common knowledge book

1,= = and equals the difference between null and ""2,15 top Java Multi-threaded face questions and answers(1), use of join in threadIn many cases, the main thread is generating and starting a child thread, and if a lot of time-consuming operations are to be performed in the sub-thread, the main thread will often end up before the child thread, but if the main thread finishes processing the other transactions, it needs to use the result of the child thread, that is, the main thread needs to wait

attribute ' flatten ' The right usage >>> a = [[1,3],[2,4],[3,5],["abc","Def"]] >>> a1 = [Y for X in a for y in x] >>> A1 [1, 3, 2, 4, 3, 5, ' abc ', ' def '] or (not understood) >>> a = [[1,3],[2,4],[3,5],["abc","Def"]] >>> flatten = lambda x: [Y for L in X for y in flatten (l)] if Type (x) is list else [x] >>> Flatten (a) [1, 3, 2, 4, 3, 5, ' abc ', ' def '] third, used in t

store the actual data in heap memory.At the same time, declare an array name or object name in the stack memory, and store the first address in the heap memory;Therefore, when you use the array name to assign a value, the address is passed.The reference data type is a pass-through address⑷ Data type Conversions① Automatic Conversionchar = "int"byte = "short =" int = "LONG =" float = "Double"From low to high turn, waste storage space② Coercion Type conversionsTurn from high to bottom, loss of da

:" + window.screen.availHeight;s + = "screen available working area width:" + window.screen.availWidth;s + = "Your screen settings are" + window.screen.colorDepth + "bit color";s + = "your screen settings" + Window.screen.deviceXDPI + "pixels/inch";}Various widths:1. In CSS, the box model of the Web page, Width: is not included padding border marginIn JS, Offsetwidth:width+padding+border;Clientwidth:width + padding (varies with browser size)So in JS use offsetwidth carefully, because once the la

Shell Common knowledge First, the user login into the system after the system environment variables $HOME user's own directory$PATH directory to search for when executing commands$TZ Time Zone$MAILCHECK every few seconds to check for new letters$PS 1 The prompt number at the command column$PS 2 When the command is not finished, the Shell asks for the prompt number to be entered againSearch path for $MANPA

Tags: mongodb modifying primary shards Db.runcommand ({"Moveprimary": "DB", To: "Shard ID"});2. Merging data blocks (data blocks need to be empty)Db.runcommand ({"Mergechunks": "Db.collection", Bounds:[min_shardkey,max_shardkey]})3. Get Data block sizeDb.runcommand ({"DataSize": "Db.collection", "Keypattern": {shardkey:1}, Min:{sharkey:min},max:{shardkey:max}})4. Get the split policyDb.runcommand ({splitvector: "Db.collection", Keypattern: {sharkey:1}, maxchunksizebytes:10008864})5

storage engine, the index is. Myi file: Data exists. MyD file ... frm is a table structure file. the difference between MyISAM and InnoDB can be found in detail: http://www.oschina.net/question/17_4248 MyISAM Deleting the Delete does not really remove the database file, it needs to execute optimize table to clear the fragment. Table-level locks. The data that exists in the INNODB will be in a file ibdata1 outside. In the MyD file. It's not like MyISAM. Support transactions. Foreign keys are sup

calendar;d) gettime in DateFormat ()38. With regard to null and "" It is correct to say () ABDA) null is a constantb) "" is a String objectc) NULL can call methodD) "" Method can be calledThe substring () method in the String class describes the correct () ABA) Gets a portion of the stringb) return a new stringc) return a new string arrayD) This method has no return value40. The following about the code string str = new String ("abc"); The correct description is () BCA) Only one string object w

advisable to modify, you can use the inheritance mechanism to extract the dependency information into the parent engineering module for Unified management.Implementing inheritance1. Create parent Project: Package mode set to Pom2. The parent project has been used in the sub-projectIf the groupId and version of the sub-project and the parent project are duplicated at this time, you can delete3. Managing dependencies in the parent projectDependencies tags in the Parent project, enclosed in depend

Knowledge about common AIX commands lsvg-o rootvg # lspv hdisk0 # oslevel-rssaxlate-l hdisk3lsattr-El kernel-El proc0lscfg-vl kernel-vl pdisk * errpt-aj paste error code errclear 0 (Remember to back up) cp/var/adm/errlog/home/ww/err/errlog errclear 0 lsdev-Cc restart-ad/dev/hdisk0bootlist-m normal-o bootlist-m normal hdisk0 hdisk1 rmt0 fdView the installation media content: installp-q-d/dev/cdrom-ldu www.2c

table. Hash is particularly suitable for storing objects. 1) hset/mget/hmset/hmet/hgetall/hdel 　　 2) query length hlen 3) whether a key has a key hexists key1 key2 4) obtain all key hkeys keys 5) obtain all value hvals keys 6) add hincrby/hincrbyfloat 7) If yes, the hsetnx key primary key sub-value is assigned.Zset (sorted set: ordered set) Redis zset and set are also set of string elements, and repeated members are not allowed. The difference is that each element is associated with a score of

