Catching ClassCastException in a generic method when doing generic cast

问题

Suppose I have a method

@SuppressWarnings("unchecked")
public <T extends Number> T getNumber() {
   try {
      return (T)number; 
   } catch (ClassCastException e) {
      return null;
   }
}

Assuming number is an instance of Integer, invoking method like

Float f = getNumber();

results into a ClassCastException.

I know (somehow) that this is because of type erasure but could someone provide more profound explanation why the exception escalates up to the assignment level and is not catchable inside method?

NOTE: I do have the version public <T extends Number> T getNumber(Class<T> classT) that checks the type from classT but was hoping to get rid of passing the classT and stopped wondering the above problem.

回答1:

After type erasure, return (T)number becomes return (Number)number (since Number is the type bound of T), which doesn't throw an exception (since number is an instance of Integer).

On the other hand, the assignment

Float f = getNumber();

is compiled to

Float f = (Float) getNumber();

since getNumber() returns a Number, which can't be assigned to a Float variable without a cast.

This cast throws the ClassCastException when getNumber() is not a Float.

4.6. Type Erasure

Type erasure is a mapping from types (possibly including parameterized types and type variables) to types (that are never parameterized types or type variables). We write |T| for the erasure of type T. The erasure mapping is defined as follows...

The erasure of a type variable (§4.4) is the erasure of its leftmost bound.

来源：https://stackoverflow.com/questions/47548573/catching-classcastexception-in-a-generic-method-when-doing-generic-cast