Map定义
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}
- 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，查不到则返回 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，查不到则返回 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>
