关于 Dart 的重定向构造函数的疑问

上代码

class Point extends Test { num x; num y; num z; num get a { return this.x + this.y; } set a(num v) { x = v - y; } Point.fromJson(Map jsonMap) : this.x = jsonMap['x'], y = jsonMap['y'], super.fromJson({}) { print(this.x); print('In Point.fromJson(): ($x, $y, $z)'); } Point.aliasB(Map jsonMap, num z) : this.z = z, super.fromJson({}) { Point.fromJson(jsonMap); } Point(Map jsonMap, this.z) : super(jsonMap) { print('In Point(): ($x, $y, $z)'); x = jsonMap['x']; y = jsonMap['y']; print('In Point(): ($x, $y, $z)'); } } class Test { num x; num y; Test(Map map) : x = map['x'], y = map['y'] { print('In Parent.Test(): ($x, $y)'); } Test.fromJson(Map map) : this(map); } main(List<String> args) { new Point({'x': 1, 'y': 2}, 3); new Test({'x': 1, 'y': 2}); } 

执行结果

In Parent.Test(): (null, null) In Point(): (null, null, 3) In Point(): (1, 2, 3) In Parent.Test(): (1, 2) 

疑问：

1：Test 构造函数冒号右边不是赋值了吗 为啥是 null ？接受的参数打印出来是有值的

2：如果我把 Test 类的成员变量 x,y 改名字不跟子类一样，就能打印出来，这又是为啥？

希望有大佬能解答一下菜鸟的疑问