Dart 集合类型Map

发布于 2022-04-17  150 次阅读

定义

map以键值对存值

不指定泛型

var map1 = {'aa': 'aaa', 'bb': 'bbb', 'cc': true}; //直接赋值初始值
print(map1); //{aa: aaa, bb: bbb, cc: true}

Map map2 = {'a': 'a1', 'b': 'b1'}; //直接赋值初始值
print(map2); //{a: a1, b: b1}

map2['c'] = 'c1';
print(map2); //{a: a1, b: b1, c: c1}

var map3 = new Map();
map3['a'] = 'a3';
map3[2] = 'b3';
print(map3); //{a: a3, 2: b3}
print(map3[3]); //null 如果不存在，则为null
map3[3] = null; //key存在 value存在 value的值为null
print(map3); //{a: a3, 2: b3, 3: null}

指定泛型

var map4 = <String, String>{};
Map<int, String> map5 = new Map(); //new也可以省略
map5[1] = 'a5';
map5[2] = 'b5';
print(map5); //{1: a5, 2: b5}

Map<String, int> map6 = {'a6': 1, 'b6': 2};
print(map6); //{a6: 1, b6: 2}

Map常见属性

Map<String, int> map7 = Map();
map7['a7'] = 1;
map7['b7'] = 2;
map7['c7'] = 3;
print(map7.length); //3 长度
print(map7.isNotEmpty); //true  是否不为空
print(map7.isEmpty); //flase  是否为空
print(
    '${map7.keys},${map7.keys.runtimeType}'); // (a7, b7, c7),_CompactIterable<String>    Map的所有key
print(
    '${map7.values},${map7.values.runtimeType}'); //(1, 2, 3),_CompactIterable<int>    Map的所有value
print(
    '${map7.entries},${map7.entries.runtimeType}'); // (MapEntry(a7: 1),
MapEntry(b7: 2), MapEntry(c7: 3)),MappedIterable<String, MapEntry<String, int> Map的所有键值对

Map常见方法

  • 新增一个键值对
Map<String, int> map8 = Map();
map8['a8'] = 1;
print(map8); //{a8: 1}
  • 修改一个键值对
Map<String, int> map9 = {'a9': 1, 'b9': 2};
map9['a9'] = 9;
print(map9); //{a9: 9, b9: 2}
  • update(K key, V update(V value), {V ifAbsent()}) 根据指定的Key对应的value做出修改，同时Map本身也会被修改
Map<String, int> map10 = {'a10': 1, 'b10': 2, 'c10': 3};
var resultMap10 = map10.update('b10', (value) => value * 2);
print(resultMap10); //4
print(map10); //{a10: 1, b10: 4, c10: 3}
  
var resultMap101 = map10.update('c', (value) => (value * 2),
    ifAbsent: () => (10)); //如果key不存在，但是有ifAbsent参数，返回idAbsent函数的值，并添加到map中
print('$resultMap101,${resultMap101.runtimeType}'); //10,int
print(map10); //{a10: 1, b10: 4, c10: 3, c: 10}
  • updateAll(V update(K key, V value)) 根据参数内的函数，修改map
Map<String, int> map11 = {'a11': 2, 'b11': 3, 'c11': 4};
var resultMap11 = map11.updateAll((key, value) {
  return value * 2;
});
print(map11); //{a11: 4, b11: 6, c11: 8}
map11.updateAll((key, value) {
  if (key == 'a11') {
    return value * 2;
  }
  if (key == 'c11') {
    return value + 1;
  }
  return 7; //如果没有这行，b11对应的value为null 这里可以看出updateall会影响所有的键值对
});
print(map11); //{a11: 8, b11: 7, c11: 9}
  • remove() 删除一个key
Map<String, int> map12 = {'a12': 2, "b12": 1};
map12.remove('a12');
print(map12); //{b12: 1}
map12.remove('c12'); //删除一个不存在的key，毫无影响，无报错无警告
print(map12); //{b12: 1}
  • removeWhere(bool predicate(K key, V value)) 根据函数条件批量删除key
Map<String, int> map13 = {'a13': 3, 'b13': 4, 'c13': 1};
map13.removeWhere((key, value) => value > 3);
print(map13); //{a13: 3, c13: 1}
  • containsKey() 是否包含某个key contrainsValue()是否包含某个value
Map<String ,int> map14 = {'a14':1};
bool resultMap14 = map14.containsKey('a11'); //false
bool resultMap141 = map14.containsValue(1); //true
  • forEach(void f(K key, V value)) 遍历Map ,遍历时不可add或者remove
Map<String, int> map15 = {'a15': 1, 'b15': 2, 'c': 3, 'd': 4, 'e': 5};
map15.forEach((key, value) {
  print('$key,$value');
  /*
  a15,1
  b15,2
  c,3
  d,4
  e,5
  */
  });
  map15.forEach((key, value) {
  //  if(key == 'a15'){value = 8;}
  map15['a15'] = 8;
});
print(map15); //{a15: 8, b15: 2, c: 3, d: 4, e: 5}
   
复制代码
  • map() 遍历每个键值对 根据参数函数，对keyvalue做出修改，转换成其他泛型Map
Map<String,int> map16 = {'a16':7,"b16":5,'c16':4};
Map<int,String> map17 = map16.map((key, value) {
     return MapEntry(value, key);
});
print(map17);
  • addAll() 两个Map合并，类型需要一致 ，且如果key相同，则会覆盖value
Map<String,int> map18 = {'a18':1,'b18':7,'a19':2};
Map<String,int> map19 = {'a19':9};
map18.addAll(map19);
print(map18); //{a18: 1, b18: 7, a19: 9}
  • addEntres(key,value) 两个Map合并，类型需要一致 ，且如果key相同，则会覆盖value
Map<String,int> map20 = {'a20':2,'b20':3};
Map<String,int> map21 = {'a21':5,'b21':9};
map20.addEntries(map21.entries);
print(map20); //{a20: 2, b20: 3, a21: 5, b21: 9}
  • putIfAbsent() 存在key则返回value，查不到则返回值 不修改Map
Map<String,int> map22 = {'a22':3,'b22':4};
var resultMap22 = map22.putIfAbsent('a22', () => 2); //存在key则返回value，查不则返回 2 不修改Map
print('$resultMap22,$map22');//3,{a22: 3, b22: 4} 
 
var resultMap221 = map2.putIfAbsent('a2', () => 1);
print('$resultMap221,$map22'); //1,{a22: 3, b22: 4} //存在key则返回value，查不到返回 1 不修改Map
  • cast() 泛型提升为其父祖类
Map<String,int> map23 = {'a33':3};
Map<Object,Object> map24 = map23.cast();
print('$map23,${map23.runtimeType},$map24,${map24.runtimeType}'); //{a33:3},_InternalLinkedHashMap<String, int>,{a33: 3},CastMap<String, int,Object, Object>
  • 清除所有键值对，对所有类型map都适用
Map<String,int> map25 = {'a25':2,'b25':3};
map25.clear();
print('$map25,${map25.runtimeType}'); //{},_InternalLinkedHashMap<String, int>

谢谢你能看完呀~~~