19、Java面向对象编程：子类、继承与封装

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Java面向对象编程：子类、继承与封装

1. 隐藏超类字段

在编程中，有时子类可能会定义与超类中同名的字段，这种情况被称为字段隐藏。

假设 PlaneCircle 类需要知道圆心到原点 (0, 0) 的距离，我们可以添加一个实例字段来保存这个值：

public double r;

在构造函数中添加以下代码来计算该字段的值：

this.r = Math.sqrt(cx*cx + cy*cy);  // 勾股定理

这里的新字段 r 与 Circle 超类中的半径字段 r 同名，此时 PlaneCircle 的 r 字段就隐藏了 Circle 的 r 字段。

要引用不同类中的同名字段，可以使用以下方式：
- r 和 this.r ：引用 PlaneCircle 类的字段。
- super.r ：引用 Circle 类的字段。
- 也可以通过将对象强制转换为适当的超类类型来引用隐藏字段，例如 ((Circ

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