Do you really need this? You could write something very simple instead like:

 public class UserListSource {
     private String userList = null;
     private long lastFetched;
     private static long MAX_AGE = 1000 * 60 * 5; // 5 mins

     public String get() {
        if(userList == null || currentTimeMillis() > lastFetched + MAX_AGE) {
             userList = fetchUserListUsingThirdPartyApi();
             fetched = currentTimeMillis();
        }
        return userList;
     }
 }