Dart -- Map

var map1 = {'aa':'aaa','bb':22,'cc':true};//直接赋初始值
Map map3 = {"a":"a1","b":"b1"};//直接赋初始值
map3["c"] ="c";
var map2 = new Map();
map2['dd']=3.3;
map2[2] = 'ddd22';
var map333 = <String,String>{};
Map<int,String> map4 = new Map();
Map<String,int> map5 = {"a":1,"b":2};

方法

• update() 对指定的key的value做出修改

      Map<String,int> map23 = {"a":1,"b":2,"c":3};
    //key存在  根据参数函数修改其值,key不存在  报错
    int result3 = map23.update("a", (value)=>(value*2));
  //key不存在  但有ifAbsent参数 返回ifAbsent函数的值  并添加到map中
    int result4 = map23.update("d", (value)=>(value*2),ifAbsent: ()=>(10));  
  

• updateAll() 根据参数函数的规则，批量修改map

  Map<String,int> map24 = {"a":1,"b":2,"c":3};
  map24.updateAll((String key,int value){
    return value*2;
  });
  

• remove() 删除一个key

   Map<String,int> map9 = {"a":1,"b":2,"c":3,"d":4,"e":5};
   map9.remove("b");
  

• removeWhere() 根据条件批量删除

   Map<String,int> map10 = {"a":1,"b":2,"c":3,"d":4,"e":5};
  //删除掉 符合参数函数的keyvalue对
   map10.removeWhere((key,value)=>(value>3));
  

• containsKey() 是否包含key

• containsValue() 是否包含value值

• forEach() 遍历时可以修改value值，遍历时，新增或删除key 都会报错

   Map<String,int> map12 = {"a":1,"b":2,"c":3,"d":4,"e":5};
   map12.forEach((String key,int value){
   //      print("$key  $value");
  });
  

• map() 遍历每个元素 根据参数函数，对keyvalue做出修改，可转换成其他泛型的Map

  Map<String,int> map19 = {"a":1,"b":2,"c":3};
  Map<int,String> map20 = map19.map((String key,int value){
   return new MapEntry(value, key);
  });
  

• clear() 清空map

• addAll() 整体合并另一个map 泛型要一致，key相同时value值后者覆盖前者，前者不存在时则添加进来

• addEntries()合并两个map 如果key有重复，被合并的map的value覆盖前者

• putIfAbsent() 存在key就获取值，不存在则添加到map 然后返回值

  Map<String,int> map18 = {"a":1,"b":2,"c":3};
  int result = map18.putIfAbsent("a", ()=>(2));//存在
  print(result);//1   获取key的值
  print(map18);//{a: 1, b: 2, c: 3}   map不变
  int result2 = map18.putIfAbsent("d", ()=>(2));//不存在
  print(result2);//2   获取新的key的value
  print(map18);//{a: 1, b: 2, c: 3, d: 2}   map改变
  

• cast() 泛型类型提升为其父祖类

  Map<String,int> map21 = {"a":1,"b":2,"c":3};
  Map<Object,Object> map22 = map21.cast();