Java: Unchecked cast from X to Y / how to implement castOrNull

问题

I have implemented this function:

 static <X,Y> Y castOrNull(X obj) {
  try {
   return (Y)obj;
  }
  catch(ClassCastException e) {
   return null;
  }
 }

This gives me the compiler warning:

Type safety: Unchecked cast from X to Y

Which I don't exactly understand. Isn't the try/catch which I am doing here a check for it? Can I ignore the warning?

Will my function work as expected or not? How would I implement it correctly?

I also tried with a obj instanceof Y check but that doesn't work because of the way Java handle generics.

Btw., this function seems quite useful to me (to make some other code more clean). I wonder if such a function may already exist in Java?

One example where I want to use it:

    void removeEmptyRawStrings() {
        for(Iterator<Entity> e = entities.iterator(); e.hasNext();) {
            RawString s = castOrNull(e.next());
            if(s != null && s.content.isEmpty()) e.remove();
        }
    }

I have cases like these quite often in my code. And I think this is more readable and simpler than anything else. But please give me a better suggestion if you have any about how to make that code even more simple.

回答1:

So the problem here is that the generic parameter Y when used for dynamic casting is treated as Object. It will never throw a CCE. You get a CCE thrown in the calling method, as you have broken static type safety.

Also X is entirely pointless here:

Almost certainly the correct solution is not to attempt anything like this. null is bad. Casting is bad.

However, if you are determined to write nonsense, you can pass the Class object:

public static <T> T evilMethod(Class<T> clazz, Object obj) {
    try {
        return clazz.cast(obj);
    } catch (ClassCastException exc) {
        return null;
    }
}

回答2:

I'm not entirely sure it will work as expected. (Depends on what you expect of course :-) but this code will for instance result in a java.lang.ClassCastException (ideone):

public class Main {

    public static void main(String[] args) {
        Integer o = Main.<String, Integer>castOrNull("hello");
    }


    public static <X, Y> Y castOrNull(X obj) {
        try {
            return (Y) obj;
        } catch (ClassCastException e) {
            return null;
        }
    }
}

@Tom Hawtin got the "correct" solution.

回答3:

You can suppress the warning in this method if you know for sure that it's not a problem by annotating it with @SuppressWarnings("unchecked")

回答4:

Thanks to the way java generics where designed this code wont work at all. Generics are only useful for compile time type checking as the classes don't use generic type information at runtime.

Your code will be compiled to this:

 static Object castOrNull(Object obj) {
  try {
   return (Object)obj;//FAIL: this wont do anything
  }
  catch(ClassCastException e) {
   return null;
  }
 }

The cast to Object will never fail, and the compiled code has no access to the generic types present at compile time. Since the cast does not happen the way it should you receive a warning for an unchecked operation.

来源：https://stackoverflow.com/questions/3933609/java-unchecked-cast-from-x-to-y-how-to-implement-castornull

标签

java

casting

instanceof

classcastexception