Map简介
键值对的集合,您可以使用其关联的键从中检索值。
普通的 HashMap是无序的(不保证顺序),LinkedHashMap 按键插入顺序迭代,而像 SplayTreeMap 这样的排序映射按排序顺序迭代键。
1,添加元素 addEntries()
final planets = <int, String>{1: 'Mercury', 2: 'Venus',
3: 'Earth', 4: 'Mars'};
final gasGiants = <int, String>{5: 'Jupiter', 6: 'Saturn'};
final iceGiants = <int, String>{7: 'Uranus', 8: 'Neptune'};
planets.addEntries(gasGiants.entries);
planets.addEntries(iceGiants.entries);
print(planets);
// {1: Mercury, 2: Venus, 3: Earth, 4: Mars, 5: Jupiter, 6: Saturn,
// 7: Uranus, 8: Neptune}
2,更新组元素 update()
final planetsFromSun = <int, String>{1: 'Mercury', 2: 'unknown',
3: 'Earth'};
// Update value for known key value 2.
planetsFromSun.update(2, (value) => 'Venus');
print(planetsFromSun); // {1: Mercury, 2: Venus, 3: Earth}
final largestPlanets = <int, String>{1: 'Jupiter', 2: 'Saturn',
3: 'Neptune'};
// Key value 8 is missing from list, add it using [ifAbsent].
largestPlanets.update(8, (value) => 'New', ifAbsent: () => 'Mercury');
print(largestPlanets); // {1: Jupiter, 2: Saturn, 3: Neptune, 8: Mercury}
3,更新所有元素的值 updateAll()
final terrestrial = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
terrestrial.updateAll((key, value) => value.toUpperCase());
print(terrestrial); // {1: MERCURY, 2: VENUS, 3: EARTH}
4,删除指定的元素 removeWhere
final terrestrial = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
terrestrial.removeWhere((key, value) => value.startsWith('E'));
print(terrestrial); // {1: Mercury, 2: Venus}
5,查找 [key] 的值,如果不存在,则添加新条目。 putIfAbsent
final diameters = <num, String>{1.0: 'Earth'};
final otherDiameters = <double, String>{0.383: 'Mercury', 0.949: 'Venus'};
for (final item in otherDiameters.entries) {
diameters.putIfAbsent(item.key, () => item.value);
}
print(diameters); // {1.0: Earth, 0.383: Mercury, 0.949: Venus}
// If the key already exists, the current value is returned.
final result = diameters.putIfAbsent(0.383, () => 'Random');
print(result); // Mercury
print(diameters); // {1.0: Earth, 0.383: Mercury, 0.949: Venus}
6,将所有键值对添加到此映射中。 addAll
final planets = <int, String>{1: 'Mercury', 2: 'Earth'};
planets.addAll({5: 'Jupiter', 6: 'Saturn'});
print(planets); // {1: Mercury, 2: Earth, 5: Jupiter, 6: Saturn}
7,从映射中删除 [key] 及其关联值(如果存在)。 remove
final terrestrial = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
final removedValue = terrestrial.remove(2); // Venus
print(terrestrial); // {1: Mercury, 3: Earth}
8,从映射中移除所有条目。 clear
final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
planets.clear(); // {}
9,循环应用于映射的每个键值对。 forEach
final planetsByMass = <num, String>{0.81: 'Venus', 1: 'Earth',
0.11: 'Mars', 17.15: 'Neptune'};
planetsByMass.forEach((key, value) {
print('$key: $value');
// 0.81: Venus
// 1: Earth
// 0.11: Mars
// 17.15: Neptune
});
10,此映射是否包含给定的 [key]。 containsKey
final moonCount = <String, int>{'Mercury': 0, 'Venus': 0, 'Earth': 1,
'Mars': 2, 'Jupiter': 79, 'Saturn': 82, 'Uranus': 27, 'Neptune': 14 };
final containsUranus = moonCount.containsKey('Uranus'); // true
final containsPluto = moonCount.containsKey('Pluto'); // false
11,此映射是否包含给定的 [value]。 containsValue
final moonCount = <String, int>{'Mercury': 0, 'Venus': 0, 'Earth': 1,
'Mars': 2, 'Jupiter': 79, 'Saturn': 82, 'Uranus': 27, 'Neptune': 14 };
final moons3 = moonCount.containsValue(3); // false
final moons82 = moonCount.containsValue(82); // true
12,创建一个 Map 实例,其中键和值是从 [iterable] 计算的。 fromIterable
final numbers = <int>[1, 2, 3];
final map = Map<String, int>.fromIterable(numbers,
key: (item) => item.toString(),
value: (item) => item * item);
print(map); // {1: 1, 2: 4, 3: 9}
final numbers = <int>[1, 2, 3];
final map = Map.fromIterable(numbers);
print(map); // {1: 1, 2: 2, 3: 3}
13,创建一个映射,将给定的 [键] 与给定的 [值] 相关联。 fromIterables文章来源:https://www.toymoban.com/news/detail-736619.html
final rings = <bool>[false, false, true, true];
final planets = <String>{'Earth', 'Mars', 'Jupiter', 'Saturn'};
final map = Map<String, bool>.fromIterables(planets, rings);
print(map); // {Earth: false, Mars: false, Jupiter: true, Saturn: true}
14,创建一个映射,将给定的 [键] 与给定的 [值] 相关联。 unmodifiable文章来源地址https://www.toymoban.com/news/detail-736619.html
final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
final unmodifiableMap = Map.unmodifiable(planets);
unmodifiableMap[4] = 'Mars'; // Throws
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