此Java代码是否有可能出现ClassCastException / IllegalStateException？

Question

I was a bit confused with both classcastexception and illegalstateexception, as it seems similar in most of the cases.

Here i come across with an issue in this java code

 class consumer
    {
         LOC----
    }

     public class provider extends consumer
       {
        public static void main(String args[])
        {
            consumer st=new provider();     
            provider est=(provider)st;
        }
     }

I was stuck with this part. According to me, it should generate an exception (classcast or illegalstate). but there is no exception in execution, there might be a classcastexception or an illegalstateexception. But the code executes without any exception, Reason???

I was a bit week in exception handling, trying hard to master exception handling. can any one explain why it doesn't show error at the lines

    consumer st=new provider();     
    provider est=(provider)st;

Also explain classcast and illegalstate exceptions...

Answer 1

Why do you expect a ClassCastException here? st is a provider , you explicitely created one, so assigning a provider object to a reference of type provider is OK.

The only thing is that the compiler does not 'know' that st will point to a provider object at runtime. Because the static type of st is consumer , the explicit cast to provider is required in the second line. But that's nothing special...

Answer 2

consumer st=new provider();

Here you are pointing to a provider object (RHS) and telling - "This is a consumer"(reference) , Which is true because provider extends consumer.

  provider est=(provider)st;

Here, you are pointing to a provider object (RHS) and telling- "This is a provider"(reference). Which is also correct. So, you should/will not get any error.

Answer 3

The ClassCastException is thrown by the JVM when you try to cast an object to a type which is unreachable in this hierarchy.

As per your code, there is no reason for such exception and here there is why:

1) You have two classes: consumer and provider , and the latter extends the former:

public class provider extends consumer

2) Your code is instantiating a provider class and assigning it to a consumer variable:

consumer st=new provider();

There is no reason here for any kind of exception, since provider extends consumer , all instances of a provider class are also a consumer , so you can use the consumer type to reference any kind of provider .

3) You are casting a provider instance into a ... (drum roll) provider instance!:

provider est=(provider)st;

The variable st is of consumer type, true; but you initialised it as provider before (remember the new provider() statement). So, internally it's a provider not a consumer so when make that cast nothing wrong happens.

IllegalStateExceptions work in a different way, some classes from the JVM can throw it due to a number of reasons. Even your code can throw it attending to your specific needs. The reason why is it thrown depends on each class implementation.

Answer 4

A ClassCastException is thrown when objects of incompatible types are casted. For instance

consumer c = new consumer();
provider p = (provider) c;

the above code will throw a classCastException as an object of type consumer can never be anprovider object.

In your code you are creating an object of type provider and assigning that to a reference of type consumer . This is perfectly legal because all methods that would be in a consumer object will be in provider object. so there won't be any exception thrown. It is called an Upcast .

When you are downcasting that is casting a an object from parent class to child class there may be a ClassCastException thrown if the object being cast is of the parent class type and the reference is of the child class type.

provider p = (provider) c; // will give you a ClassCastException

as the methods in the class Child will not be present in the parent class which will cause a meltdown at runtime if a child specific method is called on a Parent type Object.

Answer 5

nothing wrong with program ..so it will run without any exception... But if you will try without excception then it will give error at compile time... try this program for class cast exeption

class consumer
{
int a=0;
void method(Object ob)
{
    consumer st=(consumer)ob;
}

}
public class provider
{
public static void main(String args[])
{
    consumer st=new consumer();     
    provider est=new provider();
    st.method(est);
}
}