Json for Java API Learning

Json for Java API Learning
First, declare:This article is a blog of many netizens. by referring to their blog, I have roughly understood some common Json in java classes and methods in projects, as well as my personal understanding of json.My personal understanding of json

In recent studies, I have read a lot of materials and learned some basic knowledge about json. Here is a simple summary:

What is json?

No. json (JavaScript Object Notation) is a simple data exchange format. Before that, I did not know that this is actually not as abstract as I thought, the following examples will be clearer.

Data Structure in json

Json only has two basic data structures: MAP, object, and Array.

1. Map

Similar to the Map in java, here we start with {and end with}. The content of the internal object is the situation of key-value pairs, which are separated by signs, different objects are separated by commas, for example: {name: hujie, age: 21} official illustration :( http://www.json.org /)

2. Array

This is similar to an array, so json is often called a json array. Here we end with [starting with]. The values in the middle are separated by commas, for example: [value1, value2, value3]

The official illustration is:

The value in the json array can be a value of many data types.

Here, the string, number, and so on do not need to be explained. Who is the truth? I can understand the specific timing, but I cannot explain it.

Knowledge point:

1. json is not hard to understand. Simply put, json is a data exchange format.

2. json has only two structures. One is object and the other is array. The two can be nested with each other. {} is an object and [] is an array.

Conversion between json and JavaScript

There is a dynamic script execution function in javaScript: eval (), which is a global javaScript Object. The eval () function can only accept the original string as a parameter. If it is not a string, it will be returned as is, otherwise, the calculation result of the string is returned (if there is a result, no result is returned) eval () case:

<Script language = javascript type = text/javascript> alert (eval (x = 10; y = 10; x * y;); // 100, execute alert (eval (2 + 3) as a js script; // 5 alert (eval (x + 10); // 20 var str = {}; // alert (eval ('+ str +') When no attribute exists in curly brackets); // The [object Object] alert (eval (str) dialog box is displayed )); // pop-up: undefinedstr = {name: 'Hj '}; // The curly braces contain an attribute alert (typeof eval (' + str + ')')); // pop-up: object, return alert (eval (str) as an object; // pop-up: hjalert (eval ('+ str + ')'). name); // pop-up: hjalert (eval (str ). name); // pop-up: undefinedstr = {name: 'Hj ', age: 21 }; // alert (eval ('+ str +') when multiple attributes exist in curly brackets ')')); // object alert (eval ('+ str + ')'). name); // hjalert (eval ('+ str + ')'). age); // 21 // alert (eval (str); // an error occurs during running, when multiple attributes are run without parentheses, an error // curly braces {} is used to declare an object. to reference this object, we must add, because the declaration statement can only be executed and cannot be returned // The purpose of being included in () is to convert the declaration statement into an expression, so that the value of the expression can be calculated, and return </script>

Note that the generated variables in the script executed by eval belong to the scope that calls eval.

<Script language = javascript type = text/javascript> eval (var str = 'global';); function test () {eval (var str = 'local ';); alert (str); // local} test (); alert (str); // global </script>

The following describes how to convert json strings and javaScript objects into javaScript objects:

<script language=javascript type=text/javascript>    var jsonpeople = [{'name':'hj','age':21},{'name':'darkrake','age':21}];var jspeople = eval('('+jsonpeople+')');alert(jspeople[0].name+:+jspeople[0].age);alert(jspeople[1].name+:+jspeople[1].age);</script>

Comparison of json and XML formats

Simple comparison:

XML:

 

      
       
    
     hj
            21    
       
       
    
     darkrake
            21    
   
  

Json string format:

 

 

{peoples:{people:[{'name':'hj','age':21},{'name':'darkrake','age':21}]}}

Here is a little more complex, multi-layer nesting:

 

 

      
   
    
Anhui Province
       
           
    
     
Lu'an
                
    
     
Jin 'an district
                
    
     
Shucheng
            
           
           
    
     
Hefei
                
    
     
Feidong
                
    
     
Feixi
            
   
      
      
   
    
Beijing
       
           
    
     
Beijing
                
    
     
Dongcheng District
                
    
     
Haidian District
            
       
  

{Address: {Province: [{ProvinceName: Anhui, City: [{CityName: lu'an, town: [Jin 'an district, Shucheng]}, {CityName: Hefei, town: [feidong, feixi]}] },{ ProvinceName: Beijing, City: {CityName: Beijing, town: [Dongcheng District, Haidian District] }}}

 

Common classes and methods of json in Java APIs

JSONObject class (usually used in combination with JSONArray)
Case:

Package javaJsonAPI; import org. json. JSONObject; public class JsonArrayTest {/** indicates that I have not found the jsonjarpackage on the official website http://www.json.org/. here, my solution is to download the source file provided on the official website *. in java *, a project named org. the json package imports all the source files. In fact, it is no different from importing the jar package in classpath */public static void main (String args []) {JSONObject jsonObject = new JSONObject ({'name': 'darkrake', 'age': 21}); System. out. println (jsonObject. get (name); System. out. println (jsonObject. get (age ));}}

JSONArray case:

