Dart map

定义

不指定泛型

	var map1 = {'aa':'aaa','bb':22,'cc':true};//直接赋初始值
   //print(map1);//{aa: aaa, bb: 22, cc: true}
   
   Map map3 = {"a":"a1","b":"b1"};//直接赋初始值
   //print(map3);//{a: a1, b: b1}
   
   var map2 = new Map();
   map2['dd']=3.3;
   map2[2] = 'ddd22';
//print(map2);//{dd: 3.3, 2: ddd22}
//print(map2[3]);//null   如果不存在，则是null
   map2[3]= null;//key存在  value存在 value的值为null
//print(map2.length);//3    长度

指定泛型

var map1 = {};
   Map map2 = new Map();
   map2[1]="a";
   map2[2] = "b";
//print(map2);//{1: a, 2: b}
   Map map3 = {"a":1,"b":2};
//print(map3);//{a: 1, b: 2}

属性

Map map6 = {"a":1,"b":2};
//print(map6.length);//2  长度
//print(map6.isNotEmpty);//true  是否不为空
//print(map6.isEmpty);//false   是否为空
//print(map6.keys);//(a, b)   key的集合
//print(map6.values);//(1, 2)  value的集合
//print(map6.entries);//(MapEntry(a: 1), 	MapEntry(b: 2))   map迭代的键值对集合

方法

增加

 Map map7 = {"a":1,"b":2,"c":3,"d":4,"e":5};
    //新增一个key value
    map7["f"] = 6;//新增一个不存在的key
//print(map7);//{a: 1, b: 2, c: 3, d: 4, e: 5, f: 6}

修改

直接修改

Map map8 = {"a":1,"b":2,"c":3,"d":4,"e":5};
    map8["a"] = 11;
    //print(map8);//{a: 11, b: 2, c: 3, d: 4, e: 5}

update() 对指定的key的value做出修改

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

updateAll() 根据参数函数的规则，批量修改map

 Map map = {"a":1,"b":2,"c":3};
     map.updateAll((String key,int value){
       return value*2;
     });//
 	//print(map);//{a: 2, b: 4, c: 6}
 
 Map map2 = {"a":1,"b":2,"c":3};
   map2.updateAll((String key,int value){
     if(key=="a"){return 10;}
     if(key=="b"){return 20;}
     return value*2;
   });//
//print(map2);//{a: 10, b: 20, c: 6}

删除

remove() 删除一个key

Map map = {"a":1,"b":2,"c":3,"d":4,"e":5};
map.remove("b");
//print(map);//{a: 11, c: 3, d: 4, e: 5}

removeWhere() 根据条件批量删除

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

包含

containsKey() 是否包含key

Map map = {"a":1,"b":2,"c":3,"d":4,"e":5};
//print(map.containsKey("a"));//true   是否包含key
//print(map.containsKey("aa"));//false  是否包含key

containsValue() 是否包含value值

 Map map = {"a":1,"b":2,"c":3};
//print(map.containsValue(1));//true
//print(map.containsValue(4));//false

遍历

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

合并

addAll() 整体合并另一个map 泛型要一致

Map map = {"a":1,"b":2,"c":3};
    Map other = {"a":1,"c":4,"d":7};
    map.addAll(other);//key相同时value值后者覆盖前者，前者不存在时则添加进来
//print(map);//{a: 1, b: 2, c: 4, d: 7}

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

	Map map = {"a":1,"b":2,"c":3};
    Map map1 = {"a":1,"b":4,"d":3,"e":5};
    map.addEntries(map1.entries);
//print(map);//{a: 1, b: 4, c: 3, d: 3, e: 5}