What is the order of Print Out about NEW Keyword in Inheritance?

Question

It's actually part of one interview question I got confused.

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

 class B extends A
 {
     public B()
     {
         System.out.println("B") ;
     }
 }


 A a1 = new B();
 System.out.println() ;
 A a2 = (A) new B() ;

So the question is what is the print out?

At first I thought it should print out like

B
A

B
A

But after I run at home, it gives

A
B

A
B

I understand it's inheritance and then upcasting B to A, and it's legal syntax as well, but why is A print before B?

I would expect this if the constructor was `super(); System.out.println("B");` but now I am confused. — RaminS, Mar 31 '16 at 19:36
@Gendarme: Without any explicit `super(...)` or `this(...)`, the `super()` is implicit. — Jon Skeet, Mar 31 '16 at 20:02

score 7 · Answer 1 · answered Mar 31 '16 at 19:36

why is A print before B?

Because the body of the superclass constructor is executed before the body of the subclass constructor, basically.

Think of your B() constructor as implicitly:

public B()
{
    super(); // This invokes A's constructor
    System.out.println("B") ;
}

The full details are in JLS 12.5. In particular:

This constructor does not begin with an explicit constructor invocation of another constructor in the same class (using this). If this constructor is for a class other than Object, then this constructor will begin with an explicit or implicit invocation of a superclass constructor (using super). Evaluate the arguments and process that superclass constructor invocation recursively using these same five steps.

Would this also happen if it was `B b1 = new B();`? Casting to `A` is totally irrelevant? — RaminS, Mar 31 '16 at 19:40
@Gendarme: Yup, totally irrelevant. It's just "the process of creating a new instance of B". — Jon Skeet, Mar 31 '16 at 20:02

score 2 · Answer 2 · answered Mar 31 '16 at 19:35

2

The constructor of the parent class A is called before that of the subclass B. In other words, it is equivalent to:

public B() {
    super();
    System.out.println("B");
}

answered Mar 31 '16 at 19:35

M A

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What is the order of Print Out about NEW Keyword in Inheritance?

2 Answers2