If you define your own classes, you are responsible for implementing your own hashCode method.
In Java, hash table are implemented as arrays of linked lists. Each list is called a bucket.
Hash tables can be used to implement several important data structures. The simplest among them is the set type. A set is a collection of elements without duplicates. The add method of a set first tries to find the object to be added, and adds it only if it is not yet present. The Java collections library supplies a HashSet class that implements a set based on a hash table. The contains method is redefined to make a fast lookup to find if an element is already present in the set. It checks only one bucket and not all elements in the collection. You would only use a HashSet if you don't care about the ordering of the elements in the collection.
The TreeSet class is similar to the hash set, with one added improvement. A tree set is a
sorted collection.
The Java library supplies two general-purpose implementations for maps: HashMap and TreeMap. Both classes ipmlemnet the Map interface.
A hash map hashes the keys, and a tree map uses a total ordering on the keys to organize
them in a search tree.
(Text above extracted from Core Java)
Hashtable is a java public synchronized class and a member of Collections Framework in Javawhich is used to implement the mapping of Java objects as keys and value associations. In order to be members of a hashtable data structure object, both the key and its associated values must be non-null objects. Hashtable class is an extension of Dictionary class belonging to the java.util library and it implements Map, Cloneable and Serializable interfaces.
The objects used as keys of a hashtable object implement "hashCode" and "equals" methods in order to store and retrieve objects from the hashtable data structure object
(Text above extracted from http://it.toolbox.com/wiki/index.php/Hashtable_Class)
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