Learn Flutter from the Dart start series (I) basic data types

1 introduction of the Dart

Dart is an open source, general-purpose programming language. It was originally developed by Google as a replacement for the front-end JS, but as JS exploded, Dart’s position became awkward and tepid. That is, until Flutter comes to Dart’s rescue again. Dart has a lot in common with JS, but it also has some unique features. Let’s take a look at Dart in this series of articles.

2 Variable Declaration

var

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Dart, like JavaScript, uses var to declare variables, but the type of a variable is determined once it is assigned to Dart and cannot be changed otherwise it will return an error.

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var a = 1;

a = "123"; // The compiler will warn

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Final and const

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The use of final and const in Dart is no different from that in other languages. Final means that a variable can only be assigned once and cannot be changed after the assignment or an error will be reported. Const represents a compile-time constant. Variable types that are final or const can be omitted.

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final a = 1;                 //final int a = 1

const b = "abc";     //final String b = "abc"

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The dynamic and Object

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Object is the base class of all objects, and all types are subclasses of Object, so all variables can be declared as Object. Variables declared by Object and Dynamic can also be assigned to any Object. Object and Dynamic are similar to var in this respect, except that the type of variables declared by Object and Dynamic can be changed after assignment.

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 dynamic a = 1;

  a = "123";

  print(a); // No error, output 123



  Object b = 2;

  b = "abc";

  print(b); // no error, output ABC

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The difference between Object and Dynamic is that Object can only use Object methods. The Object compiler for dynamic declarations provides all possibilities.

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dynamic a = 1;     

a = "123";         

print(a.length); 3 / / output

                   

Object b = 2;      

b = "abc";         

print(b.length); Error: The getter'length' isn't defined for the class 'Object'.

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3 Basic data type

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Dart uses five common data types: Number, String, Boolean, List, and Map.

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Number

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Number contains two data types, int for integer and double for floating point. An integer can be automatically converted to a floating-point type when declared, but a floating-point type cannot be automatically converted to an integer.

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double a = 1; // No problem

Int b = 1.1; / / complains

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• Common properties and methods

int a = 1;  

int b = -1;            

Double c = 1.4;

print(a.isEven); \ \false

print(a.isNaN); \ \false

print(a.isOdd); \ \true

print(a.toString()); \ \ 1

print(c.floor()); \ \ 2

print(c.round()); \ \ 1

print(c.ceil()); \ \ 1

print(b.abs()); \ \ 1

print(a.remainder(2)); \ \ 1

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String

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String values in Dart can be expressed in single, double, or triple quotes. Single-line strings are represented by single or double quotation marks. Triple quotes are used to represent multi-line strings. Dart can concatenate strings with + and also supports interpolation of ${}.

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String a = "hello";         

 String b = "world";         

 print(a + ""+ b); \\ Output hello world

                             

 int c = 1;                  

 print("This is ${c}"); \\ Output This is 1

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• Commonly used methods and properties

String a = "Hello";           

String b = "a b c d";         

print(a.isEmpty); \ \false

print(a.length); 5 \ \

print(a.substring(2));        \\llo

print(a.toLowerCase());       \\hello

print(a.toUpperCase());       \\HELLO

print(b.split(""));          [a, b, c, d]

print(a.indexOf("o")); 4 \ \

print(a.startsWith("H")); \ \true

print(a.replaceAll("l"."o"));  \\heooo

print(a.contains("ll")); \ \true

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Boolean

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Dart Boolean has only two values true and false.

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List

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A List in Dart is similar to an array in other languages, being an ordered collection of objects. You can declare either a List of fixed length or a List of indefinite length.

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List list1 = new List(3); // Declare a List of fixed length

List list2 = new List(); // Declare an indefinite List



/ / the List initialized

  list1[0] = 0;

  list1[1] = 1;

  list1[2] = 2;

  print(list1); Output [0, 1, 2]

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• List Common attributes and methods

List list1 = new List(3); // Declare a List of fixed length

List list2 = new List(); // Declare an indefinite List



/ / the List initialized

  list1[0] = 0;

  list1[1] = 1;

  list1[2] = 2;

  print(list1.length); / / 3

  print(list1.first); / / 0

  print(list1.last); / / 2

  print(list1.reversed); / / (2, 1, 0]

  print(list1.isEmpty);                 //false

  Map map = list1.asMap();      

  print(map); //{0:0, 1:1, 2:2}

  list2.add(1);                              

  print(list2); / / [1]

  list2.clear();

  print(list2); / / []

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Map

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A Map object is a simple key/value pair. Keys and values in a Map can be of any type. A Map is a dynamic collection. In other words, a Map can grow and shrink a Map object at run time as a simple key/value pair. A Map can be declared either by a literal or by a constructor.

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Map map = {"name" : "Joe"."sex" : "male"}; // declare by literal

Map map1 = new Map(); // declare by constructor

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• Common attributes and methods of Map

 Map map = {"name" : "Joe"."sex" : "male"}; // declare by literal

Map map1 = new Map(); // declare by constructor

print(map.length); \ \ 2

print(map.keys); \ \ ["name"."sex"]

print(map.values); \ \ ["Joe"."male"]

print(map.containsKey("name")); \ \true

print(map.containsValues("John")); \ \false

print(map.isEmpty); \ \false

print(map.remove("name")); \ \"Joe"

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The last

Today, we briefly introduced Dart variable declarations and simple data types, but you can write them yourself. Paper come zhongjue shallow, realize this to practice. How we feel good, “like” is the biggest support for more articles we can pay attention to wechat public number QStack, pursue the purest technology, enjoy the joy of programming.