1. Hashing in java.util

2. HashMap
A map is a collection that holds pairs (key,value), or entries.
HashMap is an implementation of the interface Map (read main section 5.5, p.275)
Class hierarchy in java.util is as follows:
Object AbstractMap HashMap

3. HashMap Uses chaining as collision resolution technique
HashMap permits null values and the null key.
HashMap implementation is not synchronized, i.e. If multiple threads access this map concurrently, and at least one of the threads modifies the map structurally, then other threads may have unsynchronized access to the map.

4. HashMap
An instance of HashMap has two parameters that affect its performance: initial capacity and load factor (default value =0.75).
When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the capacity is roughly doubled by calling a rehash method.

5. HashMap Methods in class HashMap include: Constructor methods:
Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
HashMap(int ic)
Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).

6. HashMap Constructor methods (continue): HashMap(int ic, float lf)
Constructs an empty HashMap with the specified initial capacity and load factor.
HashMap(Map m)
Constructs a new HashMap with the same elements from the specified Map m.

7. HashMap Methods void clear() boolean containsKey(Object key)
Removes all mappings from this hash map.
boolean containsKey(Object key)
Returns true if this map contains a mapping for the specified key.
boolean containsValue(Object value)
Returns true if this map maps one or more keys to the specified value.

8. HashMap Methods (continue)
Set entrySet()
Returns a set containing all the pairs (key, value) in the hash map.
Object get(Object key)
Returns the object associated with key.
boolean isEmpty()
Return true if this map contains no elements, false otherwise.

9. HashMap Methods (continue)
Object put(Object key, Object val)
Put the pair(key,val) in the hash map; return a value associated with key if there is any in the hash map, null otherwise.
void putAll(Map m)
Add objects from map m to the current hash map.
Void rehash()
A protected method to increase the capacity of the hashtable; the method is called automatically when the number of keys in the hashtable is greater than the product of the load factor and the current capacity.

10. HashMap Methods (continue)
Object remove(Object key)
Remove the pair (key, corresponding value) from the hash map and return the value associated currently with key in the hash map.
int size()
Returns the number of objects in this map

11. HashSet A set is an object that stores unique elements.
HashSet is an implementation of the interface Set
Class hierarchy in java.util is as follows:
Object AbstractCollection AbstractSet HashSet

12. HashSet Methods in class HashSet include: Constructor methods:
Creates an empty HashSet with initial capacity (16) and load factor (0.75).
HashSet(int ic)
Constructs an empty HashSet with the specified initial capacity and the default load factor (0.75).

13. HashSet Constructor methods (continue): HashSet(int ic, float lf)
Constructs an empty HashSet with the specified initial capacity and load factor.
HashSet(Collection c)
Constructs a new HashSet with the same elements from the specified Collection c.

14. HashSet Methods boolean add(Object ob) void clear()
Adds the specified element to this set if it is not already present
void clear()
Remove all the objects from the hash set
boolean contains(Object ob)
Return true if the hash set contains the object ob
boolean isEmpty()
Returns true if this set contains no elements.

15. HashSet Methods (continue)
boolean remove(Object ob)
Removes the specified element from this set if it is present.
int size()
Returns the number of elements in this set (its cardinality).

16. Hashtable This is another implementation of the interface Map.
It is roughly equivalent to HashMap except that it is synchronized and does not permit null keys or null values.
Class hierarchy in java.util is as follows:
Object Dictionary Hashtable
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