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Dart series: built in types in dart language

2021-11-25 17:49:56 What about the procedure

brief introduction

Like all programming languages ,dart There are built-in language types , These built-in types inherit from Object, Of course, these built-in types are dart The basis of language , Only by mastering these built-in types can we use dart Language is handy .

Let's talk about it today dart The built-in type of language .

Null

stay dart of use null To represent null . that null and Null What does it matter ?

Null Is a class , Let's take a look at Null The definition of :

       
class Null {
factory Null._uninstantiable() {
throw UnsupportedError('class Null cannot be instantiated');
}

external int get hashCode;

/** Returns the string `"null"`. */
String toString() => "null";
}
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You can see Null Type of string It means null. And the corresponding null It's a keyword , It corresponds to Null class .

Numbers

dart The corresponding class of numbers in is num, He has two subcategories , Namely int and double.

int No greater than 64 bits The integer of . because dart Can run on different platforms , Therefore, the scope represented by different platforms is also different .

On native platform , such as android perhaps IOS platform ,int It can reach -2^63 To 2^63 – 1. But in web Environment , The representable range is -2^53 To 2^53 – 1.

double The corresponding represents the floating point type .

For numbers , The basic operators are like +, -, / and * It's all defined in num Class . Of course, there are other regular operators .

If more complex operations are needed , You can use dart:math library .

Here are some examples of the use of numbers :

       
int age =18;
int number= 20;
double money = 10.1;
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character string

String is a type that is often used .dart The class corresponding to the string in is String. It can also be expressed directly in literal terms as follows :

       
var name ='jack';
var site ="www.flydean.com";
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Strings can be expressed in single or double quotation marks .dart The string in is UTF-16 code .

dart In the string in , You can also bring variable values , His format is ${expression}.

       
var age=10;
var words ='your age is ${age}!';
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Two strings can be used To see if it's equal , The character comparison is the corresponding character coding sequence , If the character encoding sequences are equal , Then the corresponding strings are equal .

String connection can use +.

       
var hello ="hello " + "word";
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Another way to create a string is to use three single quotes or three double quotes .

       
var string1= '''
this is a string!
''';

var string2 = """
this is string again!
""";
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By default string The character representation in is the character itself , If you want to convert it to its original meaning , You can prefix the string with r:

       
var string3 =r'this is line one \n this is line two';
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Boolean value

Boolean values are in dart of use bool To express .bool The value is only represented by two strings , Namely true and false.

because dart It's type safe , That is, when you need to use bool When it comes to type , No other type can replace .

For example, we need to determine whether the string is empty , It can be judged that :

       
var name='';
if(name.isEmpty){
do something
}
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list

dart The list in... Is used List To express , Of course, you can also directly use the following literal to express :

       
var list = [1, 2, 3];
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List index from 0 Start , With length-1 end .

from dart 2.3 Start , The extension character is introduced … And nullable extenders …? , The extender can be used to put a known list Expand to its corresponding element , Thus, it is convenient to build list The combination of :

       
var list = [1, 2, 3];
var list2 = [0, ...list];
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dart Provides a very magical function , Is that you can build list Used in the process of if and for sentence , To generate dynamically list The elements in :

       
var nav = [
'Home',
'Furniture',
'Plants',
if (promoActive) 'Outlet'
];
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perhaps :

       
var listOfInts = [1, 2, 3];
var listOfStrings = [
'#0',
for (var i in listOfInts) '#$i'
];
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set and map

dart The set in is used Set To express .

set Represents a collection of non repeating elements , As shown below :

       
var names = {'jack', 'mark', 'max'};
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dart The mapping in Map To express .

Map The creation and set Is very similar , But it includes key and value:

       
var students = {'jack':18, 'mark':19, 'max':20};
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You can see ,set and map It's very similar , So here comes the question , How to express an empty set perhaps map Well ?

because set The elements in are single , and map The element in is a key value pair , So we can say :

       
var names = <String>{};
var gifts = Map<String, String>();
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But if you don't specify the type , Then the default is to create a map:

       
var map = {};
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To get map The value in , It can be used in this way :

       
var gifts = Map<String, String>();
gifts['first'] = 'partridge';
gifts['second'] = 'turtledoves';
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map and set All support extensions … And nullable extensions …? , It also supports if and for operation .

This article has been included in  ​ ​http://www.flydean.com/02-dart-buildin-type/​

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