Hash table, generic

1 Set

1.1 HashSet Use

 

2、 Hash table

2.1 summary

* Hash table structure It can be understood as The first node of the Federation is saved in the array , For preservation k and v Key operation
*
*hash Algorithm ： Is a secure encryption mechanism , You can convert variable length data into fixed length data , And there is no guarantee of its uniqueness , Also known as hash conflict
*
* Again Java The middle finger is hashCode Method
*
* Generate multiple times for an object hash value , The value must be the same , Multiple objects can also generate the same hash value , It's called hash conflict
*
*k Do not repeat ,v Can be repeated
*
* Add process ：   
*                        1 According to the... To be added key, Call his hashCode Method , Generate hash value
*                        2 Through a certain algorithm , according to hash Value generates the subscript of the array
*                        3 Judge whether the subscript is , Is there any data , If there is no data, save the key value pair mapping relationship to the array
*                        4 If the subscript is in , There's data , Call key Of equals Method , Compare with all the corresponding data . If it's not equal , Then add it to the tail of the linked list
*                        5 If the corresponding linked list , adopt equals When comparing methods , Found the same data , that key Not adding , however value Value will replace the original value value
*                         Through the addition process, we know , Will automatically call the hashCode and equals. So when saving custom types , Attention should be paid to method overrides
*                         Again 1.8 There is a new change in , In order to improve the query efficiency , The optimization of red black tree and capacity expansion is introduced
*                                             Because the efficiency and performance of linked list re query are poor So if the number of linked lists in each array is greater than or equal to 7, The linked list will be converted into a red black tree
*                                             The default initialization capacity of the array is 16 
*                                             
stay Java There is no concept of hash table , Wrap the hash table into HashMap and HashTable

 

 

2.2  HsahSet 

When we use HashSet When , In fact, it is equal to using again HashMap

When adding data , Even though it's called HashSet Of add Method , But the essence is to call map Of put Method

Ps : stay map in ,put Is an add operation

and map in What needs to be saved is k and v The mapping relationship , So in set There is a variable in that holds value Value

So let's go on set When adding , It's just the operation map Medium key,value We no longer care about

3  Map

3.1  summary

map Is chaotic , And save to yes k-v The mapping relationship between key value pairs of , among K Can't repeat ,V repeatable

HsahMap： At the bottom is a hash table

TreeMap ： At the bottom are red and black trees   It must be sorted according to certain rules

The mapping relationship ： such as goods And the number of purchases Or statistics  

3.2  Inheritance system

 3.3 Usage method

 3.4  HashMap

// establish map

                  HashMap map = new HashMap();

                  // add to K-V

                  map.put("A", "one");

                  map.put("B", "two");

                  map.put("C", "three");

                  map.put(65,100);

                  map.put('A', "2222");

                  // key repeat , Don't add ,value Replace

                  map.put("A", "2222");

                  // Support K and V all null, But it doesn't make sense

                  map.put(null,null);

                  // Number

                  System.out.println(map.size());

                  // get : according to K obtain V Value

                  System.out.println(map.get("A"));

                  // To determine whether or not to include a key

                  System.out.println(map.containsKey(65));

                  // To determine whether or not to include a value

                  System.out.println(map.containsValue("one"));

                  // according to key Delete The mapping relationship , Return the corresponding value value

                  map.remove(65);

                  // Get all value, And put it in the collection to return

                  Collection values = map.values();

                  for (Object object : values) {

                          System.out.println(object);

                  }

                  System.out.println("======");

                  // keySet : Get all key, Package to set Object and return

                  Set keys = map.keySet();

                  for (Object object : keys) {

                          System.out.println(object +":"+map.get(object));

                  }

                  // hold map Convert to set

                  // Entry Class , Save the K and V Two variables , hold map Medium k and v Convert to entry Class

                  // So we just need to save entry object , It's like saving k and v

                  Set set  = map.entrySet();

                  for (Object object : set) {

                          // C=three

                          System.out.println(object);

                          // Convert to entry type

                          Entry entry = (Entry) object;

                          // obtain k and v

                          System.out.println(entry.getKey()+" : "+entry.getValue());

                  }

3.5  TreeMap

 

 4  Generic

4.1 summary

 4.2 Usage mode

 4..3 Be careful

Generic types cannot write basic types   Only reference types can be written

If you want to save basic types , You need to write the corresponding packing class type

 4.4  Custom generics

 

 5、 Example