ClassCastException [英] ClassCastException

问题描述

我在Java中有两个类：

i have two classes in java as:

class A {

 int a=10;

 public void sayhello() {
 System.out.println("class A");
 }
}

class B extends A {

 int a=20;

 public void sayhello() {
 System.out.println("class B");
 }

}

public class HelloWorld {
    public static void main(String[] args) throws IOException {

 B b = (B) new A();
     System.out.println(b.a);
    }
}

在编译时不会给出错误，运行时它显示一个错误：
线程main中的异常java.lang.ClassCastException：A不能转换为B

at compile time it does not give error, but at runtime it displays an error : Exception in thread "main" java.lang.ClassCastException: A cannot be cast to B

推荐答案

这是因为 new A（）的编译时表达式类型为 A - 其中可以是对 B 的实例的引用，因此允许转换。

This happens because the compile-time expression type of new A() is A - which could be a reference to an instance of B, so the cast is allowed.

但是，在执行时，引用只是一个 A 的实例，所以它不能转换。 A 的实例不是 B 的实例。如果引用确实指向 B 或子类的实例，则强制转换仅适用。

At execution time, however, the reference is just to an instance of A - so it fails the cast. An instance of just A isn't an instance of B. The cast only works if the reference really does refer to an instance of B or a subclass.

这篇关于ClassCastException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！