Java泛型类强制转换异常 [英] Java generics class cast exception

问题描述

我正在尝试创建一个处理可比类的类.我归结为尝试实例化该类时给出错误的最简单代码.我收到几个编译警告(未经检查的强制转换)，但是当我运行此程序时，它将引发类广播异常.我确实查看了有关该主题的其他一些问题，但是没有遇到有用的东西.

I am trying to create a class that processes comparables. I boiled down to the simplest code that gives an error when I try to instantiate the class. I get a couple of compile warnings (unchecked cast) but when I run this program it throws a classcast exception. I did look at some of the other questions on this topic but didnt come across something useful.

public class GD<Item extends Comparable<Item>> {
   private Item[] data;
   private final int MAX_SIZE = 200;
   public GD() {
      data = (Item[]) new Object[MAX_SIZE];
   }

   public static void main(String[] args) {
     GD<String> g = new GD<String>();
   }
}

推荐答案

问题出在这里:

data = (Item[]) new Object[MAX_SIZE];

您要实例化 Object 的数组，然后尝试将其强制转换为 Item 的数组，这会引发异常，因为 Object 不会扩展您的 Item 类，因为它没有实现 Comparable .相反，您想要的是:

You are instantiating an array of Object and then you try to cast it as an array of Item, which throws an exception because Object does not extend your Item class, because it does not implement Comparable. What you would like instead is:

data = new Item[MAX_SIZE];

但是您不能这样做，因为 Item 是泛型类型.如果要动态创建此类型的对象(或对象数组)，则需要将 Class 对象传递给GD类的构造函数:

But you can't do this because Item is a generic type. If you want to create objects (or arrays of objects) of this type dynamically, you need to pass the Class object to your GD class's constructor:

import java.lang.reflect.Array;

public class GD<Item extends Comparable<Item>> {
   private Item[] data;
   private final int MAX_SIZE = 200;
   public GD(Class<Item> clazz) {
      data = (Item[]) Array.newInstance(clazz, MAX_SIZE);
   }

   public static void main(String[] args) {
     GD<String> g = new GD<String>(String.class);
   }
}

这篇关于Java泛型类强制转换异常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！