An entity that can be multiplexed using a {@link Selector}. <p> Every selectable is associated with exactly one {@link SelectableChannel}. Selectables may be interested in three kinds of events: read events, write events, and timer events. A selectable will express its interest in these events through the {@link #isReading()}, {@link #isWriting()}, and {@link #getDeadline()} methods. <p> When a read, write, or timer event occurs, the selectable must be notified by calling {@link #readable()}, {@link #writeable()}, or {@link #expired()} as appropriate.