从Java中的基类访问子类字段 [英] Access subclass fields from a base class in Java

问题描述

我有一个名为 Geometry 的基类，其中存在一个 子类：

< public class Geometry
{
String shape_name;
String material;

public Geometry（）
{
System.out.println（New geometry object created。
}
}


和一个子类：

  public class Sphere extends Geometry 
 {
 Vector3d center; 
 double radius; 
 
 public Sphere（Vector3d coords，double radius，String sphere_name，String material）
 {
 this.center = coords; 
 this.radius = radius; 
 super.shape_name = sphere_name; 
 super.material = material; 
} 
} 
  

我有一个ArrayList包含所有几何对象，我想迭代它来检查文本文件中的数据是否被正确读取。这是我的迭代器方法到目前为止：

  public static void check（）
 {
 Iterator< Geometry> ; e = objects.iterator（）; 
 while（e.hasNext（））
 {
几何g =（几何）e.next（）; 
 if（g instanceof Sphere）
 {
 System.out.println（g.shape_name）; 
 System.out.println（g.material）; 
} 
} 
} 
  

如何访问和打印出现 半径和中心字段？
提前感谢：）

解决方案

如果你想访问子类的属性，以投射到子类。

  if（g instanceof Sphere）
 {
 Sphere s = ） G; 
 System.out.println（s.radius）; 
 .... 
} 
  

这不是最OO的方式做一些事情，虽然：一旦你有更多的子类的几何，你将需要开始投射到每个类型，这很快就变成了一个混乱。如果要打印对象的属性，您应该在您的几何对象上有一个名为print（）的方法或者沿着这些行的方法，它将打印对象中的每个属性。像这样：

  
 class Geometry {
 ... 
 public void print ）{
 System.out.println（shape_name）; 
 System.out.println（material）; 
} 
} 
 
类形状extends几何{
 ... 
 public void print（）{
 System.out.println半径）; 
 System.out.println（center）; 
 super.print（）; 
} 
} 
  

这样，你不需要你可以在你的while循环中调用g.print（）。

I have a base class called Geometry from which there exists a subclass Sphere:

public class Geometry 
{
 String shape_name;
 String material;

 public Geometry()
 {
     System.out.println("New geometric object created.");
 }
}

and a subclass:

public class Sphere extends Geometry
{
 Vector3d center;
 double radius;

 public Sphere(Vector3d coords, double radius, String sphere_name, String material)
 {
  this.center = coords;
  this.radius = radius;
  super.shape_name = sphere_name;
  super.material = material;
 }
}

I have an ArrayList that contains all Geometry objects and I want to iterate over it to check whether the data from a text file is read in correctly. Here is my iterator method so far:

public static void check()
 {
  Iterator<Geometry> e = objects.iterator();
  while (e.hasNext())
  {
   Geometry g = (Geometry) e.next();
   if (g instanceof Sphere)
   {
    System.out.println(g.shape_name);
    System.out.println(g.material);
   }
  }
 }

How do I access and print out the Sphere's radius and center fields? Thanks in advance :)

解决方案

If you want to access properties of a subclass, you're going to have to cast to the subclass.

if (g instanceof Sphere)
{
    Sphere s = (Sphere) g;
    System.out.println(s.radius);
    ....
}

This isn't the most OO way to do things, though: once you have more subclasses of Geometry you're going to need to start casting to each of those types, which quickly becomes a big mess. If you want to print the properties of an object, you should have a method on your Geometry object called print() or something along those lines, that will print each of the properties in the object. Something like this:

class Geometry {
   ...
   public void print() {
      System.out.println(shape_name);
      System.out.println(material);
   }
}

class Shape extends Geometry {
   ...
   public void print() {
      System.out.println(radius);
      System.out.println(center);
      super.print();
   }
}

This way, you don't need to do the casting and you can just call g.print() inside your while loop.

这篇关于从Java中的基类访问子类字段的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！