getUserAndApply(uuid, user -> {
    Node tempGroup = api.getNodeFactory().makeGroupNode(api.getGroup("tempGroup")).build();
    user.hasPermission(tempGroup).asBoolean()
});

public void getUserAndApply(UUID playerUuid, Consumer<User> action) {
    User user = api.getUser(playerUuid);
    if (user != null) {
        // user is already loaded, just apply the action
        action.accept(user);
        return;
    }

    // ok, user isn't online, so we need to load them.
    // once the user is loaded, this callback will be executed on the main thread.
    api.getStorage().loadUser(playerUuid)
            .thenAcceptAsync(wasSuccessful -> {

                // for whatever reason, the user could not be loaded.
                // this might be because the database is not accessible, or because
                // there was some other unexpected error.
                if (!wasSuccessful) {
                    return;
                }

                // ok, so the user *should* be loaded now!
                User loadedUser = api.getUser(playerUuid);
                if (loadedUser == null) {
                    // nope, still not loaded.
                    return;
                }

                // apply the action now they're loaded.
                action.accept(loadedUser);

                // tell LuckPerms that you're finished with the user, and that
                // it can unload them.

                api.cleanupUser(loadedUser);
            }, api.getStorage().getSyncExecutor());
}

 Node tempGroup = api.getNodeFactory().makeGroupNode(api.getGroup("tempGroup")).setExpiry(10L).build();

User user = ...;
InheritanceNode groupNode = InheritanceNode.builder("group-name-here").build();
Tristate hasGroup = user.data().contains(groupNode, NodeEqualityPredicate.IGNORE_VALUE_OR_IF_TEMPORARY);