java - 继承和创建对象

Question

假设 Java 中有以下类:

TeachingAssistant extends Graduate

我可以制作一个Graduate 的object 并在TeachingAssistant 中使用吗？示例

TeachingAssistant TA1 = new TeachingAssistant();
Graduate grad1 = new Graduate();
grad1 = TA1; //can I do this?
TA1 = grad1;// can I do this as well?

还假设A 类扩展 B 并实现 C我可以在 A 中创建 B 和 C 的对象吗？

Answer 1

给定

class TeachingAssistant extends Graduate{}
class Graduate{}

这总结了你能做什么/不能做什么(以及为什么)

TeachingAssistant t = new TeachingAssistant(); //OK
Graduate g = new Graduate(); //OK
Graduate gt = new TeachingAssistant(); //OK
TeachingAssistant tg1= new Graduate(); //Compile error: a Graduate is NOT ALWAYS a TeachingAssistant
TeachingAssistant tg2= (TeachingAssistant) new Graduate(); //Runtime error: as previous example
TeachingAssistant tg3= (TeachingAssistant)gt; //OK: gt is an object of type TeachingAssistant 
                                              //referenced by reference of type Graduate