// Java sucks

// old version without generics
public Set<User> getUsersOld(String userIds) {
    Set<User> users = Sets.newHashSet();
    if (userIds == null) return users;
    for (String userId : userIds.split(",")) {
        userId = userId.trim();
        User user = userManager.get(userId);
        if (user != null) users.add(user);
    }
    return users;
}

// new version with generics
public Set<User> getUsersNew(String userIds) {
    final UserManager fUserManager = userManager; // retarded
    return getEntities(userIds, ",",
        new GetCallback<User>() {
            User get(key) {
                fUserManager.get(key);
            }
        });
}

protected <T> Set<T> getEntities(String keys, String delimiter, GetCallback<T> fetchCallback) {
    Set<T> ts = Sets.newHashSet();
    if (keys == null) return ts;

    for (String v : keys.split(delimiter)) {
        v = v.trim();
        T t = fetchCallback.get(v);
        if (t != null) ts.add(t);
    }

    return ts;
}

interface GetCallback<T> {
    <T> T get(String key);
}