泛型方法参数的getClass（）强制转换安全

Question

Is this cast safe?

private <T> void foo(T value) {
    final Class<T> aClass = (Class<T>) value.getClass();
} 

Does a more elegant way exist to do this?

Is it possible to avoid unchecked cast warning?

Answer 1

You can suppress the compiler warning. See What is SuppressWarnings ("unchecked") in Java?

Compiler is warning you for a reason. value can contain an object of some subtype of T and there are important differences between Class<Parent> and Class<Child> . In your case, you can see that you are not doing anything unsafe, but the compiler just follows the rules.

Imagine if the method was slightly different and there was a way to get another value of type T :

<T> void foo(T a, T b) {
    final Class<T> aClass = (Class<T>) a.getClass();

    // What can go wrong?  casting T to T
    T c = aClass.cast(b);
}

// but what if the two parameters are of different classes at runtime
foo(5, 5.5);   // ClassCastException!

Or what if we allow Class<T> to escape as a return value:

<T> Class<T> foo(T val) {
    final Class<T> aClass = (Class<T>) val.getClass();
    return aClass;
}

// uhoh is declared as Class<Number> but contains Class<Integer> at runtime
Class<Number> uhoh = foo(5);

// Ok to cast Double to Number but not to Integer
Number num = uhoh.cast(5.5);  // ClassCastException!