Summary of Java collection framework (3)

Dragoncl 2021-01-24 11:52:39
summary java collection framework


Link The interface is concluded , Next comes set Interface


Set Interface
features :
– Set The interface stores a unique set of , Disordered objects
– ( The order of deposit and withdrawal is not necessarily the same )
– The way to manipulate data is not List similar ,Set Interface does not exist get() Method
– The element value of the corresponding position cannot be obtained by subscript , Because of disorder

Implementation class :
1、HashSet:
- use Hashtable Hash table storage structure
– advantage : Add fast , Fast query speed , Delete fast
– shortcoming : disorder
2、LinkedHashSet
Using hash table storage structure , At the same time, use the linked list to maintain the order
Orderly ( Add order )
3、TreeSet
– Use a binary tree ( Red and black trees ) Storage structure of
– advantage : Orderly ( The ascending order after sorting ) Query speed ratio List fast
– shortcoming : The query speed is not HashSet fast

HashSet
First of all, let's talk about hash surface , According to the definition : According to the key code value (Key value) Data structures that are accessed directly . in other words , It accesses records by mapping key values to a location in a table , To speed up the search . This mapping function is called the hash function , The array that holds the records is called a hash table .

hashSet operation :
1、 Every time hash Tables need to be redefined equals and hashcode, To ensure that the same object in the table is unique , When setting elements , If it's a custom object , It looks for equals and hashcode Methods , without , The comparison is the address .
2、 HashSet How to ensure the uniqueness of the elements ?
answer : It's through two methods of elements ,hashCode and equals Method to accomplish
If the element's HashCode Same value , To judge equals Is it true
If the element's hashCode Different values , Can't call equals Method


TreeSet
Definition :
It is from _n(n>=1_) Finite nodes make up a set with hierarchical relations , Call it "it". “ Trees ” Because it looks like an upside down tree , That is to say, it is root up , And leaf down . It has the following characteristics :

 Each node has zero or more child nodes ; A node without a parent is called a root node , Each non root node has only one parent node ; Except for the root node , Each sub node can be divided into several disjoint sub trees .

features :
– Use a binary tree ( Red and black trees ) Storage structure of ( Tree interface can refer to the data structure book , There is no detailed explanation here )
– advantage : Orderly ( The ascending order after sorting ) Query speed ratio List fast
– shortcoming : The query speed is not HashSet fast

Notice there's one here Comparable Interface
What does the above algorithm depend on to determine the “ size ” The order ?
• All can “ Sort ” All of the classes are implemented java.lang.Comparable Interface ,
Comparable There is only one method in the interface

 public int compareTo(Object obj);
The method :
Return positive number this > obj
Returns the complex number this < obj
return 0 this == obj

Realized Comparable Interface class through the implementation of comparaTo Method to determine
The sort method of this kind of objects .

Comparator classification ( Locate ):

 Internal comparator Defined in the class of the element , adopt comparable Interface to implement
External comparator Defined in the current class , By implementing comparator Interface to implement , But to pass the comparator into the collection
 public class SetDemo implements Comparator<Person> TreeSet treeSet = new TreeSet(new SetDemo());

Be careful :
1、 An external comparer can be defined as a tool class , At this point, if all the rules that need to be compared are consistent , You can reuse
2、 The internal comparer can only be used when storing the current object
3、 If both exist at the same time , Listen to the external comparator
4、 When using comparators , Not invoke equals Method

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本文为[Dragoncl]所创,转载请带上原文链接,感谢
https://javamana.com/2021/01/20210124115156626B.html

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