• If you declare equals properly, you need also to declare hashCode.
• java.util.Hashtable's WWW pages state that: ‘to successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method and the equals method’.
• So a class should override the following methods from java.lang.Object:

  public int hashCode();
  public boolean equals(Object pObject);
  

• hashCode method is used by the following methods:

  Hashtable	contains, containsKey, get, put, remove
  HashMap	containsKey, containsValue, get, put, remove
  HashSet	add, contains, remove

• Few books that teach Java mention hashCode.