[fluent] dart data type (VaR data type | object data type)

Dart In language dynamic , var , Object ,

3

3

3 Type of data ;

One 、 var data type

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var It is also a keyword for defining variables , Use var Declare variables , It's also By the system according to the assignment Automatically Infer the data type of the variable ; This usage is similar to dynamic The variables defined by keywords are similar ;

Code example :

  varDemo(){ 
        //  Statement  var  Variable     var d = "CSDN";    //  Print  var  The runtime type of the variable     print(d.runtimeType);    //  Print  var  A variable's value     print(d);  }

Running results : d The type of the variable is automatically inferred as String type ;

StringCSDN

Two 、 var The data type cannot be changed

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var And dynamic The difference between variables is , once var After the dynamic data type of the variable of type is defined , The number is The data type cannot be modified , If var After the type variable is assigned a string , Assign it again int type , At this time, an error will be reported during compilation ,

3、 ... and 、 Object type

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Object yes Base classes for all objects , Some methods are defined , Definition Object Type of variable after , You can call the Object Method ;

When you assign a value to it , Sure Assign the type value of its subclass , Such as string value , amount to Polymorphic usage ;

Object Methods defined in class :

class Object { 
      const Object();  external bool operator ==(other);  external int get hashCode;  external String toString();  @pragma("vm:entry-point")  external dynamic noSuchMethod(Invocation invocation);  external Type get runtimeType;}

Code example :

  objectDemo(){ 
        //  Definition  Object  Type variable     Object d = "CSDN";    //  call  Object  Object method     //  Print  var  The runtime type of the variable     print(d.runtimeType);    //  Print  var  A variable's value     print(d);  }

Running results : Variable d The runtime type of is String type ;

StringCSDN

Object Variables and dynamic The difference between variables is , Object The data type is certain , Namely Object type , Object Type variable cannot be called Object A method that does not exist in a class ;

Four 、 Complete code example

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Complete code example :

import 'package:flutter/material.dart';class DartType extends StatefulWidget { 
      @override  _DartTypeState createState() => _DartTypeState();}class _DartTypeState extends State<DartType> { 
      @override  Widget build(BuildContext context) { 
        dynamicDemo();    varDemo();    objectDemo();    return Container();  }  dynamicDemo(){ 
        dynamic d = "CSDN";    //  Print  dynamic  The runtime type of the variable     print(d.runtimeType);    //  Print  dynamic  A variable's value     print(d);    //  call  dynamic  Variable method ,  Static compilation cannot check for errors ,  The runtime will report an error     //d.getName();    //  by  dynamic  Variable assignment  int  data     d = 666;    //  Print  dynamic  The runtime type of the variable     print(d.runtimeType);    //  Print  dynamic  A variable's value     print(d);  }  varDemo(){ 
        //  Statement  var  Variable     var d = "CSDN";    //  Print  var  The runtime type of the variable     print(d.runtimeType);    //  Print  var  A variable's value     print(d);    //  Will have been automatically inferred as  String  Type of  d  Assign a value to a variable  int  Type values     //  At this time, errors will appear in the compilation     //d = 666;  }  objectDemo(){ 
        //  Definition  Object  Type variable     Object d = "CSDN";    //  call  Object  Object method     //  Print  var  The runtime type of the variable     print(d.runtimeType);    //  Print  var  A variable's value     print(d);    //  call  Object  If there is no method, an error will be reported     //d.getName()  }}

Running results :

StringCSDNint666StringCSDNStringCSDN

5、 ... and 、 Related resources

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Reference material :

• Dart Developer website : https://api.dart.dev/
• Flutter Chinese net ( unofficial , The translation is very good ) : https://flutterchina.club/ , http://flutter.axuer.com/docs/
• Flutter Official website : https://flutter.dev/ ( Quilt wall )
• official GitHub Address : https://github.com/flutter
• Flutter Related issues : https://flutterchina.club/faq/ ( It is recommended to watch it at the introductory stage )

Blog source download :

• GitHub Address : https://github.com/han1202012/flutter_app_hello ( Keep updating with the progress of the blog , There may not be the source code of this blog )

• Blog source snapshot : https://download.csdn.net/download/han1202012/15113321 ( The source code snapshot of this blog , You can find the source code of this blog )