Day_ 16 set

1. Array encapsulation

1.1 Adding, deleting, modifying and querying arrays

Inquire about

change

add to  

Delete  

 1.2 Use

 

 2. aggregate

Java A collection is a set of data that enables a program to store and manipulate elements that are not fixed .

2.1 Inheritance system  

Collection Is a collection , The two direct sub interfaces are List and set

List characteristic : Orderly repeatable , Ensure that the data is added in the same order as the data is taken out

Set characteristic : disorder Do not repeat , There is no guarantee that the data is added and taken out in the same order

List There are three subcategories :

1. ArrayList : At the bottom is an array , Queries and changes are extremely efficient

2. LinkedList : The bottom layer is a two-way linked list , Adding and deleting are more efficient

3. Vector : The bottom layer is also an array , Is thread safe , obsolete , It is not recommended to use , Has been ArrayList Instead of

Set There are two subclasses :

1. HashSet : At the bottom is a hash table

2. TreeSet : At the bottom is a binary tree

2.3 Collection

Collection As the parent of the collection class , therefore ,collection The method in , Is a method that all collection classes have

Common methods : 

 

Use

 2.4 Iterator

Iterator iterator : An iterator is a pattern , It can separate the traversed object from the traversed object , We no longer care about the underlying data structure , How to store data? Just get the iterator object , You can traverse

 collection in Provides a iterator() Method Used to get iterator objects                  aggregate .iterator();

   In an iterator , There are three ways :

1 boolean  hasNext() : Determine whether there are elements under the cursor , The default point is to the top , It doesn't point to the first element

2 E next() : Move the iterator cursor down one bit , And take out the element

3 remove () : Delete the currently executed element , Will also delete the in the set

Be careful :

Once the iterator is created , Collection cannot be added or deleted , If you add and delete , Need to regenerate iterators

enhance for loop forEach It's short for iterator

 

2.5 contains   remove

contains(Object o) : Determine whether an element is included

remove(Object o ) : Deletes the specified element

These two methods , The bottom layer will call equals Methods for comparison

 

 2.6 forEach

enhance for loop forEach It's short for iterator

 

2.7 List

2.7.1 ArrayList

List characteristic : Orderly repeatable , Ensure that the data is added in the same order as the data is taken out

List There are three subcategories : 

1. ArrayList : At the bottom is an array , Queries and changes are extremely efficient , The default initialization capacity is 10, Expand to the original 1.5 times , Non-thread safety , When creating objects , No capacity , Namely 0 , When adding data for the first time , To initialize capacity

 2.LinkedList : The bottom layer is a two-way linked list , Adding and deleting are more efficient

3. Vector : The bottom layer is also an array , Is thread safe , obsolete , It is not recommended to use , Has been ArrayList Instead of , The default initialization capacity is 10, Expand to the original 2 times

Be careful : 

Only reference data types can be saved in a collection , If you add a basic type , Automatic packing will be carried out first to the corresponding packing class , Then polymorphic transformation occurs to Object type

usage :