The difference between jdk and jre
jre is the environment for running Java programs, and a tool for compiling Java code in jdk, including some simple libraries
The difference between Oracle jdk and open jdk
The former is updated slowly and the latter is updated quickly, the former is after the acquisition, and the latter is before the acquisition
The difference between Java and javax
javax is extended, and then the two are combined
How the thread is started
Implement the Runnable interface and inherit the Thread class
If a class inherits the Thread class, it is not suitable for multiple threads to share resources, and the implementation of the Runnable interface can facilitate resource sharing.
Types of collections, which are ordered and unordered
List, set, map, queue,
list are ordered, set and map unordered
set bottom layer is map, map is the
only value of key and value set, the value of list may not be unique, the subscript of map is unique, and the value is not unique
Hashmap bottom layer
The bottom layer is a hash table
before jdk1.8, using array + linked list form
after jdk1.8, conflicts less than 8 use array + linked list form, >=8 use red-black tree
Why not balanced binary tree? Because the balanced binary tree is still a linear tree under certain circumstances,
then if you set the capacity, it will be in the form of a power exponent.
MySQL tuning
emmm
The characteristics of the transaction
The four characteristics, acid
ACID, refer to the abbreviation of the four basic elements for the correct execution of database transactions. Including: Atomicity, Consistency, Isolation, Durability
GC garbage collection algorithm
Commonly used are 标记清除, 复制, 标记整理and 分代收集algorithm
Mark clear
The mark removal algorithm is divided into two stages: "marking" and "clearing". Mark all objects that need to be recycled, and collect them uniformly after marking. This routine is very simple and has shortcomings. The subsequent algorithms are all improved on this basis.
In fact, it is to mark the dead object as free memory, and then record it in a free list. When we need a new object, the memory management module will look for free memory from the free list to allocate it to the new object.
Human words: Mark the dead and useless objects and record them in a free list. When you want to new an object, take it from the free memory.
Copy algorithm
Divide the memory into 2 sides equally, one side is empty and the other side is used. Use the side that is used for normal use. Put the trash, unused, and still-used ones together. Then when they are full, remove the trash, put the still-used ones on the other side, and empty your own side.
Tag sorting algorithm
The replication algorithm will have certain efficiency problems when the object survival rate is high. The marking process is still the same as the "mark-sweep" algorithm, but the subsequent steps are not to directly clean up the recyclable objects, but to make all surviving objects to one end Move, and then directly clean up the memory outside the boundary
Generational collection algorithm
This algorithm does not have any new ideas, but divides the memory into several blocks according to the life cycle of the object. Generally, the Java heap is divided into the new generation and the old generation, so that the most appropriate collection algorithm can be adopted according to the characteristics of each generation. In the new generation, a large number of objects are found dead each time a garbage collection, and only a few survive, then the replication algorithm is selected, and the collection can be completed only by paying a small amount of the replication cost of the surviving objects. In the old age, because the object has a high survival rate and no extra space for its allocation guarantee, it is necessary to use the "mark-clean" or "mark-sort" algorithm for recycling.
Human words: It is to divide the memory, put different types in no batch of spaces, and then which spaces are cleared according to the above algorithm
Three paradigm
The first normal form: that is, the columns of the table are atomic and cannot be decomposed, that is, the information in the columns cannot be decomposed.
