Map

• The Map Interface 


  • It deals with collections of ordered pairs.
• Think of the pair as consisting of a key K (to search for) and an associated value V. 


  For example, the key might be a student ID number and name
• The Map interface takes a base type for the key and a base type for the value.
• Use the put method to add a key/value pair to the collection and the get method to retrieve the value for a given key.
• The base types must be objects and cannot be primitive data types.
• public Base_Type_Value put(Base_Type_Key k, Base_Type_Value v)

  • Associates the value v with the key k.
• public Base_Type_Value put(Base_Type_Key k, Base_Type_Value v)


  Returns the previous value for k or null if there was no previous mapping.
• public Base_Type_Value get(Object k)

  • Returns the value mapped to the key k or null if no mapping exists.
• public Base_Type_Value remove(Object k) 


  • Removes the mapping of key k from the map if present.
• public Base_Type_Value remove(Object k)

  Returns the previous value for the key k or null if there was no previous mapping.
• public boolean containsKey(Object k) 


  • Returns true if the key k is a key in the map.
• public boolean containsValue(Object v) 


  • Returns true if the value v is a value in the map.
• public boolean hashCode() 


  • Returns the hash code value for the calling object.
• public boolean isEmpty()

  • Returns true if the map contains no mappings.
• public Set <Base_Type_Key> keySet() 


  • Returns a set containing all of the keys in the map.
• public Collection <Base_Type_Values> values() 


  • Returns a collection containing all of the values in the map.
• The HashMap class implements the Map interface and is used to store a map from a key object to a value object.
• class HashMap uses the hash table algorithm.
• HashMap is able to quickly retrieve the value object when given the key object.