When inserting objects into a type-safe heterogeneous container, why do we need the class reference?

Question

I'm checking out the heterogeneous container pattern from Bloch's Effective Java and I'm trying to determine why the class reference is needed when inserting objects into the heterogeneous container. Can't I use instance.getClass() to get this reference? Isn't JPA's entity manager an example of this?

interface BlochsHeterogeneousContainer {

    <T> void put(Class<T> clazz, T instance);

    <T> T get(Class<T> clazz);
}

interface AlternativeHeterogeneousContainer {

    // Class<T> not needed because we can use instance.getClass()
    <T> void put(T instance);

    <T> T get(Class<T> clazz);
}

Answer 1

No you can't do that, as it won't give you class of reference type in case of inheritance, rather the class of actual object type.

Consider this example:

Number num = new Integer(4);
System.out.println(num.getClass());

this will print:

class java.lang.Integer

and not java.lang.Number.