Suppose the following classes in Java :
TeachingAssistant extends Graduate
Can I make an object ofGraduate and use in TeachingAssistant? Example
TeachingAssistant TA1 = new TeachingAssistant();
Graduate grad1 = new Graduate();
grad1 = TA1; //can I do this?
TA1 = grad1;// can I do this as well?
Also suppose class A extends B and implements C can I make objects of B and C in A?
Given
class TeachingAssistant extends Graduate{}
class Graduate{}
This sums up what you can/cannot do (and why)
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
I believe there is a few concepts here you mentioned about.
Composition vs Inheritance
Also suppose class A extends B and implements C can I make objects of B and C in A?
I am going to make a assumption to say that C is a interface as you mentioned about implementing it. No, it is impossible to instantiate C in A. It is possible to declare object B in A as class attribute, but the use case of this is rare and you should reconsider your design solution if you are doing it.
Upcasting
grad1 = TA1; //can I do this?
Yes, you can do this, but the correct syntax should be:
grad1 = (Graduate) TA1;
Downcasting
TA1 = grad1;// can I do this as well?
You cannot do this. This will leads to compilation error.
