Java learning: Map Interface

Map collection

Collection Interface:  defines a single set of specifications Collection <E>

• A single storage element per element

Map interface: defines a set of specifications of the double row Map <K, V>

• One pair each storage element

java.util.Map <K, V> set

Map of feature set:

1. Map is set a double-row set , an element contains two values (a key, a value)
2. Map elements of the collection, key and value data type may be the same or different
3. Map elements of the collection, Key is not allowed to repeat, value can be repeated
4. Map elements in the collection, key and value is one to one

java.util.HashMap <K, V> collection implements Map <K, V> Interface

HashMap set of features:
1.HashMap collection bottom is a hash value: special fast query

• Before JDK.8: singly linked list arrays +
• After JDK.8: Array + singly linked list / black trees (chain length of more than 8) : increase the speed of the query

2.HashMap set is an unordered collection, storage and order of the elements may be inconsistent extraction elements

java.util.LinkedHashMap <k, v> set extends HashMap <K, V> set

LinkedHashMap features:

1. The bottom layer is set LinkedHashMap hash table + chain (guaranteed iteration order)
2. LinkedHashMap collection is a ordered collection , storage and order of the elements of the extraction elements is consistent with

Map interface common method

public V put (K key, V value): adding the specified value of the key to the Map specified collection.

Return value: v

• When the key-value pairs stored, key will not be repeated, the return value is null V
• When stored key-value pairs, key duplication, the new value will replace the duplicate value map, the return value is replaced by the value

public V remove (Object key): the value corresponding to the specified key to delete the set of elements in the Map, the return value of the element is deleted.
Return value: V

• key presence, v return value is deleted
• key does not exist, v returns null

V GET public (Object Key) , acquires a value corresponding to the set in Map with a specified key.
return value:

• key is present, the value corresponding to the return value
• key does not exist, return null

boolean containsKey (Object key)  determines whether the collection contains the specified key

• Contains returns true, it does not include returns false

 

The first traversal Map collection: find a way through the key value

Map collection methods:

• Set <K> keySet () Returns a Set view of the keys contained in this map.

Implementation steps:

1. Use Map collection keySet (), the set of all the Map key taken out, stored in a Set collection.
2. Set traverse the collection , get Map in the collection of each Key
3. By Map collection methods get (key), pass through to find the key value

Entry key object

Map.Entry <K, V>: Map interface has one internal Entry Interface
effect: when creating a collection Map, it will create a Map Entry object in the collection, and to record the key value (key object, mapping between keys and values)

 

The second approach Map traverse the set: Use the Entry Object traversal

Set <Map.Entry <K, V> entrySet ()  Returns a Set view of the keys contained in this map.
Implementation steps:

1. Set <Map.Entry <K, V >> entrySet ():  the plurality of objects inside the Map Entry taken out of storage to a set of collection Set
2. Traversal Set Collection: get every Enter Object Set collection
3. Get Entry object Method: getKey () Gets key, getValue () Get value

 

Custom type key storage HashMap

Key: the Person type

• Person type must override hashCode method and the equals method to ensure a unique key

value: String type

• Can repeat

java.util.linkedHashMap<K,V> extends HashMap<K,V>

 

Hash table and linked list a list of the Map interface, with a predictable iteration order.

The underlying principle:

• + Chain hash table (sequential recording element)

Hashtable collection

java.util.Hashtable <K, V> collection implements Map <k, V> Interface

 

• Hashtable: the bottom is a hash table, is a collection of thread-safe, is a collection of single-threaded, slow
• HashMap: the bottom is a hash table, is a thread safe collection is a collection of multi-threaded, high speed
• HashMap set (the set of all previous studies): store a null value, null key
• Hashtable collection: can not store a null value, null key
• Hashtable and Vector collection of collections as being more advanced collection (HashMap, ArrayList) replaced after jdk1.2 version
• Hashtable subclass properties are still active in the arena calendar
• Properties and is a unique set of I / O stream combined set