Dart（五）Map的属性和方法实例
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;