Mysql common knowledge point bitsCN.com --Stored Procedure and common process control syntax /* This code creates a stored procedure named "p4" and sets two int-type varchar parameters, s1, s2, and s3, or other data types, two internal variables x1 and x2 are created. DELIMITER is used to modify the DELIMITER. DELIMITER $ refers to changing the default DELIMI

= [[UIViewALLOC] init];selectedbg. backgroundcolor=[UicolorPurplecolor];Cell. Selectedbackgroundview= selectedbg;Proxy method /*** Called when a row is selected (click)*/-(void) TableView: (UITableView *) TableView Didselectrowatindexpath: (Nsindexpath *) Indexpath{Xmgwine *wine = Self.winearray[indexpath.row];NSLog (@ "clicked:%@", wine.name);NSLog (@ "Selected:%zd", Indexpath.row);}/*** Called when a row is unchecked*/-(void) TableView: (UITableView *) TableView Diddeselectrowatindexpath: (N

Summary of common Python knowledge points and python Summary 1. Set Basic Data Type A. set is an unordered and non-repeating element set. Class set (object): "set ()-> new empty set object set (iterable)-> new set object Build an unordered collection of unique elements. "def add (self, * args, ** kwargs): # real signature unknown" "Add an element to a set, add the element This has no effect if the element i

Common (software) plugins:1.http://v3.bootcss.com/css/(bootstrap Chinese manual) 2.http://jquery.cuishifeng.cn/(jquery Chinese manual) 3.[font Awesome] (http://fontawesome.io/) (icon manual) 4.[jquery Lazy Load] (https://github.com/tuupola/jquery_lazyload) 5. [Sweetalert] (Https://github.com/t4t5/sweetalert)/[sweetalert2] (https://github.com/limonte/sweetalert2) 6.[toastr] (http://codeseven.github.io/toastr/) http://thevectorlab.net/flatlab/toastr.ht

remove the appropriate element from the collection.3) Comparable interface:Implementations that can be used for comparisons, classes that implement the comparable interface can determine how the class objects are sorted by implementing the Comparato method.Iii. the difference between HashMap and Hashtable1. HashTable:Thread-safe, multiple threads can share a hashtable, and multiple threads cannot share hashmap without proper synchronization.Java 5 provides Concurrenthashmap, an alternative to H

be used to extract text, but can only be used to specify the object of a multi-select structure or quantifier.They are not numbered in terms of $.Example: (1|one)(?:and|or)(2|two)After such a match, $1 or ' one ' contains ' 2 ' or ' one 'ResourcesHttps://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/RegExpHttps://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/String/matchHttps://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Glo

+ K-clears the character of the cursor to the end of the line# CTRL + T-swap the first two characters of the cursor# Ctrl + Y-paste the characters that were removed from the previous Ctrl+u class command# CTRL + P-Previous command# CTRL + N-Next command# Ctrl + V-Input control word such as CTRL + V. # CTRL + F-Move the cursor back one character# CTRL + B-cursor moves forward one character# Ctrl + H-delete a character before the cursor# N+# N+# CTRL + S-Suspend current shell# CTRL + Q-Re-enable#

queue.. Clear (): (void) empties the queue.. Empty (): (BOOL) returns whether the queue is empty.Priority Queue 1:priorit Y_ Queue type name > variable name;The larger the element is, the higher the priority.Priority Queue 2 : Priorit Y_ Queue type name ,vector type name >,greater type name > > variable name;The smaller the element, the higher the priority.Priority Queue 3 : Priorit Y_ Queue type name ,vector type name , Custom Compare struct (struct body with bool overloaded parentheses ope

acceptiptables-a input-p tcp-j reject or iptables-a INPUT- M state--state new-m tcp-p TCP--dport 80-j accept20, write a script for nginx log statistics, get the top 10 IP access (nginx log path:/home/logs/nginx/ Default/access.log). Cd/home/logs.nginx/defaultsort-m-K 4-o access.logok access.1 access.2 access.3 ..... cat Access.logok |awk ' {print $} ' |sort-n|uniq-c|sort-nr |head-1021. Write down the meanings of the following commands (1) maxkeepaliverequests 100 Maximum number of requests to