UPDATE/{id}

update.java

@Put(value = "{id}", consumes = MediaType.APPLICATION_JSON)
public Task updateTask(@PathVariable("id") String id, Task task)
    {
    String  description = task.getDescription();
    Boolean completed   = task.getCompleted();

    if (description != null)
        {
        return tasks.update(id, Task::setDescription, description);
        }
    else if (completed != null)
        {
        return tasks.update(id, Task::setCompleted, completed);
        }

    throw new IllegalArgumentException("either description or completion status must be specified");
    }