Question

如何将类的实例强制转换为子类并添加属性以便不抛出ClassCastException？例如：

public class Shape {
  private int length;
  private int width;
  public Shape(int length, int width) {
    this.length = length;
    this.width = width;
  }
}
public class Solid extends Shape {
  private int height;
  public Solid (int length, int width, int height) {
    super(length, width);
    this.height = height;
  }
}
public class Test {
  public static void main(String[] args) {
    Shape shape = new Shape(1, 2);
    //Do something to change the shape instance to solid instance.
    Solid solid = (Solid) shape;//Makes it does not throw ClassCastException.
    System.out.println(shape instanceof Solid);//Makes it print true.
    System.out.println(shape == solid);//Makes it print true.
  }
}

我知道我可以创建一个Solid的新实例并从旧实例导入属性，但我想将属性添加到旧实例，所以比较==返回true。有什么想法吗？

Answer 1

您可以通过向Solid添加一个接受Shape作为参数的构造函数来接近您想要的内容：

public Solid (Shape shape) {
    this(shape.getLength(), shape.getWidth(),0);
}

并且测试是：

Shape shape = new Shape(1, 2);
shape = new Solid(shape);
System.out.println(shape instanceof Solid);//prints true.
Solid solid = (Solid) shape;
System.out.println(shape == solid);//prints true.

将属性添加到超类实例以使其成为子类实例

1 个答案: