Dart（五）Map的属性和方法实例

转  https://www.jianshu.com/p/3da39cd895a1

声明

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} map3["c"] ="c"; //print(map3);//{a: a1, b: b1, c: c} 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 map333 = <String,String>{}; Map<int,String> map4 = new Map(); map4[1]="a"; map4[2] = "b"; // print(map4);//{1: a, 2: b} Map<String,int> map5 = {"a":1,"b":2}; // print(map5);//{a: 1, b: 2} 

属性

    Map<String,int> 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

新增一个key-value

    Map<String,int> 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} 

修改一个key的value

    Map<String,int> 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<String,int> map23 = {"a":1,"b":2,"c":3}; int result3 = map23.update("a", (value)=>(value*2));//key存在 根据参数函数修改其值 // print(result3);//2 // print(map23);//{a: 2, b: 2, c: 3} // int result4 = map23.update("d", (value)=>(value*2));//key不存在 报错 int result4 = map23.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<String,int> map24 = {"a":1,"b":2,"c":3}; map24.updateAll((String key,int value){ return value*2; });// // print(map24);//{a: 2, b: 4, c: 6} 

Map<String,int> map25 = {"a":1,"b":2,"c":3}; map25.updateAll((String key,int value){ if(key=="a"){return 10;} if(key=="b"){return 20;} return value*2; });// // print(map25);//{a: 10, b: 20, c: 6} 

remove() 删除一个key

    Map<String,int> map9 = {"a":1,"b":2,"c":3,"d":4,"e":5}; map9.remove("b"); //print(map9);//{a: 11, c: 3, d: 4, e: 5} 

removeWhere() 根据条件批量删除

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

containsKey() 是否包含key

    Map<String,int> map11 = {"a":1,"b":2,"c":3,"d":4,"e":5};
//    print(map11.containsKey("a"));//true   是否包含key
//    print(map11.containsKey("aa"));//false  是否包含key

containsValue() 是否包含value值

    Map<String,int> map17 = {"a":1,"b":2,"c":3}; // print(map17.containsValue(1));//true // print(map17.containsValue(4));//false 

forEach() 遍历

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

//遍历时修改value值 Map<String,int> map13 = {"a":1,"b":2,"c":3}; map13.forEach((String key,int value){ // print("$key $value"); map13["c"] = 4; // a 1 // b 2 // c 4 }); 

//遍历时，新增或删除key 都会报错 Map<String,int> map14 = {"a":1,"b":2,"c":3}; map14.forEach((String key,int value){ // print("$key $value"); // map14["d"] = 4;// 报错 // map14.remove("a");// 报错 }); 

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); }); // print(map20);//{1: a, 2: b, 3: c} 

clear() 清空map

    Map<String,int> map15 = {"a":1,"b":2,"c":3}; map15.clear(); // print(map15);//{} 

addAll() 整体合并另一个map 泛型要一致

    Map<String,int> map16 = {"a":1,"b":2,"c":3}; Map<String,int> other = {"a":1,"c":4,"d":7}; map16.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(map16);//{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<String,int> map26 = {"a":1,"b":2,"c":3}; Map<String,int> map27 = {"a":1,"b":4,"d":3,"e":5}; map26.addEntries(map27.entries); // print(map26);//{a: 1, b: 4, c: 3, d: 3, e: 5} 

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(); map22["d"]=33;