Package javaJsonAPI; import org. json. JSONArray; public class JsonArrayTest {/** indicates that I have not found the jsonjarpackage on the official website http://www.json.org/. here, my solution is to download the source file provided on the official website *. in java *, a project named org. the json package imports all the source files. In fact, it is no different from importing the jar package in classpath */public static void main (String args []) {String strs [] = {one, two, three}; JSONArray myjson = new JSONArray (strs); for (int I = 0; I <myjson. length (); I ++) {System. out. println (myjson. getString (I ));}}}

Advanced: Use JSONObject in combination with the JSONArray class

Package javaJsonAPI; import org. json. JSONObject; public class JsonArrayTest {/** indicates that I have not found the jsonjarpackage on the official website http://www.json.org/. here, my solution is to download the source file provided on the official website *. in java *, a project named org. json package, which imports all the source files, is actually no different from importing the jar package in classpath */public static void main (String args []) {String str = {address: {Province: {ProvinceName: Anhui, City: [{CityName: lu'an, own: [Jin 'an district, Shu Cheng]}, {CityName: Hefei, own: [feidong, feixi]}] }}; JSONObject jo = new JSONObject (str); String ProvinceName = jo. getJSONObject (address )//. getJSONObject (Province ). get (ProvinceName ). toString (); // System. out. println (ProvinceName); String cityName = jo. getJSONObject (address ). getJSONObject (Province )//. getJSONArray (City ). getJSONObject (0 ). get (CityName )//. toString (); System. out. println (+ cityName); String town1 = jo. getJSONObject (address ). getJSONObject (Province )//. getJSONArray (City ). getJSONObject (0 ). getJSONArray (town )//. get (0 ). toString (); System. out. println (+ town1); String town2 = jo. getJSONObject (address ). getJSONObject (Province )//. getJSONArray (City ). getJSONObject (0 ). getJSONArray (town )//. get (1 ). toString (); System. out. println (+ town2 );}}

JSONStringer

-> Quickly construct a JSON string and write it to the file.

Basic Methods:

JSONStringer is a subclass of JSONWriter;

JSONStringer is constructed by object (). key (). value (). key (). value (). endObject;

JSONString (): Constructor

Object () indicates to start an object, that is, add {;

EndObject () indicates that an object is ended, that is, adding };

Array () indicates to start an array, that is, to add [;

EndArray () indicates to end an array, that is, to add a];

Key () indicates adding a key;

Value () indicates adding a value;

Note: Through this method, "," between objects and "," between values of Array arrays are automatically added.

Simple case:

 

Package javaJsonAPI; import org. json. JSONStringer; public class JsonArrayTest {/** indicates that I have not found the jsonjarpackage on the official website http://www.json.org/. here, my solution is to download the source file provided on the official website *. in java *, a project named org. the json package imports all the source files. In fact, it is no different from importing the jar package in classpath */public static void main (String args []) {JSONStringer js = new JSONStringer (); js. array (); js. object (); js. key (name); js. value (hj); js. key (age); js. value (21); js. endObject (); js. object (); js. key (name); js. value (darkrake); js. key (age); js. value (21); js. endObject (); js. endArray (); System. out. println (js. toString ());}}

Small Comprehensive case:

 

 

Package javaJsonAPI; import org. json. JSONArray; import org. json. JSONObject; import org. json. JSONStringer; public class JsonArrayTest {/** indicates that I have not found the jsonjarpackage on the official website http://www.json.org/. here, my solution is to download the source file provided on the official website *. in java *, a project named org. the json package imports all the source files. In fact, it is no different from importing the jar package in classpath */public static void main (String args []) {JSONStringer js = new JSONStringer (); // create book1JSONObject book1 = new JSONObject (); book1.put (name, Java); JSONArray ja1 = new JSONArray (); string str1 [] = {LiuWANJUN, XXX}; ja1.put (str1); book1.put (author, ja1); book1.put (price, ￥108 ); // create book2JSONObject book2 = new JSONObject (); book2.put (name, JavaScript); JSONArray ja2 = new JSONArray (); String str2 [] = {LiSongFeng, CaoLi }; ja2.put (str2); book2.put (author, ja2); book2.put (price, ￥108); js. object (); js. key (Book); js. array (); js. value (book1); js. value (book2); js. endArray (); js. endObject (); System. out. println (js. toString ());}}

{  Book: [    {      author: [        [          LiuWANJUN,          XXX        ]      ],      price: ￥108,      name: Java    },    {      author: [        [          LiSongFeng,          CaoLi        ]      ],      price: ￥108,      name: JavaScript    }  ]}

Convert to XML:

 

 

  
   LiuWANJUNXXX
   
    ￥108
   
   
    Java
   
  
  
   LiSongFengCaoLi
   
    ￥108
   
   
    JavaScript
   
  

 

Summary:

1. JSON is a data exchange format, which is widely used in web projects. In particular, AJAX loads data in one step, usually in json format, rather than traditional XML;

2. JSON has only two structures: Object and Array.

3. I don't know if I wrote anything wrong. I also learned and understood it based on others' learning schemes. I will have time next week to study the JSON application in the previous section, here, we mainly write operations on the Java background.