Human words: each field can't be divided anymore, it is already the smallest element
Second normal form: In the case of satisfying the first normal form, a primary key is the second normal form
Third normal form: In the case of satisfying the second normal form, non-primary key fields cannot depend on each other
What engine does MySQL use
Innodb: default, support transaction security, support row-level locks, used to add and modify,
By default, it
supports transactions, is transaction safe, provides row-level locks and foreign key constraints, and has a buffer pool for buffering data and indexes
Applicable scenarios: used for transaction processing, with ACID transaction support, applied to tables that perform a large number of insert and update operations
MyISAM: does not support transactions, foreign keys, locks, used for checking
Does not support transactions, does not support foreign key constraints, does not support row-level locks, the entire table needs to be locked during operation, but the number of rows in the table is saved, so the execution is particularly fast when executing select count(*) from tablename
Applicable scenarios: used to manage non-transactional tables, provide high-speed retrieval and full-text retrieval capabilities, suitable for tables with a large number of select operations, such as log tables
MEMORY: for precise search
The table is created using the content in the memory, and each memory actually corresponds to only one disk file. Because it exists in memory, the memory access speed is very fast, and the engine uses a hash index, which can be located at one time. It does not need to find the branch node from the root node like a B-tree, so the access speed is very fast for precise query, but it is not When searching accurately, such as like, this kind of range search, hash will not work. In addition, once the service is shut down, the data in the table will be lost because it is not stored in the disk.
Applicable scenarios: Mainly used for tables with infrequent content changes, or as an intermediate lookup table. Be cautious when updating the table because the data is not written to the disk, and consider the storage of the data before shutting down the service
The difference between == and equals
In the basic types (int, float), == and equals are the same
In the reference type (String), ==compare the addresses of the two, equalscompare the value in the address
String 、Stringbuffer 、Stringbuilder
The principle of the three, the difference
In the storage char array of String, there is a final limitation, that is, it will not be inherited by other classes. For
example, to modify things in String, its essence is not to modify, but to create a new char[] array, assign it to it, and then assign char[] to String
Stringbuffer has no final restriction, there is synchronization lock
Stringbuilder has no final restriction, no synchronization lock
Which are safe and which are not safe
String and Stringbuffer are safe because String has final and Stringbuffer has synchronization lock.
Stringbuilder is not safe because there is no final and synchronization lock.
Respective scope of application
Single-threaded data volume is large, using Stringbuilder
multi-threaded data volume is large, using Stringbuffer
How to remember?
String has a final all know it
Stringbuffer Well, there is buff, so there 锁and 处理多线程
Stringbuilder still buildin nothing, therefore 不安全, we can only deal with 单线程大things
MySQL data types
Numerical type
tinyint smallint mediumint int bigint float double mecimal
Where the precision mecimal>double>float
Time type
date time datetime timestamp
Character type
char varchar BLOB text
Java basic types
布尔、byte、short、int、long、float、double、char
The difference between tcp and udp protocol
UDP is used for network calls and TCP is used for SMS sending
Speed
udp is faster than tcp
In number
udp: one-to-one, one-to-many
tcp: one-to-one, point-to-point
reliability
TCP is reliable, because the 3-way handshake
udp is unreliable, just send, no handshake
3-way handshake
Client-send data packet with SYN flag-one handshake-server server
-send data packet with SYN/ACK flag-two handshake-
client client-send data packet with ACK flag –Three handshake – server
Why do you need to shake hands
The purpose of the three-way handshake is to establish a reliable communication channel. When it comes to communication, it is simply the sending and receiving of data. The main purpose of the three-way handshake is to confirm that the sending and receiving between themselves and the other party is normal.
The first handshake: Client cannot confirm anything; Server confirms that the other party is sending normally, and it is receiving normally
Second handshake: Client confirmed: sending and receiving are normal by itself, sending and receiving by the other party are normal; Server confirming: sending and receiving by the other party is normal, and receiving by itself is normal
The third handshake: Client confirmed: sending and receiving by itself is normal, sending and receiving by the other party is normal; Server confirming: sending and receiving by itself is normal, and sending and receiving by the other party are normal
Therefore, the three-way handshake can confirm that the dual sending and receiving functions are normal, and one is indispensable.
The difference between HashMap and TreeMap
Implementing the NavigableMap interface gives TreeMap the ability to search for elements in the collection.
Implementing the SortMap interface gives TreeMap the ability to sort the elements in the collection according to the key. The default is to sort by key in ascending order