Shulou(Shulou.com)06/02 Report--

This article mainly introduces "what are the basic knowledge points of Java". In the daily operation, I believe that many people have doubts about the basic knowledge points of Java. The editor consulted all kinds of materials and sorted out simple and easy-to-use methods of operation. I hope it will be helpful for you to answer the questions of "what are the basic knowledge points of Java?" Next, please follow the editor to study!

Packages commonly used in JDK

Java.lang: this is the basic class of the system, such as String, Math, Integer, System, and Thread, which provides common functions.

Java.io: here are all the input and output related classes, such as file operations, etc.

Java.net: there are network-related classes, such as URL,URLConnection, etc.

Java.util: this is the system helper class, especially the collection class Collection,List,Map and so on.

Java.sql: this is the class for database operation, Connection, Statememt,ResultSet, etc.

The difference between Get and Post

1.get is to get data from the server, and post is to transfer data to the server.

The amount of data transmitted by 2.get is small and cannot be greater than 2KB. Post transmits a large amount of data and is generally regarded as unrestricted by default.

The security of 3.get is very low, while that of post is high. But the execution efficiency is better than the Post method.

4. You can only use post, not get, when uploading files.

The concrete embodiment of Java Polymorphism

Object-oriented programming has four characteristics: abstraction, encapsulation, inheritance, and polymorphism.

There are four forms of polymorphism:

1. Interfaces and inheritance of interfaces.

two。 Classes and inheritance of classes.

3. Reload.

4. Rewrite.

Among them, overloading and rewriting are the core.

Overloading: overloading occurs in the same class, if there are multiple parties with the same name

Method, but the parameter type and number of the method are not the same, so the method is repeated.

Here we go.

Rewriting: rewriting occurs when the subclass inherits the relationship of the parent class, and the method quilt in the parent class

Class inheritance, method name, return value type, parameters are exactly the same, but the method body is not

Similarly, then the method in the parent class is overridden by the subclass.

StringBuffer StringBuilder String distinguishes √

String string constants are immutable in two different spaces when using string concatenation

StringBuffer string variable variable thread safe string concatenation appends directly after the string

StringBuilder string variable variable non-thread safe string concatenation is appended directly after the string

The execution efficiency of 1.StringBuilder is higher than that of StringBuffer and that of String.

2.String is a constant and immutable, so StringBuffer and StringBuilder are mutable for each + = assignment. The append method is used for string concatenation and appended on the original basis, so the performance is higher than String. Because StringBuffer is thread-safe and StringBuilder is thread-safe, StringBuilder is efficient.

Higher than StringBuffer.

3. For the concatenation of strings with large amount of data, StringBuffer,StringBuilder.

The execution efficiency of Stringbuilder is higher than that of stringbuffer and higher than that of string.

String is a variable that is immutable, so create a new object when you assign a + = value.

Both Stringbuffer stringbuilder are mutable. When concatenating strings, you can use the appent method to append to the original, so the performance is higher than string, and because stringbuffer is readily safe, but stringbuilder is thread-unsafe, so stringbuilder is less efficient than stringbuffer.

Stringbuffter or stringbuilder can be used for big data string concatenation.

The difference between Hashtable and HashMap √

HashMap is not thread-safe, HashTable is thread-safe.

HashMap allows null (null) keys and values (key), while HashTable does not.

The performance of HashMap is better than Hashtable.

Map

1.Map is a key-value storage interface. There are two concrete implementations under Map, which are HashMap and HashTable.

2.HashMap is thread-safe and HashTable is thread-safe, so HashMap is more efficient than HashTable.

3.HashMap allows a key or value to be empty, while HashTable does not allow a key or value to be empty.

Map is a storage interface for key values, and two implementations under map are hashmap and hashtable.

Hashmap thread-unsafe hashtable is thread-safe, so hashmap is more efficient than hashtable

Hashmap allows null keys, but hashtable does not allow null.

Nine implicit objects

Input / output objects: request response out

Scope communication object: session application pageContext

Servlet object: page config

Error object: exception

Forword (request forwarding) and Redirect (redirect)

1. From data sharing

Forword is a continuation of a request that allows you to share request data

Redirect starts a new request and cannot share request data.

2. From the address bar

The Forword forwarding address bar does not change.

The Redirect forwarding address bar has changed

VIII. Summary of JQurey

Jquery is a lightweight js framework with cross-browser features and good compatibility.

And encapsulated a lot of tools, easy to use.

Commonly used are: selector, dom operation, ajax (ajax cannot cross-domain), special effects, utility class

Characteristics of XML and Json

Xml features:

1. There is and only one root node

2. The carrier of data transmission

3. All tags need to be customized

4. Plain text file

Json (JavaScript Object Notation) features:

There are two formats for json:

Json object (that is, key-value pairs are stored in {}, separated by colons between keys and values

Separated by commas between key-value pairs)

Json array (that is, multiple json objects are stored in [], separated by commas between json objects)

One of the carriers of data transmission (which can be nested between the two)

Difference:

Xml needs to use more characters to describe the data in the same format.

