java - ClassCastException

Question

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);
    }
}

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

score 16 · Accepted Answer

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.

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.

score 7 · Accepted Answer

B extends A and therefore B can be cast as A. However the reverse is not true. An instance of A cannot be cast as B.

If you are coming from the Javascript world you may be expecting this to work, but Java does not have "duck typing".

score 1 · Accepted Answer

首先这样做：

  A aClass = new B();

现在进行显式转换，它将起作用：

   B b = (B) aClass;

这意味着显式转换必须需要隐式转换。elsewise 不允许显式转换。

score 0 · Accepted Answer

一旦创建了子类的对象，就不能将其类型转换为超类。看看下面的例子

假设： Dog 是继承自 Animal(SuperClass) 的子类

正常类型转换：

Dog dog = new Dog();
Animal animal = (Animal) dog;  //works

错误的类型转换：

Animal animal = new Animal();
Dog dog = (Dog) animal;  //Doesn't work throws class cast exception

下面的 Typecast 确实有效：

Dog dog = new Dog();
Animal animal = (Animal) dog;
dog = (Dog) animal;   //This works

编译器在运行时检查它的语法内容已实际验证

java - ClassCastException

4 回答 4

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