Java flow-control

Question

Can anyone explain why this code is giving output as null? When I try to call new A() instead of new B(), it is printing the current date.

class A
{
    Date d = new Date();

    public A()
    {
        printDate();
    }

    void printDate() 
    {
        System.out.println("parent");
        System.out.println(d);
    }
}

class B extends A
{
    Date d = new Date();

    public B()
    {
        super();
    }

    @Override
    void printDate()
    {
        System.out.println("child");
        System.out.println(d);
    }
}

public class Test 
{
    public static void main(String[] args) 
    {
        new B();
    }
}

Answer 1

new B() invokes the constructor of B, which invokes the constructor of A. A's constructor calls printDate(), which, due to the overriding, executes B's printDate(), which prints the value of d variable of B. However, d variable of B is not initialized yet (it will only be initialized after the constructor of A is executed). Therefore it is still null (which is the default value for reference variables).

On the other hand, when you create an instance of A (new A()), printDate of A is called, and it prints the d variable of A, which was initialized prior to the constructor of A being executed.

In case it's not clear, B.d does not override A.d, it just hides it. Only methods can be overridden.

Answer 2

Declare Date as Static

static Date d = new Date();
    public B(){
        super();
    }
    @Override
    void printDate(){
        System.out.println("child");
        System.out.println(d);
    }