How many objects were created where B is inherited from A?

Here are two classes, 

class A
{
}
class B extends A
{
}

and A is inherited from B.What confuses me is that How many objects were created when I use the following code

A a =  new B();

I know when I create an instance of B ,it will first call A's constructor and then call the constructor of B's.
Was there an instance of A been created when call A's constructor?
How many objects were created in 

A a = new B();
Jon Skeet
people
quotationmark

It creates a single object, which is an instance of B. It's already a B when the A constructor executes, as you'll see if you change your code to:

class A {
    public A() {
        System.out.println(getClass());
    }
}

class B extends A {
}

...

A a = new B(); // Prints B in the A constructor

Basically, the constructor isn't what creates an object - it's what initializes an object in the context of that class.

So you can think of the steps as:

  • Create object of type B
  • Initialize the object in the context of A (field initializers, constructor body)
  • Initialize the object in the context of B (field initializers, constructor body)

(with constructor chaining up the inheritance tree evaluating constructor arguments, of course... while the constructors are sort-of called going up the chain too, as the first part of any constructor has to chain to the same class or a superclass, the main part of the constructor body happens top-down).

For rather more detail about exactly what happens, see JLS 15.9.4 and JLS 12.5.

people

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