85 lines
1.9 KiB
Dart
85 lines
1.9 KiB
Dart
///
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/// build in data types
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/// - All data types in Dart are Objects.
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///
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main(List<String> args) {
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// Numbers
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var num1 = 100; // type 'int'
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int num2 = 200;
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double num3 = 30.0;
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// float num4; [Error] dart语言没有float关键字，使用double
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print(
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"Numbers: num1=$num1, num2=$num2, num3=$num3, num1+num2=${num1 + num2}");
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//num1 = num3; [Error] A value of type 'double' can't be assigned to a variable of type 'int'
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num1 = num2; // 'var' 定义的变量首次赋值后类型就不可以改变了, num1为'int'类型
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print('');
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// Strings
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String name = "Dart";
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var company = "Google"; // type 'String'
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print("$company $name");
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// 多行文本
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String lines = 'hello \n'
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'dart \n'
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'Strings';
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print(lines);
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// 字符串可以用单引号，也可以用双引号
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var s1 = ' "Hello" ';
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var s2 = " 'Dart' ";
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var s3 = " \"Google\" ";
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print("$s1, $s2, $s3");
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print('');
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// bool
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bool isValid = true;
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var isAlive = false; // type 'bool'
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print("$isValid $isAlive");
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print('');
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// dynamic
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int number; // 未赋值的变量默认值为'null'
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print(number == null ? 'null' : 'not null');
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number = 123;
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dynamic obj = number;
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print("obj is 'int', value is $obj");
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obj = "Hello";
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print("obj is 'String', value is $obj");
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print('');
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// final
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final langName = 'Dart';
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//langName = 'Java'; [Error] a final variable, can only be set once.
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// const
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const PI = 3.14;
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print('$langName, $PI');
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print('');
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var c1 = Circle(20.0);
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var c2 = Circle(30.0);
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print("c1: ${c1.radius}");
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print("c2: ${c2.radius}");
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print("PI: ${Circle.PI}");
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}
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class Circle {
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//const PI = 3.14; [Error] Only static fields can be declared as const.
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// 常量必须在定义时赋值
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static const PI = 3.14;
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// final 的变量可以在构造函数中赋值
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final double radius;
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Circle(this.radius);
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}
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