带有通用可变参数的 ClassCastException？

Question

I have a generic interface

{
void evaluate( TYPE... things );
}

I have a generic method with the signature

<TYPE> void genericmethod( INTERFACE<TYPE> interfase, TYPE thing )
{
//the following line throws the ClassCastException
interfase.evaluate( thing );
}

PCard is a name in my project.

So the call in the end is

INTERFACE<PCard> interfase = new WorkingImplementation<PCard>();
PCard pcard = new PCard();

this seems to throw a ClassCastException in the genericmethod, when it tries to call turn the single pcard into a varargs array.

genericmethod( interfase, pcard );

The problem is relieved when there is no call to a genericmethod and you just call the interface directly.

interfase.evaluate( pcard );

What causes the Exception (internally)? (my personal guess is java is overwhelmed)

the Exception message:

java.lang.ClassCastException: class [Ljava.lang.Object; cannot be cast to class [Llib.cardgame.CG$PCard; ([Ljava.lang.Object; is in module java.base of loader 'bootstrap'; [Llib.cardgame.CG$PCard; is in unnamed module of loader 'app')

Answer 1

Short answer: Generics and arrays don't mix. Use List<? extends TYPE> things List<? extends TYPE> things instead.

Generics are merely a compiler trick. They don't exist at runtime. Arrays have a distinct type at runtime, like String[] or Number[] . But since a generic type doesn't exist at runtime, it's not possible for the compiler to generate code that will make an array of a generic type. (A call to a varargs method creates an array implicitly to hold the varargs arguments.)

In your case, the compiler emits a warning about this, and generates code that's as close to what you want as possible: the generated code for the implicit array that is created whenever a varargs method is called will be Object[] (since TYPE , I'm guessing, has no upper bound).

If you turn on all compiler warnings, you will be notified of this. Addressing all compiler warnings properly (meaning, do not use @SuppressWarnings ) will guarantee that you won't get obscure surprises like the one you're getting now.

I recommend Gilad Bracha's generics tutorial . I have found it invaluable for understanding how generics work.