constructor - 如何在构造函数中初始化最终类属性？

Question

在 Java 中你可以这样做:

class A {    
    private final int x;

    public A() {
        x = 5;
    }
}

在 Dart 中，我尝试过:

class A {    
    final int x;

    A() {
        this.x = 5;
    }
}

我收到两个编译错误:

The final variable 'x' must be initialized.

和

'x' can't be used as a setter because its final.

有没有办法在 Dart 的构造函数中设置最终属性？

Answer 1

您无法在构造函数主体中实例化最终字段。有一个特殊的语法:

class Point {
  final num x;
  final num y;
  final num distanceFromOrigin;

  // Old syntax
  // Point(x, y) :
  //   x = x,
  //   y = y,
  //   distanceFromOrigin = sqrt(pow(x, 2) + pow(y, 2));

  // New syntax
  Point(this.x, this.y) :
    distanceFromOrigin = sqrt(pow(x, 2) + pow(y, 2));
}