java抽象类，构造器未在超类中调用，为什么？

Question

Below is a simple example. I have two abstract classes, A and B , one concrete class, C , which includes the abstract method.

When I create a new C , I expected to see the constructors of A and B being called. Any idea why they are not called?

//Q.java
class Q {
    abstract class A {
        A() {
            System.out.println("in A");
        }
        public abstract void sayHi();
    }
    abstract class B extends A {
        B() {
            super();
            System.out.println("in B");
        }
    }
    class C {
        C() {
            super();
            System.out.println("in C");
        }
        public void sayHi() {
            System.out.println("Hi!");
        }
    }

    Q() {
        C Ccc = new C();
    }

    public static void main(String[] args) {
        Q z = new Q();
    }
}

Answer 1

Because the super class of c is Object

Further class names start with upper case letters A,B,C,etc

If you want to see both a and b invoked do this:

class c extends b

Answer 2

Your class c doesn't extend class b or a , so it can't call their constructors using super() and calls Object constructor instead.

Try this :

class c extends b {
    public c(){
        super();
        //...
    }
}

Answer 3

you should extends the abstract classes from class c .

class c extends b {

Without extending b explicitely, it is equivalent to

class c extends Object {

and, hence call to super invokes Object constructor.

Answer 4

Just make:

class c extends b{

It will work now.. you have not extended b in c..

Answer 5

See, we have the super class Object which all classes extend implicitly. so if we have public class A{} then it is equivalent to writing public class A extends Object{} . But if you want to extend another class of your own then you have to explicitly mention it as public class B extends A{} else java will treat Object as your super class. (NOTE: even it this case Object is a super class to 'B' but this is because it is a super class to 'A' and hence 'B'). So if you don't explicitly mention the 'B extends A' then class B will have Object as its only super class. More over you dont need mention super() in the sub class constructor. This too is called implicitly.

Answer 6

C and B are not related in any way. super class of C is java.lang.Object by default

C should extend B like

class C extends B{ .. }

In such a case, it will call the super class constructors.

Also, you do not need to call super() explicitly as it is implicitly there.