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?? 什么鬼

## ??= 短路賦值運(yùn)算符, TS ?

```dart

void main() {

dynamic d;

print("d = $nhkv1sx5v02o");

// null || 1;

// ??= 短路賦值運(yùn)算符, TS ?

d ??= 1;

print("d ??= $nhkv1sx5v02o");

// var n = null;

// "" ?不好使

// var n = "";

// n ??= "string";

// print("n ??= ${n}");

// 0 ?不好使

// var n = 0;

// n ??= 7;

// print("n ??= ${n}");

// false ?不好使

// var n = false;

// n ??= true;

// print("n ??= ${n}");

print("\n");

int i = 0;

print("i = ${i}");

i += 3;

print("i += ${i}");

i -= 1;

print("i -= ${i}");

i *= 3;

print("i *= ${i}");

// int ?, /= 必須是 double 類型,

// i /= 2;

// print("i /= ${i}");

i %= 2;

print("i %= ${i}");

// double, ? /= 必須是 double 類型

double dd = 2.0;

dd /= 2;

print("dd /= ${dd}");

// int => double, ? / 必須是 double 類型

int x = 3;

// x = x / 2;

print("x / 2 = ${x / 2}");

}

```

??= 什么鬼

## ??= 短路賦值運(yùn)算符, TS ?

```dart

void main() {

dynamic d;

print("d = $nhkv1sx5v02o");

// null || 1;

// ??= 短路賦值運(yùn)算符, TS ?

d ??= 1;

print("d ??= $nhkv1sx5v02o");

// var n = null;

// "" ?不好使

// var n = "";

// n ??= "string";

// print("n ??= ${n}");

// 0 ?不好使

// var n = 0;

// n ??= 7;

// print("n ??= ${n}");

// false ?不好使

// var n = false;

// n ??= true;

// print("n ??= ${n}");

print("\n");

int i = 0;

print("i = ${i}");

i += 3;

print("i += ${i}");

i -= 1;

print("i -= ${i}");

i *= 3;

print("i *= ${i}");

// int ?, /= 必須是 double 類型,

// i /= 2;

// print("i /= ${i}");

i %= 2;

print("i %= ${i}");

// double, ? /= 必須是 double 類型

double dd = 2.0;

dd /= 2;

print("dd /= ${dd}");

// int => double, ? / 必須是 double 類型

int x = 3;

// x = x / 2;

print("x / 2 = ${x / 2}");

}

```

math operator

list.asMap

keys, values

```dart

print("map.keys = ${map.keys}");

print("map.values = ${map.values}");

print("map.entries = ${map.entries}");

```

[], length

isEmpty, isNotEmpty

Keys, Values, Entries

containsKey(), containsVlaue()

remove()

forEach()

```dart

void main() {

dynamic map = {"a": 1, "b": 2, "c": 3, "str": "string", "bool": true};

print("map = $map");

const mc = {"key": 1, "str": "string", "bool": true};

print("mc = $mc");

// var m = new Map("key", 1);

// var m = new Map({"key": 1});

var m = new Map();

// m.addEntries({"key": 3});

// m.addEntries();

m.addAll({"key": 3});

print("m = $m");

}

```

```dart

/*

?void forEach(void f(E element)) {

? ?for (E element in this) f(element);

?}

*/

```

forEach() 不是 List 專有的，只是一個(gè)通用迭代器方法

```dart

/**

*

* @author xgqfrms

* @license MIT

* @copyright xgqfrms

* @created 2019-01-01

*

* @description

* @augments

* @example

*

*/

void main() {

dynamic list = [1, 2, 3, "string", false];

print("list = $list");

list.forEach(print);

print("\n");

list.forEach((key, i) => {

print("list = $key, $i")

// print("list = $key");

// Error: Expected '}' before this.

});

}

// forEach() 不是 List 專有的，只是一個(gè)通用迭代器方法

/*

list = [1, 2, 3, string, false]

list = 1

list = 2

list = 3

list = string

list = false

*/

```

[], length

add(), insert()

remove(), clear()

indexOf(), lastIndexOf()

sort(), sublist()

shuffle(), asMap(), forEach()

list === array

```dart

void main() {

String s = "string";

print("s = $s");

String replaceAll = "string string".replaceAll("s", "S");

print("replaceAll = $replaceAll");

String replaceFirst = "string string".replaceFirst("s", "s");

print("replaceFirst = $replaceFirst");

String replaceFirstMapped = "string string".replaceFirstMapped("s", (match) => null);

print("replaceFirstMapped = $replaceFirstMapped");

String replaceAllMapped = "string string".replaceAllMapped("s", (match) => null);

print("replaceAllMapped = $replaceAllMapped");

String replaceRange = "string string".replaceRange(0, 1, "S");

print("rreplaceRange = $replaceRange");

}

```

方法

contains(), substring()

startsWith(), endsWith()

indexOf(), lastIndexOf()

toLowerCase(), toUpperCase()

trim(), trimLeft(), trimRight()

split(), replace???()

屬性 length， isEmpty, isNotEmpty