/*
* the known knowledge:
* assume G is a generic class, then to G And G , regardless of whether the class A and class B has inheritance relationships
* G And G Are regarded as two independent of each other without any relationship types, namely G Object and G Object reference not each assignment
* like a Sting type object reference can't be assigned to a Date object, and vice versa
*
* but G Object and G Object references can be assigned to G<?> Object
* because G<?> The equivalent of G And G The parent class, G<?> Even can be regarded as a G