What is popular is json-based data transmission.

The hierarchy of xml is clearer than json.

What they have in common:

Xml and json are the carriers of data transmission, and have the characteristics of cross-platform and cross-language.

Request.getSession (), reqeust.getSession (false) and request.getSession (true)

GetSession () / getSession (true): returns the session if it exists, otherwise create a new session and return the object

GetSession (false): returns the session when session exists, otherwise returns null

The difference between Page and PageContext

Page is a servlet object; using the this keyword, its scope is on the same page.

PageContext is a scoped communication object; setAttribute () and getAttribute () are usually used to set and get the value of the stored object.

Ajax summary

Full name of AJAX: asynchronous JavaScript and XML (Asynchronous JavaScript And XML)

The core of Ajax is the JavaScript object XmlHttpRequest (XHR).

Advantages of Ajax:

Improve user experience (UE)

Improve the performance of the application

Perform a local refresh

AJAX is not a new programming language, but a technology for creating better, faster and more interactive Web applications.

two。 With AJAX, our JavaScript can use JavaScript's XMLHttpRequest object to communicate directly with the server. Through this object, our JavaScript can exchange data with the Web server without reloading the page, which can be partially refreshed.

3. AJAX uses asynchronous data transfer (HTTP requests) between the browser and the Web server, so that the web page can request a small amount of information from the server instead of the whole page, reducing the burden on the server and improving the performance of the site.

AJAX can make Internet applications smaller, faster, more friendly, and have a good UE.

5. Ajax is based on standardization and widely supported technology, and does not require plug-ins and download Mini Program

The size and scope of four scopes in JSP9 large occult objects

Four scopes from large to small: appliaction > session > request > page

Application: global scope, shared by the whole application. The life cycle is: the application starts to stops.

Session: session scope, when the user first accesses, a new session is generated, and the server can remember the session state later.

Request: request scope, which is a request from the client.

Page: a JSP page.

The above scope of action makes it smaller and smaller, the life cycle of request and page is short, the difference between them is that a request can contain multiple page pages (include,forward).

List,Set,Collection,Collections √

[if! supportLists] 1. [endif] List and Set are interfaces, and they both inherit from interface Collection,List is an ordered repeatable set, while Set is an unordered and unrepeatable set. Collection is the top-level interface of the collection, and Collections is a utility class that encapsulates many static methods about collection operations, and cannot be instantiated because the constructor is private.

The 2.List interface implementation class has ArrayList,LinkedList,Vector. ArrayList and Vector are implemented based on arrays, so the query is fast, while the speed of adding and deleting is slower. LinkedList is based on chained storage structure, so it is slower when querying but faster when adding and deleting. And because Vector is thread-safe, it is less efficient than ArrayList.

Both List and set belong to the interface. They both inherit from collection.

List is an ordered repeatable set that is unordered and unrepeatable.

Collection is the top-level interface of collections, and collections is a static utility class that encapsulates the operations of many collections because it is privatized, so it cannot be instantiated.

The interface implementation classes of List are divided into arraylist vertor linkedlist arraylist and verson, which are implemented based on arrays. It is said that query is faster, but the speed of addition and deletion is slower. Linkelist is based on chained storage, so it is slower to query, faster to add and delete Sudoku, and arraylist is more efficient because of vertor thread safety.

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Basic data types of java

Data type size

Byte (byte) 1 (8 bits)

Shot (short Integer) 2 (16 bits)

Int (Integer) 4 (32 bit)

Long (long Integer) 8 (32 bit)

Float (floating point) 4 (32 bit)

Double (double) 8 (64 bit)

Char (character type) 2 (16-bit)

Boolean (Boolean) 1 bit

Attach:

Is String a basic data type? (String is not a basic data type)

What is the length of String? is there a limit? (length is affected by memory size)

Bubbling sort

Public class Sort {

Public static void sort () {

Scanner input = new Scanner (System.in)

Int sort [] = new int [10]

Int temp

System.out.println ("Please enter 10 sorted data:")

For (int I = 0; I < sort.length; iTunes +) {

Sort [I] = input.nextInt ()

}

For (int I = 0; I < sort.length-1; iTunes +) {

For (int j = 0; j < sort.length-I-1; Jake +) {

If (sort [j] < sort [j + 1]) {

Temp = sort [j]

Sort [j] = sort [j + 1]

Sort [j + 1] = temp

}

}

}

System.out.println ("the order after arrangement is:")

For (int iTuno Bandi)

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